May/June 2024

Which unit counts as an SI base unit?

SI units

What do the terms mass and weight of an object on the Earth mean?

Physical quantities

A thin horizontal plate with an area of $0.036\,\text{m}^2$ lies below the surface of a liquid that has a density of $930\,\text{kg m}^{-3}$. The force acting on one side of the plate as a result of the liquid pressure is $290\,\text{N}$. What is the depth of the plate below the liquid surface?

Density and pressure

Spheres $X$ and $Y$ make up an isolated system. Sphere $Y$ has a greater mass than sphere $X$. At the start, sphere $Y$ is at rest. Sphere $X$ undergoes an elastic collision with sphere $Y$. Before the collision, the speed of sphere $X$ is $u$. Which statement must be correct?

Linear momentum and its conservation

A $0.10\,\text{kg}$ ball is propelled towards a vertical bat that is stationary. The ball strikes the bat with a horizontal velocity of $20\,\text{m s}^{-1}$. It then rebounds and departs from the bat with a horizontal velocity of $15\,\text{m s}^{-1}$. What is the change in momentum of the ball?

Momentum and Newton's laws of motion

An isolated object with negligible weight is subject to two coplanar forces of equal magnitude. In which diagram is the object in equilibrium?

Equilibrium of forces

A beam $PQ$ of uniform density lies horizontally on a support at point $S$. A rope is fixed to one end of the beam. The rope makes an angle of $60^{\circ}$ with the vertical and applies a force $F$, measured in newtons, to the beam. What is the moment, in $\text{N m}$, of the force $F$ about point $S$?

Turning effects of forces

The diagram illustrates the dimensions of an elastic cord that is used to launch a stone. When the cord is stretched into the arrangement shown, the tension in the cord is $T$. Which force is exerted on the stone by the elastic cord?

Equilibrium of forces

A box with weight $40\,\text{N}$ is moved across level ground through a distance of $2.4\,\text{m}$ by a horizontal force of $20\,\text{N}$. It is then raised vertically at constant velocity by a height of $1.6\,\text{m}$ using a vertical force. What is the total work done on the box by these two forces?

Gravitational potential energy and kinetic energy

Which of the following statements about efficiency is correct?

Energy conservation

A plane wave with amplitude $A$ falls on a surface of area $S$, arranged so that it is perpendicular to the wave’s direction of travel. The rate at which energy reaches the surface is $E$. The wave amplitude is then raised to $2A$, while the surface area is reduced to $\tfrac{1}{2}S$. What energy per unit time reaches this smaller surface?

Progressive waves

Which one of the following may have the same units as force?

SI units

A steel ball is descending through oil at a constant speed. Which graph shows how the gravitational potential energy $E_p$ and the kinetic energy $E_k$ of the ball change with time?

Gravitational potential energy and kinetic energy

A force of $0.80\,\text{N}$ stretches a spring to a length of $90\,\text{mm}$. The same spring has a length of $115\,\text{mm}$ when a force of $1.30\,\text{N}$ is applied. The spring obeys Hooke’s law. Determine the spring constant of the spring.

Elastic and plastic behaviour

A metal wire is used in an experiment to see how it responds as the tensile force applied to it is varied. The wire has a constant cross-sectional area. Which graph has a gradient equal to the Young modulus of the metal?

Stress and strain

For a wire, Hooke’s law is obeyed for a tension $F$ and extension $x$. The Young modulus for the material of the wire is $E$. Which expression gives the elastic potential energy stored in the wire?

Stress and strain

A plane-polarised wave of amplitude $A$ is directed normally onto a polarising filter. The transmission axis of the filter is at angle $\theta$ to the plane of polarisation of the incident wave. What is the amplitude of the wave that emerges from the filter?

Polarisation

An electromagnetic wave is moving through a vacuum. What values could the wavelength and period of the electromagnetic wave be?

Progressive waves

Light with frequency $6.7 \\times 10^{14}\\,\\text{Hz}$ travelling through vacuum is incident perpendicularly on a diffraction grating containing $4.0 \\times 10^5$ lines $\\text{m}^{-1}$. What is the angle between the neighbouring second-order and third-order intensity maxima?

The diffraction grating

A siren on a police car that is moving produces sound waves of frequency $440\,\text{Hz}$. A fixed observer detects a sound frequency of $494\,\text{Hz}$. The speed of sound in air is $340\,\text{m s}^{-1}$. What are the possible speed and direction of travel of the car?

