May/June 2010

In base units, the SI unit of potential difference, the volt, is written as

Potential difference and power

Two equal masses move towards one another on a frictionless air track with speeds of $60\,\text{cm s}^{-1}$ and $40\,\text{cm s}^{-1}$. On collision, they remain stuck together. What is the speed of the combined masses after the collision?

Linear momentum and its conservation

A brick with a weight of $20\,\text{N}$ is placed on an inclined plane. The component of the brick’s weight acting parallel to the plane is $10\,\text{N}$. In addition, a frictional force of $4\,\text{N}$ acts on the brick. Calculate the acceleration of the brick down the plane. Take the acceleration of free fall $g$ to be equal to $10\,\text{m s}^{-2}$.

Momentum and Newton's laws of motion

The diagram depicts two identical spheres $X$ and $Y$. At the start, $X$ is moving at speed $v$ straight towards $Y$. $Y$ is at rest. The spheres undergo an elastic collision. What happens?

Linear momentum and its conservation

Forces of $3\,\text{N}$, $4\,\text{N}$ and $5\,\text{N}$ are applied at a single point on an object. The directions of the forces may be changed. What is the magnitude of the smallest resultant force from these forces?

Equilibrium of forces

An object submerged in a liquid in a tank experiences an upthrust. What physical cause gives rise to this upthrust?

Density and pressure

An upward force of $1000\,\text{N}$ is required to raise the crane hook at constant speed. The crane is then employed to lift a load of mass $1000\,\text{kg}$ with a speed of $0.50\,\text{m s}^{-1}$. What proportion of the power produced by the crane’s motor is spent in raising the hook and the load? Take the acceleration of free fall $g$ as $10\,\text{m s}^{-2}$.

Energy conservation

A constant force $F$ acts on a car of mass $m$ and moves the car up the slope by a distance $s$ at constant velocity $v$. The slope makes an angle $\alpha$ with the horizontal. Which expression gives the efficiency of the process?

Gravitational potential energy and kinetic energy

At sea level, atmospheric pressure is $100\,\text{kPa}$. Sea water has a density of $1020\,\text{kg m}^{-3}$. At what depth in the sea is the total pressure $110\,\text{kPa}$?

Density and pressure

As ice melts, it gets smaller in volume. Which row shows the correct result for ice changing into water?

Density and pressure

In stress–strain tests on metal wires, the stress axis is usually labelled in units of $10^8\,\text{Pa}$, while the strain axis is given as a percentage. This is illustrated for one specific wire in the diagram. Determine the Young modulus for the material of the wire.

Stress and strain

Pressure multiplied by volume has the same SI base units as

SI units

A force compresses a spring. The graph plots the compressing force $F$ against the spring’s length $L$. What is the spring constant of this spring?

Elastic and plastic behaviour

Which graph shows the force-extension relationship for a rubber band when it is stretched very close to breaking?

Elastic and plastic behaviour

Electromagnetic waves from an unidentified source in space were observed to be strongly diffracted as they passed through gaps of about $10^{-5}\,\text{m}$. What type of wave are they most likely to be?

Electromagnetic spectrum

When monochromatic light is used, an interference pattern of fringes appears on a screen that is positioned a distance $D$ from two slits separated by $a$. The fringe spacing is $x$. Now both $a$ and $D$ are doubled. What is the fringe spacing in the new pattern?

Interference

Diagram 1 presents a ripple tank experiment where plane waves are diffracted by passing through a narrow slit in a metal sheet. Diagram 2 shows the same tank, but here the slit is wider. For both cases, the wave pattern arriving at the slit and the pattern leaving it are displayed. Which action would make the waves in diagram 1 diffract less, so that the emergent pattern becomes closer to the one shown in diagram 2?

Diffraction

The diagram represents a steel wire secured at one end and kept under tension at the other end by a weight suspended over a pulley. A vibration generator is fixed to the wire close to the clamped end. A stationary wave with one loop is formed. The frequency of the vibration generator is $f$. Which frequency should be selected to create a stationary wave with two loops?

Stationary waves

Which row shows the conditions in which forces act on a charged particle in a uniform electric field?

Potential difference and power

The diagram illustrates two points $P$ and $Q$ on a circle of radius $r$, separated by an angle of $90^\circ$. A positive point charge placed at the centre of the circle produces an electric field of magnitude $E$ at both $P$ and $Q$. Which expression represents the work done in transferring a unit positive charge from $P$ to $Q$?

