Work, Energy, and Power

A hammer with 10 J of kinetic energy hits a nail and pushes it 5.0 mm into a plank. Both the hammer and nail come to rest after the collision. What is the approximate average force that acts on the nail while it moves through 5.0 mm?

• 0.050 N

• 2.0 N

• 50 N

• 2000 N

The change in gravitational potential energy ∆E of an object of mass m when moving through height ∆h near the surface of the Earth is given by the equation shown.

∆E = mg∆h

Which equation is needed as part of the derivation of this expression?

• kinetic energy = 1/2 x mass x (speed)^2

• moment = force x distance

• weight = mass x acceleration of free fall

• work done = power x time

A racing car has an output power of 300 kW when travelling at a constant speed of 60 m s^–1. What is the total resistive force acting on the car?

• 5 kN

• 10 kN

• 50 kN

• 100 kN

A ball drops onto a horizontal surface and bounces elastically.

What happens to the kinetic energy of the ball during the very short time that it is in contact with the surface?

• Most of the kinetic energy is lost as heat and sound.

• The kinetic energy decreases to zero and then returns to its original value.

• The kinetic energy remains constant because it is an elastic collision.

• The kinetic energy remains constant in magnitude but changes direction.

An object of mass 0.30 kg is thrown vertically upwards from the ground with an initial velocity of 8.0 m s^–1. The object reaches a maximum height of 1.9 m. How much work is done against air resistance as the object rises to its maximum height?

• 4.0 J

• 5.6 J

• 9.6 J

• 15 J