Doppler effect for sound waves

The diagram illustrates the form, at a particular instant, of part of a stretched string as a wave moves along it from left to right. In which directions are the velocities of points 1, 2 and 3 on the string at this instant?

Transverse and longitudinal waves

Which type of wave cannot be longitudinal?

Transverse and longitudinal waves

An object’s velocity changes from an initial velocity $u$ to a final velocity $v$. These velocities are shown by the vectors. Which one vector shows the change in velocity of the object?

Scalars and vectors

Microwaves are produced by two sources located at points X and Y. These two waves overlap at point Z. The diagram illustrates the routes taken by the two waves. The waves released from points X and Y are coherent. What direct effect does coherence between the two waves have?

Interference

What unit is resistivity measured in?

Resistance and resistivity

A kettle is linked to a $250\,\text{V}$ mains supply. What could the power of the kettle and the current in the kettle be?

Potential difference and power

In which circuit does the output voltage $V_{out}$ rise as temperature increases?

Potential dividers

The circuit shows four resistors, each having resistance $R$. The resistance measured between point X and point Y is $120\,\Omega$ altogether. What is the value of the resistance $R$?

Kirchhoff's laws

A cell that has internal resistance is joined to a variable resistor $R$ as shown. The resistance of $R$ is then reduced step by step. What happens to the current $I$ and to the terminal potential difference (p.d.) across the cell?

Practical circuits

The diagram represents a circuit containing a cell and three resistors with resistances $R_1$, $R_2$ and $R_3$. The cell has negligible internal resistance. The total resistance of the circuit is $R_T$. Which equation for $R_T$ is correct?

Resistance and resistivity

Hydrogen and deuterium may be shown by the nuclide symbols $^1_1\text{H}$ and $^2_1\text{H}$ respectively. State one difference between hydrogen and deuterium.

Atoms, nuclei and radiation

A radioactive sample undergoes decay by releasing $\beta^-$ particles. The energy given out in the decay is identical for every nucleus that decays, yet the $\beta^-$ particles emitted possess a continuous spread of kinetic energies. Which statement accounts for the fact that the $\beta^-$ particles are emitted with a continuous spread of kinetic energies?

Atoms, nuclei and radiation

Which particle is not classified as a fundamental particle?

Fundamental particles

An object travels with initial velocity $u$. It then continues with uniform acceleration $a$ for a time $t$ until it has final velocity $v$. Which equation describes the motion of the object?

Equations of motion

What charge is carried by an anti-top quark?

Fundamental particles

Which operation results in a vector quantity?

Scalars and vectors

A thermometer may be read with an accuracy of $\pm 0.5^\circ\text{C}$. It is used to measure a rise in temperature from $40^\circ\text{C}$ to $100^\circ\text{C}$. What is the percentage uncertainty in the measurement of the temperature rise?

Errors and uncertainties

The diagram illustrates the trajectory of a golf ball. Which row describes how the horizontal and vertical components of the golf ball’s velocity change when air resistance is neglected?

Equations of motion

An aircraft travels from London to Sydney in 21 hours 40 minutes. The distance covered is $17\,000\,\text{km}$. Determine the aircraft's average speed.

Equations of motion

A golf club strikes a golf ball. The graph indicates how the force $F$ acting on the ball changes with time $t$. Which graph shows how the ball’s velocity $v$ changes with time $t$?

Momentum and Newton's laws of motion

What does the term scalar quantity mean?

Scalars and vectors

A lead pellet is fired vertically upwards into a clay block, which is at rest at the instant of impact but can move upwards freely after the impact. The pellet has a mass of $5.0\ \text{g}$ and the clay block has a mass of $95\ \text{g}$. The pellet strikes the block with an initial vertical velocity of $200\ \text{m s}^{-1}$. It becomes embedded in the block and does not come out. How high above its starting position does the block rise?

Linear momentum and its conservation

An object is acted on by two forces. Which diagram shows a couple?

Turning effects of forces

The diagram represents a uniform rod, XY, which is freely hinged to a vertical wall at end Y. The rod makes an angle of $60^{\circ}$ with the wall. At end X, a force $F$ acts at an angle of $30^{\circ}$ to the rod. The rod weighs $3.0\ \text{N}$ and is in equilibrium. Determine the magnitude of force $F$?