Potential difference and power

Two parallel plates with opposite charges are set up as shown. An electron is released from rest at the surface of the negatively charged plate. The electron moves away from the negatively charged plate and towards the positively charged plate. Which graph shows how the force $F$ on the electron changes with its distance $x$ from the negative plate?

Potential difference and power

In the diagram, the shaded region shows a uniform electric field pointing away from the observer (at right-angles into the plane of the paper). A horizontal beam of electrons enters the field and travels from left to right. By the field, in which direction is this beam deflected?

Electric current

An ion passes through a sequence of electrodes in a vacuum and is accelerated. A graph showing the power supplied to the ion is drawn against time. What does the area beneath the graph between two times represent?

Energy conservation

Using energy transfer $W$ and charge $q$, how are potential difference (p.d.) and electromotive force (e.m.f.) defined?

Potential difference and power

What unit is used for resistivity?

Resistance and resistivity

The resistance of a thermistor varies with its temperature, and the resistance of a light-dependent resistor (LDR) varies with the illumination level. In what conditions would both the thermistor and the LDR have the greatest resistance?

Resistance and resistivity

Resistors P, Q and R in the circuit each have the same resistance. The battery, which has negligible internal resistance, provides a total power of $12\,\text{W}$. What is the power dissipated as heating in resistor R?

Potential difference and power

When establishing a formula for the total resistance of three separate resistors connected in series, Kirchhoff’s laws are applied. Which physics principle is used in this derivation?

Kirchhoff's laws

A source with e.m.f. of $9.0\,\text{mV}$ and internal resistance $6.0\,\Omega$ is connected across a galvanometer with resistance $30\,\Omega$. What current flows through the galvanometer?

Practical circuits

In every resistor setup, the ammeter has a resistance of $2\,\Omega$. For which setup is the ammeter reading greatest when the same potential difference is applied across points P and Q?

Practical circuits

Which descriptions correctly apply to a $\gamma$-ray and a $\beta$-particle?

Atoms, nuclei and radiation

The chart displays several nuclides organised by their numbers of protons and neutrons. A nucleus of the nuclide $^{8}_{3}\text{Li}$ decays by emitting a $\beta$-particle. Which nuclide is formed as a result?

Atoms, nuclei and radiation

Which quantity does not remain conserved in nuclear processes?

Atoms, nuclei and radiation

A square-wave trace is displayed on the screen of a cathode-ray oscilloscope. The screen is overlaid with a grid of $1\,\text{cm}$ squares. The time-base setting is $10\,\text{ms cm}^{-1}$. What is the approximate frequency of the square wave?

Practical circuits

The diagram below shows a decay chain that includes two $\alpha$-particles and one $\beta$-particle. $^{217}_{85}\text{At} \xrightarrow{\alpha} V \xrightarrow{\alpha} W \xrightarrow{\beta} X$ Which nuclide is X?

Atoms, nuclei and radiation

A vector quantity $V$ is broken down into two perpendicular components $X$ and $Y$. The angle between $V$ and component $X$ is $\theta$. The angle between component $X$ and the vector $V$ is then increased from $0^{\circ}$ to $90^{\circ}$. In this situation, how do the magnitudes of $X$ and $Y$ vary as the angle $\theta$ is increased?

Scalars and vectors

A student determines the density of a liquid by measuring its mass and its volume. His measurements are listed below: mass of empty beaker = $(20 \pm 1)\,\text{g}$; mass of beaker + liquid = $(70 \pm 1)\,\text{g}$; volume of liquid = $(10.0 \pm 0.6)\,\text{cm}^3$. He works out the density of the liquid correctly as $5.0\,\text{g cm}^{-3}$. What is the uncertainty in this value?

Errors and uncertainties

A micrometer screw gauge is used to determine the diameter of a copper wire. Diagram 1 shows the reading when the wire is in place. The wire is then taken out and the jaws of the micrometer are closed. Diagram 2 shows the new reading. What is the diameter of the wire?

Errors and uncertainties

The diagram presents a velocity-time graph for a vehicle. Travelling at $4.0\,\text{m s}^{-1}$, the vehicle starts to accelerate when time $= 0$. What is the vehicle’s acceleration at time $= 3.0\,\text{s}$?

Equations of motion

A small steel ball is released from rest and falls freely under gravity. Which graph best shows how the height $h$ of the ball changes with time $t$?

