Units 2.3 and 2.4 Work, Power, Energy and Momentum

An object of mass m is sliding down a ramp at constant speed. During the motion it travels a distance x along the ramp and falls through a vertical distance h. The coefficient of dynamic friction between the ramp and the object is μ. What is the total energy transferred into thermal energy when the object travels distance x?

• mgh

• mgx

• μmgh

• μmgx

An object has a weight of 6.10 × 10² N. What is the change in gravitational potential energy of the object when it moves through 8.0 m vertically?

• 5 kJ

• 4.9 kJ

• 4.88 kJ

• 4.880 kJ

A boat with an output engine power of 15 kW moves through water at a speed of 10 m s^-1. What is the resistive force acting on the boat?

• 0.15 kN

• 0.75 kN

• 1.5 kN

• 150 kN

An astronaut is moving at a constant velocity in the absence of a gravitational field when he throws a tool away from him.

What is the effect of throwing the tool on the total kinetic energy of the astronaut and the tool and the total momentum of the astronaut and the tool?C

• A

• B

• C

• D

A ball of mass m collides with a wall and bounces back in a straight line. The ball loses 75 % of the initial energy during the collision. The speed before the collision is v.

What is the magnitude of the impulse on the ball by the wall?

• $\left(1-\frac{\sqrt{3}}{2}\right)mv$

• $\frac{1}{2}mv$

• $\frac{5}{4}mv$

• $\frac{3}{2}mv$

An object falls from rest from a height h close to the surface of the Moon. The Moon has no atmosphere.
When the object has fallen to height  $\frac{h}{4}$  
above the surface, what is
                   
 $\frac{kinetic\ energy\ of\ the\ object\ at\ \frac{h}{4}}{gravitational\ potential\ of\ object\ at\ h}$  ?

•  $\frac{3}{4}$  

•  $\frac{4}{3}$  

•  $\frac{9}{16}$  

•  $\frac{16}{9}$  

Child X throws a ball to child Y. The system consists of the ball, the children and the Earth. What is true for the system when the ball has been caught by Y?

• The momentum of child Y is equal and opposite to the momentum of child X.

• The speed of rotation of the Earth will have changed.

• The ball has no net momentum while it is in the air.

• The total momentum of the system has not changed.

An object is moving in a straight line. A force F and a resistive force f act on the object along the straight line.

Both forces act for a time t.

What is the rate of change of momentum with time of the object during time t ?

• F + f

• F - f

• (F + f )t

• (F - f )t

The graph shows the variation with time t of the force F acting on an object of mass 15 000 kg.

The object is at rest at t = 0.

What is the speed of the object when t = 30 s?

• 0.18 m s^–1

• 6 m s^–1

• 18 m s^–1

• 60 m s^–1

The graph is a speed versus time graph for an object that is moving in a straight line.

The distance travelled by the object during the first 4.0 seconds is

• 80 m

• 40 m

• 20 m

• 5 m