Turning effects of forces

The density of water is $1.0\ \text{g cm}^{-3}$. The density of glycerine is $1.3\ \text{g cm}^{-3}$. A student pours a volume of $40\ \text{cm}^3$ of glycerine into a container. The student then adds water to the container in order to produce a water-and-glycerine mixture. Assume that, when the two liquids are combined, the total volume of water and glycerine stays unchanged. What volume of water must be added to obtain a mixture with density $1.1\ \text{g cm}^{-3}$?

Density and pressure

The force that opposes the motion of a car is proportional to the square of the car’s speed. At a speed of $20.0\ \text{m s}^{-1}$, the force has a magnitude of $800\ \text{N}$. What useful output power must the car’s engine provide in order to keep the car moving at a constant speed of $40.0\ \text{m s}^{-1}$?

Energy conservation

A box with weight $W$ is drawn up a slope by a force $P$. The slope has length $d$, and the box gains a vertical height of $h$. The frictional force acting between the box and the slope is $F$. The diagram indicates the directions of the forces. The slope is used to lift the box vertically. Which expression represents the efficiency of the slope?

Gravitational potential energy and kinetic energy

The kinetic energy of a particle is multiplied by $4$. By what factor does its speed rise?

Energy conservation

A mass of $28\ \text{g}$ is lifted vertically by a distance of $4.6\ \text{m}$. What change occurs in the gravitational potential energy of the mass?

Gravitational potential energy and kinetic energy

A sample of metal is put under a force that rises to a maximum value and then falls again to zero. The force-extension graph for the sample is shown. As the sample shortens, it follows the same force-extension curve as when it was being stretched. What is the behaviour of the metal between X and Y?

Elastic and plastic behaviour

Two wires, P and Q, are made from the same material and are pulled with a force that increases steadily. A graph is drawn to show how the extension of each wire varies with force. Both wires have the same original length, but their diameters are different. Determine the ratio $\frac{\text{diameter of wire Q}}{\text{diameter of wire P}}$?

Stress and strain

Quantity $X$ has a percentage uncertainty of $2\%$. Quantity $Y$ has a percentage uncertainty of $4\%$. A quantity $W$ is found using the values of $X$ and $Y$. The percentage uncertainty in $W$ is $8\%$. What relationship could link $W$, $X$ and $Y$?

Errors and uncertainties

A graph showing extension against force for a spring is shown. What is the spring constant of the spring?

Elastic and plastic behaviour

A man is standing still in front of a swing. A child is seated on it and swinging. The child sounds a whistle that produces a note at a steady frequency. The man notes the frequency of the sound when the swing is at positions X, Y and Z. At which position will the man hear the greatest frequency?

Doppler effect for sound waves

A loudspeaker in a room is emitting music. The door into the room is open, and its width is $0.80\,\text{m}$. Sound waves of a range of frequencies travel through the opening and diffract. The speed of sound in air is $340\,\text{m s}^{-1}$. Which sound-wave frequency will diffract the most as it goes through the doorway?

Diffraction

A stationary sound wave is formed in the air between a loudspeaker and a wall. A microphone, linked to a cathode-ray oscilloscope (CRO), is moved along a straight line that lies directly between the loudspeaker and the wall. The CRO trace amplitude increases to a maximum at a point X, decreases to a minimum and then increases again to a maximum at a point Y. The separation between X and Y is $33\,\text{cm}$. The speed of sound in air is $330\,\text{m s}^{-1}$. Which diagram could show the CRO trace of the sound detected at X?

Stationary waves

Polarisation occurs with some waves. Which waves are unable to be polarised?

Polarisation

A guitar string is plucked. Which statement describes the waves produced?

Stationary waves

A diffraction grating with $N$ lines per metre is used to diffract light of a range of wavelengths $\lambda$. The graph illustrates how the diffraction angle $\theta$ varies with $\lambda$ for several wavelengths in the $n$th order interference pattern. What is the gradient of the graph?

The diffraction grating

A stationary sound wave is produced in the air column inside a tube open at both ends. The stationary wave contains three nodes. How many antinodes are present?