Equations of motion

A micrometer screw gauge is being used to find the diameter of a copper wire. With the wire inserted, the reading is shown in diagram 1. The wire is then taken out and the jaws of the micrometer are closed. The new reading is shown in diagram 2. State the diameter of the wire.

Errors and uncertainties

In the diagram, there are two identical spheres X and Y. At first, X travels at speed $v$ straight towards Y, while Y is at rest. The spheres then undergo an elastic collision. What occurs?

Linear momentum and its conservation

Two equal masses move towards one another on a frictionless air track with speeds of $60\ \text{cm s}^{-1}$ and $40\ \text{cm s}^{-1}$. On collision, they remain stuck together. What speed do the masses have after the collision?

Linear momentum and its conservation

Forces of $3\ \text{N}$, $4\ \text{N}$ and $5\ \text{N}$ act together at a single point on an object. The directions in which the forces act may change. What is the value of the minimum resultant force of these forces?

Equilibrium of forces

When an object is placed in a liquid inside a tank, it experiences an upthrust. What physical cause produces this upthrust?

Density and pressure

A brick with weight $20\ \text{N}$ is at rest on a sloping plane. Its weight has a $10\ \text{N}$ component acting parallel to the plane. The brick also has a frictional force of $4\ \text{N}$ acting on it. What is the acceleration of the brick down the plane? Assume that the acceleration of free fall $g$ is equal to $10\ \text{m s}^{-2}$.

Momentum and Newton's laws of motion

A constant force $F$, acting on a car of mass $m$, moves the car up the incline through a distance $s$ at constant velocity $v$. The angle between the incline and the horizontal is $\alpha$. Which expression represents the efficiency of this process?

Gravitational potential energy and kinetic energy

An upward force of $1000\ \text{N}$ is required to raise the crane hook at constant speed. The crane is then employed to raise a load with mass $1000\ \text{kg}$ at a speed of $0.50\ \text{m s}^{-1}$. What fraction of the power produced by the crane motor is devoted to lifting the hook and the load? Take the acceleration due to gravity $g$ to be $10\ \text{m s}^{-2}$.

Gravitational potential energy and kinetic energy

As ice melts, it shrinks. Which row gives the correct result for ice changing into water?

Density and pressure

The atmospheric pressure at sea level is $100\,\text{kPa}$. Sea water has a density of $1020\,\text{kg m}^{-3}$. At what depth in the sea would the total pressure be $110\,\text{kPa}$?

Density and pressure

An applied force compresses a spring. The graph gives compressing force $F$ as a function of the spring length $L$. What is the spring constant for this spring?

Elastic and plastic behaviour

In base units, the SI unit of potential difference (the volt) is expressed as

Potential difference and power

Which graph shows the force-extension relationship for a rubber band stretched to nearly its breaking point?

Elastic and plastic behaviour

In stress-strain experiments carried out on metal wires, the stress axis is commonly labelled in units of $10^8\,\text{Pa}$, while the strain axis is shown as a percentage. This is illustrated for one specific wire in the diagram. What is the value of the Young modulus for the material of the wire?

Stress and strain

The diagram depicts a steel wire fixed at one end and kept under tension at the other end by a weight suspended over a pulley. A vibration generator is connected to the wire close to the fixed end. A stationary wave with one loop is formed. The frequency of the vibration generator is $f$. Which frequency should be chosen to form a stationary wave with two loops?

Stationary waves

Diagram 1 shows a ripple tank experiment in which plane waves are diffracted as they pass through a narrow slit in a metal sheet. Diagram 2 shows the same tank with a slit that is wider. In both cases, the wave pattern arriving at the slit and the wave pattern after the slit are shown. Which action would make the waves in diagram 1 diffract less and therefore produce an emergent pattern closer to the one shown in diagram 2?

Diffraction

Electromagnetic waves from an unidentified source in space were observed to diffract strongly as they passed through gaps of the order of $10^{-5}\,\text{m}$. What type of wave are they most likely to be?

Electromagnetic spectrum

With monochromatic light, interference fringes are formed on a screen located a distance $D$ from a pair of slits with separation $a$. The fringe spacing is $x$. Both $a$ and $D$ are then doubled. What will the new fringe separation be?

