State what is meant by the principle of conservation of momentum.
The propulsion setup of a toy car uses a propeller driven by an electric motor, as shown in Fig. 3.1. The car is on horizontal ground and is first held stationary by its brakes. When the motor is turned on, it spins the propeller so that air is driven horizontally to the left. The density of the air is $1.3\,\text{kg m}^{-3}$. Assume that the air moves at $1.8\,\text{m s}^{-1}$ in a uniform cylinder of radius $0.045\,\text{m}$. Also assume that the air to the right of the propeller is stationary.
Show that the mass of air pushed to the left in a time interval of $2.0\,\text{s}$ is $0.030\,\text{kg}$.
Calculate the increase in momentum of the air mass in (b)(i).
Calculate the force that the propeller exerts on this mass of air.
Explain how Newton’s third law is involved in the motion of the air caused by the propeller.
The car has a total mass of $0.20\,\text{kg}$. Its brakes are released and it starts moving with an initial acceleration of $0.075\,\text{m s}^{-2}$. Find the initial frictional force acting on the car.