Stationary waves

Interference fringes with spacing $x$ are seen on a screen placed $1.00\,\text{m}$ from a double slit illuminated by yellow light with wavelength $600\,\text{nm}$. At what distance from the double slit would fringes with the same spacing $x$ be produced if blue light of wavelength $400\,\text{nm}$ were used?

Interference

What does the term potential difference across a component mean?

Potential difference and power

A football is kicked so that it travels straight up through the air. How do the air resistance and the resultant force on the ball change as it rises vertically?

Non-uniform motion

The diagram represents a cell with negligible internal resistance joined to a switch and two resistors with resistances $R$ and $2R$. The circuit also includes two ammeters X and Y. When the switch is open, the reading on X is $4.0\,\text{A}$. What readings are shown on X and Y once the switch has been closed?

Practical circuits

The $I$-$V$ characteristics of four components, A, B, C and D, are shown. At a potential difference of $V_1$, which component has the largest resistance?

Resistance and resistivity

A cylindrical wire has cross-sectional area $A$ and free-electron number density $n$. The wire carries current $I$, and the free electrons move with mean drift speed $v$. A second cylindrical wire has cross-sectional area $0.5A$ and free-electron number density $2n$. In this wire, the free electrons move with mean drift speed $2v$. What current is in the second wire?

Electric current

An electric current is produced by charge carriers that are moving. Which charge is impossible for a charge carrier to have?

Electric current

Which statement correctly describes Kirchhoff’s first law?

Kirchhoff's laws

Two resistors with resistances $r$ and $R$ are connected in parallel. The value of $r$ is smaller than the value of $R$. Which statement about the total resistance of the pair is correct?

Resistance and resistivity

The diagram illustrates four identical resistors arranged in a circuit. Which resistor loses the greatest power?

Potential difference and power

Nuclide X has proton number $Z$ and undergoes $\beta^+$ decay to produce nuclide Y. This decay can be shown by the equation below. $X \rightarrow Y + \beta^+ + W$. What is the proton number of Y, and which particle does the symbol $W$ stand for?

Atoms, nuclei and radiation

Which elementary particles make up a hadron?

Fundamental particles

The unstable nuclide $^{218}_{84}X$ undergoes a decay chain involving the emission of $\alpha$ and $\beta^-$ particles until the stable nuclide $^{210}_{83}Y$ is produced. How many $\alpha$ and $\beta^-$ particles are released in this decay process?

Atoms, nuclei and radiation

Which statement is false?

Physical quantities

Which of the following statements about radioactive decay is correct?

Atoms, nuclei and radiation

The diagram depicts a laboratory experiment in which a feather drops from rest through a long vertical evacuated tube of length $L$. The feather takes time $T$ to travel from the top to the bottom of the tube. How far down from the top of the tube has the feather fallen in time $0.50T$?

Equations of motion

A car moves along a straight, level road. The graph displays how the car’s velocity $v$ changes with time $t$ over $6.0\,\text{s}$ of its motion. The car’s brakes are used from $t = 1.0\,\text{s}$ until $t = 4.0\,\text{s}$. What distance does the car cover during the time the brakes are applied?

Equations of motion

In deep space, two satellites undergo an inelastic collision. What happens to the total kinetic energy and the total momentum?

Linear momentum and its conservation

What would be a sensible estimate for the momentum of a family car moving at $25\ \text{kilometres per hour}$?

Momentum and Newton's laws of motion

A ball hits a wall. Before the impact, the ball is moving with velocity $8\ \text{m s}^{-1}$ to the right. After the impact, it is moving with velocity $3\ \text{m s}^{-1}$ to the left. Determine the change in velocity of the ball in the collision?

Scalars and vectors

Which quantity is equal to $0.00005\ \text{J}$?

SI units

The diagram gives a view from above of a sprinkler system used to water a garden. The sprinkler is made from a tube of length $0.15\,\text{m}$. The tube is pivoted at its midpoint and rotates in a horizontal plane while releasing jets of water from both ends. The two water jets act in opposite directions to each other. Each water jet applies a horizontal force of $3.0\,\text{N}$ on the tube at right angles to the tube. What is the magnitude of the torque on the tube due to the water jets?

Turning effects of forces

An object is in equilibrium under three forces, all of which lie in the same plane. The diagram shows two of the forces acting on the object. The third force is not shown in the diagram. What is the missing third force?