Interference

As illustrated, two parallel plates carry opposite charges. An electron is let go from rest at the surface of the negatively charged plate. It moves from the negatively charged plate towards the positively charged plate. Which graph represents how the force $F$ on the electron changes with its distance $x$ from the negative plate?

Potential difference and power

In the diagram, the shaded region shows a uniform electric field pointing away from the observer (perpendicular into the plane of the paper). An electron beam travels horizontally into the field from left to right. In what direction does the field deflect this beam?

Electric current

The diagram depicts two points $P$ and $Q$ separated by an angle of $90^{\circ}$ on a circle with radius $r$. A positive point charge placed at the centre of the circle produces an electric field of magnitude $E$ at both $P$ and $Q$. Which expression represents the work done in moving a unit positive charge from $P$ to $Q$?

Potential difference and power

Which row gives the circumstances under which a charged particle experiences forces in a uniform electric field?

Potential difference and power

A student determines the density of a liquid by finding its mass and its volume. The measurements are summarised below. mass of empty beaker $= (20 \pm 1)\,\text{g}$ mass of beaker + liquid $= (70 \pm 1)\,\text{g}$ volume of liquid $= (10.0 \pm 0.6)\,\text{cm}^3$ He works out the density of the liquid correctly as $5.0\,\text{g cm}^{-3}$. What is the uncertainty in this value?

Errors and uncertainties

A thermistor’s resistance varies with its temperature, while a light-dependent resistor (LDR) has a resistance that varies with the illumination. In which conditions is the resistance of both a thermistor and an LDR greatest?

Resistance and resistivity

With respect to energy transfer $W$ and charge $q$, what is meant by potential difference (p.d.) and electromotive force (e.m.f.)?

Potential difference and power

For every resistor arrangement, the ammeter has a resistance of $2\,\Omega$. When the same potential difference is applied between points P and Q, which arrangement produces the greatest ammeter reading?

Practical circuits

A source with an e.m.f. of $9.0\,\text{mV}$ and an internal resistance of $6.0\,\Omega$ is connected across a galvanometer whose resistance is $30\,\Omega$. What will be the current in the galvanometer?

Practical circuits

Resistors P, Q and R in the circuit each have the same resistance. The battery, with negligible internal resistance, delivers a total power of $12\,\text{W}$. What is the power dissipated as heat in resistor R?

Potential difference and power

Kirchhoff’s laws are applied to derive a formula for the total resistance of three different resistors connected in series. Which physics principle is used in this derivation?

Kirchhoff's laws

What unit is resistivity measured in?

Resistance and resistivity

The grid displays a collection of nuclides arranged according to the number of protons and the number of neutrons in each nucleus. A nucleus of the nuclide $^8_3\text{Li}$ decays by emitting a $\beta$-particle. What nuclide is produced?

Atoms, nuclei and radiation

The decay chain below shows two $\alpha$-decays followed by one $\beta$-decay. $^{217}_{85}\text{At} \xrightarrow{\alpha} V \xrightarrow{\alpha} W \xrightarrow{\beta} X$ What nuclide is X?

Atoms, nuclei and radiation

Which descriptions are correct for a $\gamma$-ray and a $\beta$-particle?

Atoms, nuclei and radiation

An ion is speeded up by a chain of electrodes in a vacuum. A graph showing the power supplied to the ion is drawn against time. What does the area beneath the graph between two times represent?

Energy conservation

What quantity is not conserved during nuclear processes?

Atoms, nuclei and radiation

A square-wave trace is shown on the screen of a cathode-ray oscilloscope. The screen is overlaid with a grid made of $1\,\text{cm}$ squares. The time-base setting is $10\,\text{ms cm}^{-1}$. Estimate the frequency of the square wave.

Practical circuits

A vector quantity $V$ is split into two perpendicular components $X$ and $Y$. The angle between $V$ and component $X$ is $\theta$. The angle between component $X$ and the vector $V$ is increased from $0^\circ$ to $90^\circ$. As $\theta$ is increased in this manner, what happens to the magnitudes of $X$ and $Y$?

Scalars and vectors

The SI base units for the product of pressure and volume are the same as those for

Energy conservation

A small steel ball is dropped from rest and moves freely under gravity. Which graph best shows how the height $h$ of the ball varies with time $t$?

Equations of motion

The diagram presents a velocity-time graph for a vehicle. At time $= 0$, the vehicle is travelling at $4.0\,\text{m s}^{-1}$ and then starts to accelerate. What is the vehicle’s acceleration at time $= 3.0\,\text{s}$?

