Work and energy: net work, forces and kinetic energy

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• You push a heavy box in a straight line along the top of a rough

horizontal table. The box starts at rest and ends at rest. Describe the work done on

it (including sign) by each force acting on it and the net work done on it.

Determine the Concept The normal and gravitational forces do zero work

because they act at right angles to the box’s direction of motion. You do positive

work on the box and friction does negative work. Overall the net work is zero

because its kinetic energy doesn’t change (zero to zero).

• You are riding on a Ferris wheel that is rotating at constant speed. True

or false: During any fraction of a revolution: (a) None of the forces acting on you

does work on you. (b) The total work done by all forces acting on you is zero.

(c) There is zero net force on you. (d) You are accelerating.

(a) False. Both the force exerted by the surface on which you are sitting and the

gravitational force acting on you do work on you.

(b) True. Because you are rotating with a constant speed (ignoring the very brief

periods of acceleration at the beginning and end of the ride) and return, at the end

of the ride, to the same location from which you started the ride, your kinetic

energy has not changed and so the net work done on you is zero.

(c) False. The net force acting on you is the sum of the gravitational force acting

on you and the force exerted by the surface on which you are sitting.

(d) True. Because the direction you are moving is continually changing, your

velocity is continually changing and, hence, you are experiencing acceleration.

Give an example of a particle that has constant kinetic energy but is

accelerating. Can a non-accelerating particle have a changing kinetic energy? If

so, give an example.

Determine the Concept A particle moving along a circular path at constant speed

has constant kinetic energy but is accelerating (because its velocity is continually

changing). No, because if the particle is not accelerating, the net force acting on it

must be zero and, consequently, its kinetic energy must be constant.