Equilibrium of forces

An object has two forces acting on it altogether. The object is in equilibrium. Which of the following statements about the forces are correct? 1. The two forces must be of equal magnitude. 2. The two forces must act in the same direction. 3. The two forces must pass through the same point.

Equilibrium of forces

The diagram illustrates how the atoms are arranged in a specific crystal. There is one atom at each corner of a cube. Each atom has a mass of $3.5 \times 10^{-25}\,\text{kg}$. The density of the crystal is $9.2 \times 10^{3}\,\text{kg m}^{-3}$. Determine the smallest distance between the centres of two adjacent atoms?

Density and pressure

Four measuring cylinders are each filled with the same liquid to the heights shown. At which position is the pressure greatest?

Density and pressure

In many filament lamps, up to $92\,\text{J}$ of energy is released as thermal energy for each $8\,\text{J}$ of energy released as light. What is the efficiency of a filament lamp, given as the percentage of electrical energy converted into light energy?

Energy conservation

What does the term unit of power mean?

Potential difference and power

An object is situated in a uniform gravitational field. The graph indicates how the change in gravitational potential energy $\Delta E_p$ of the object depends on the vertical distance $x$ through which the object is moved from a fixed point. Which graph shows how the gravitational force $F$ acting on the object varies with distance $x$?

Gravitational potential energy and kinetic energy

A block with mass $2.0\,\text{kg}$ is let go from rest on an incline. It moves $7.0\,\text{m}$ along the slope and descends through a vertical height of $3.0\,\text{m}$. A frictional force of $5.0\,\text{N}$ acts parallel to the slope on the block. What is the block’s speed after it has fallen this distance?

Energy conservation

In an experiment, two wires are pulled tight: one is brass and the other is steel. Throughout the experiment, both wires follow Hooke’s law. The Young modulus of brass is lower than the Young modulus of steel. Which graph shows the way stress changes with strain for both wires in this experiment?

Stress and strain

When a physical quantity is measured, the result can be affected by random errors as well as systematic errors. Which statement is correct?

Errors and uncertainties

A specimen of material is pulled with a tensile force until it has been taken past its elastic limit. The tensile force is then brought down to zero. The force-extension graph is displayed. Which area shows the net work done on the specimen?

Elastic and plastic behaviour

A wire is secured at one end and a force $F_1$ is applied at the other end so that the wire stretches. In this situation, the wire stores $0.15\,\text{J}$ of elastic potential energy. The applied force is then replaced by a force $F_2$. The wire then stores $0.60\,\text{J}$ of elastic potential energy. The wire obeys Hooke’s law. What is the relationship between $F_1$ and $F_2$?

Elastic and plastic behaviour

The diagram displays a wave pattern on the screen of an oscilloscope. The y-gain is set to $3.5\,\text{mV cm}^{-1}$. What is the amplitude of the wave?

Progressive waves

A transverse progressive wave moving along a string has a wavelength of $12\,\text{cm}$ and an amplitude of $1.5\,\text{cm}$. The wave speed on the string is $48\,\text{cm s}^{-1}$. In a time of $2.0\,\text{s}$, what distance does a point on the string travel?

Progressive waves

An ambulance siren produces a sound of single frequency $f$. The ambulance moves towards a stationary observer, passes very close by, and then moves away from the observer. Which statement gives the frequency of the sound detected by the observer as the ambulance goes past?

Doppler effect for sound waves

An electromagnetic wave in a vacuum has a wavelength of $138\,\text{pm}$. Which part of the electromagnetic spectrum does it fall into?

Electromagnetic spectrum

A student carries out an investigation into microwave polarisation. The microwaves emitted by the transmitter are vertically polarised. If a metal grille is inserted between the microwave transmitter and the receiver, it behaves as a polarising filter. The voltmeter reading is proportional to the intensity of microwaves that pass through the grille. With the grille’s transmission axis vertical, the voltmeter shows $3.50\,\text{V}$. The grille is then turned through an angle $\theta$. The new voltmeter reading is $2.20\,\text{V}$. Determine $\theta$.

Polarisation

Two waves overlap. This produces a resultant wave pattern. Which statement about the two waves has to be correct?

Interference

A water wave travels through an opening in a harbour wall and undergoes diffraction. The opening is $8\,\text{m}$ across. The wave moves directly towards the opening. For which wavelength is the diffraction of the wave the greatest?