Equations of motion

the quantity whose SI base units are the same as those of pressure × volume

SI units

A liquid in a tank exerts an upthrust on an object that is immersed in it. What physical cause produces this upthrust?

Density and pressure

At a single point on an object, forces of $3\,\text{N}$, $4\,\text{N}$ and $5\,\text{N}$ act. The directions of these forces may be adjusted. What is the minimum possible value of the resultant force from these forces?

Equilibrium of forces

Two masses of equal mass move towards one another on a frictionless air track at speeds of $60\,\text{cm s}^{-1}$ and $40\,\text{cm s}^{-1}$. They become joined when they collide. What is the speed of the masses immediately after the collision?

Linear momentum and its conservation

A small steel ball is dropped from rest and moves freely under gravity. Which graph best shows how the height $h$ of the ball changes with time $t$?

Equations of motion

The diagram presents a velocity-time graph for a vehicle. At time $= 0$, the vehicle, travelling at $4.0\,\text{m s}^{-1}$, starts to accelerate. What is the vehicle’s acceleration at time $= 3.0\,\text{s}$?

Equations of motion

The atmospheric pressure at sea level is $100\,\text{kPa}$. Sea water has a density of $1020\,\text{kg m}^{-3}$. At what depth in the sea will the total pressure be $110\,\text{kPa}$?

Density and pressure

As ice melts, it contracts. Which row is correct for ice changing into water?

Density and pressure

A car of mass $m$ is pulled uphill over a distance $s$ at constant speed $v$ by a constant force $F$. The incline makes an angle $\alpha$ with the horizontal. Which expression gives the efficiency of the process?

Gravitational potential energy and kinetic energy

A force of $1000\,\text{N}$ is required to raise the hook of a crane at constant speed. The crane is then used to lift a load of mass $1000\,\text{kg}$ at a speed of $0.50\,\text{m s}^{-1}$. What amount of the power developed by the motor of the crane is used in lifting the hook and the load? Assume the acceleration of free fall $g$ is equal to $10\,\text{m s}^{-2}$.

Gravitational potential energy and kinetic energy

Which graph shows the force-extension relationship for a rubber band stretched almost to the point where it breaks?

Elastic and plastic behaviour

A vector quantity $V$ is split into two perpendicular components $X$ and $Y$. The angle between $V$ and component $X$ is $\theta$. The angle between component $X$ and the vector $V$ is then increased from $0^{\circ}$ to $90^{\circ}$. How do the magnitudes of $X$ and $Y$ vary as the angle $\theta$ is increased in this way?

Scalars and vectors

In stress-strain investigations on metal wires, the stress axis is commonly labelled in units of $10^8\,\text{Pa}$, while the strain axis is given as a percentage. The diagram shows this for one particular wire. What is the value of the Young modulus for the material of the wire?

Stress and strain

A force compresses a spring. The graph shows the compressive force $F$ against the spring’s length $L$. What is the spring constant of this spring?

Elastic and plastic behaviour

When monochromatic light is used, interference fringes appear on a screen positioned a distance $D$ from a pair of slits separated by $a$. The fringe spacing is $x$. If both $a$ and $D$ are doubled, what is the new fringe separation?

Interference

Electromagnetic waves from an unidentified source in space were seen to be strongly diffracted as they passed through gaps of the order of $10^{-5}\,\text{m}$. Which type of wave are they most likely to be?

Diffraction

The diagram depicts a steel wire fixed at one end and made taut at the other end by a weight suspended over a pulley. A vibration generator is connected to the wire close to the fixed end. A stationary wave with one loop is formed. The frequency of the vibration generator is $f$. Which frequency should be used to form a stationary wave with two loops?

Stationary waves

Diagram 1 illustrates a ripple tank experiment in which plane waves pass through a narrow slit in a metal sheet and are diffracted. Diagram 2 shows the same tank with a slit that is wider. For each case, the wave pattern arriving at the slit and the resulting pattern after the slit are displayed. Which change would make the waves in diagram 1 diffract less, producing an emergent pattern more like the one shown in diagram 2?

Diffraction

In the diagram, the shaded region shows a uniform electric field pointing away from the observer (that is, at right angles into the plane of the paper). A horizontal beam of electrons enters the field, moving from left to right. In which direction does this beam get deflected by the field?