Diffraction

Light of wavelength $5.50 \times 10^{-7}\,\text{m}$ from a laser falls normally on a diffraction grating. The diffracted light then falls on a semicircular screen, as seen from above. In total, 9 bright dots appear on the screen. The grating is placed at the centre of the semicircle. The lines in the grating are vertical. The spacing between neighbouring lines in the grating is $d$. What could be a possible value of $d$?

The diffraction grating

The Young modulus of the material used for a wire is to be determined. The Young modulus $E$ is defined by the equation shown. $E=\dfrac{4FL}{\pi d^2 x}$ A known force is used to extend the wire, and these measurements are taken. Which measurement has the greatest effect on the uncertainty in the calculated value of the Young modulus?

Stress and strain

Which quantity is obtained by multiplying charge by electric potential difference?

Potential difference and power

The current flowing through a filament lamp is increased. Which statement about the lamp is correct?

Resistance and resistivity

A student assembles the circuit below. The lamps are all identical. Which lamp dissipates the greatest power?

Kirchhoff's laws

The resistance of a metal cube is determined by placing it between two parallel plates, as illustrated. The cube has volume $V$ and is made from a material of resistivity $\rho$. The leads attached to the cube have negligible resistance. Which expression gives the electrical resistance of the metal cube between X and Y?

Resistance and resistivity

To complete a circuit, two resistors must be connected between points X and Y, as illustrated. These resistors may be arranged either in series or in parallel. Which arrangement of resistors gives the smallest resistance between X and Y?

Resistance and resistivity

The diagram represents a circuit containing a cell with internal resistance. At first, the switch is open. Which row gives the changes in currents $I_1$ and $I_2$ when the switch is closed?

Practical circuits

The diagram represents a circuit made up of a cell and three resistors $R_1$, $R_2$ and $R_3$. The cell has electromotive force (e.m.f.) $1.60\,\text{V}$ and negligible internal resistance. The current in the cell is $0.64\,\text{A}$. The potential difference across $R_1$ is $0.48\,\text{V}$. The current in $R_3$ is $0.32\,\text{A}$. Calculate the resistance of $R_2$.

Resistance and resistivity

The diagram represents a potentiometer circuit that is being used to compare the electromotive forces (e.m.f.), $E_1$ and $E_2$, of two cells. $XY$ is a resistance wire with uniform resistance. The fixed resistor has resistance $R$. A sliding contact is then moved along wire $XY$. When the sliding contact reaches position $P$, the galvanometer shows zero. The circuit is altered so that the galvanometer again shows zero, but now when the sliding contact is at a different position to the left of $P$. Which change could have been made to the circuit?

Practical circuits

Nucleus $P$ contains 144 neutrons and 93 protons. Nucleus $P$ is unstable and changes by $\alpha$-decay to produce nucleus $Q$. Nucleus $Q$ then changes by $\beta^-$ decay to produce nucleus $R$. On the graph, nucleus $P$ is shown by point $P$. Which point on the graph shows nucleus $R$?

Atoms, nuclei and radiation

The diagrams illustrate the quark composition of four different hadrons. One of these hadrons is a $\Sigma^+$ particle. Its charge is $+e$, where $e$ is the elementary charge. Which hadron could be the $\Sigma^+$ particle?

Fundamental particles

When two physical quantities are multiplied together, the result may be either a scalar quantity or a vector quantity. Which combination of two quantities gives a scalar quantity?

Scalars and vectors

Which kind of particle contains the greatest number of quarks?

Fundamental particles

The diagram presents a velocity-time graph for an object travelling in a straight line. What is the displacement over the final $2\ \text{s}$ of the motion?

Equations of motion

A block is travelling on a horizontal frictionless surface. While it moves in the direction of the force $F$, a constant force $F$ and a constant resistive force of $5.0\ \text{N}$ act on the block, as shown. The graph illustrates how the momentum of the block changes with time. What magnitude does force $F$ have?

Momentum and Newton's laws of motion

Newton’s third law states that two forces in a pair are equal in magnitude. Which statement about the two forces in such a pair is not correct?

Momentum and Newton's laws of motion

The graph illustrates how the speed of a raindrop falling straight down through air changes with time. Which statement is correct?

Non-uniform motion

Which statement correctly describes a perfectly elastic collision between two objects?

Linear momentum and its conservation