Electric current

Two parallel plates with opposite charges are set up as shown. An electron is released from rest at the surface of the negatively charged plate. The electron moves away from the negatively charged plate towards the positively charged plate. Which graph shows how the force $F$ acting on the electron changes with its distance $x$ from the negative plate?

Potential difference and power

Which row shows the conditions in which forces act on a charged particle in a uniform electric field?

Electric current

The diagram indicates two points P and Q that are separated by $90^{\circ}$, on a circle of radius $r$. A positive point charge placed at the centre of the circle produces an electric field of magnitude $E$ at both P and Q. Which expression gives the work done in moving a unit positive charge from P to Q?

Potential difference and power

The diagram depicts a square-wave trace on the screen of a cathode-ray oscilloscope. A grid made of $1\,\text{cm}$ squares overlays the screen. The time-base setting is $10\,\text{ms cm}^{-1}$. What is the approximate frequency of the square wave?

Practical circuits

What unit is used for resistivity?

Resistance and resistivity

A source with an e.m.f. of $9.0\text{ mV}$ and internal resistance of $6.0\,\Omega$ is connected across a galvanometer of resistance $30\,\Omega$. What current flows through the galvanometer?

Practical circuits

Using energy transfer $W$ and charge $q$, how are potential difference (p.d.) and electromotive force (e.m.f.) defined?

Potential difference and power

A thermistor’s resistance varies with temperature, whereas the resistance of a light-dependent resistor (LDR) varies with the illumination. In what conditions will both the thermistor and the LDR have their greatest resistance?

Resistance and resistivity

For every resistor combination shown, the ammeter has a resistance of $2\,\Omega$. When the same potential difference is applied across points P and Q, which arrangement produces the greatest ammeter reading?

Practical circuits

Resistors P, Q and R in the circuit each have the same resistance. The battery has negligible internal resistance and delivers a total power of $12\text{ W}$. What power is dissipated as heat in resistor R?

Practical circuits

While obtaining a formula for the total resistance of three different resistors connected in series, Kirchhoff’s laws are applied. Which physics principle is used in this derivation?

Kirchhoff's laws

Which quantity is not conserved during nuclear processes?

Atoms, nuclei and radiation

Which descriptions correctly describe a $\gamma$-ray and a $\beta$-particle?

Atoms, nuclei and radiation

The decay chain below shows two $\alpha$-particles and one $\beta$-particle. $^{217}_{85}\text{At} \xrightarrow{\alpha} V \xrightarrow{\alpha} W \xrightarrow{\beta} X$ Which nuclide is X?

Atoms, nuclei and radiation

A student determines the density of a liquid by recording its mass and volume. The measurements are summarised below: mass of empty beaker $= (20 \pm 1)\,\text{g}$ mass of beaker + liquid $= (70 \pm 1)\,\text{g}$ volume of liquid $= (10.0 \pm 0.6)\,\text{cm}^3$ He works out the density of the liquid correctly as $5.0\,\text{g cm}^{-3}$. What uncertainty should be assigned to this value?

Errors and uncertainties

The grid displays several nuclides organised by the number of protons and the number of neutrons in each one. A nucleus of the nuclide $^{8}_{3}\text{Li}$ undergoes decay by emitting a $\beta$-particle. What nuclide is produced?

Atoms, nuclei and radiation

A micrometer screw gauge is used to determine the diameter of a copper wire. The measurement taken with the wire in place is shown in diagram 1. The wire is then taken out and the jaws of the micrometer are closed. The updated reading is shown in diagram 2. Determine the diameter of the wire.

Errors and uncertainties

In base units, the SI unit of potential difference (the volt) is written as

Potential difference and power

An ion moves through a vacuum and is accelerated by a sequence of electrodes. A graph shows the power delivered to the ion against time. What does the area under the graph between two times represent?

Energy conservation

A brick with weight $20\,\text{N}$ is stationary on an inclined plane. Its weight has a $10\,\text{N}$ component acting parallel to the plane. The brick is also acted on by a frictional force of $4\,\text{N}$. What is the acceleration of the brick down the plane? Take the acceleration of free fall $g$ to be equal to $10\,\text{m s}^{-2}$.

Momentum and Newton's laws of motion

The diagram depicts two identical spheres X and Y. At the start, X is travelling at speed $v$ straight towards Y, while Y is at rest. The spheres undergo an elastic collision. What happens?

Linear momentum and its conservation