Page 225 - Physics

Work, energy and power

Chapter summary

1. Work is done when a force acts on an object and the object moves in the direction
of the force. Work is therefore the product of force and distance moved in the
direction of the force. Work is measured in Joules.

2. Energy is the ability to do work and is also measured in Joules.
3. Mechanical energy consists of kinetic and potential energy.
1
4. Kinetic energy is the energy an object has due to its motion. KE = mv 2
2
5. Potential energy is the energy stored in an object or energy due to its position.
PE = mgh

6. Power is the rate at which work is done. It is a measure of the rate at which energy
W
changes. P =
t

Revision exercise 9 The normal force acting on the box
Section A does NO work on the box during the
1. Choose the most correct answer. motion because it is
(i) Work is said to be done when (a) equal to the applied force.

(a) force acts upon a body, but the (b) perpendicular to the applied
body does not move. force.
(b) force acts upon a body and it (c) equal and opposite to the weight
moves it in the direction of of the box.
applied force. (d) perpendicular to the
(c) force acts upon a body but the displacement of the box.
body does not move in the (iii) The food consumed by a human
direction of the force. being produces
(d) force acts upon a body and the (a) only kinetic energy.
body moves in opposite to the (b) only heat energy.
direction of applied force. (c) both heat energy and chemical

(ii) A constant horizontal force F energy.
displaces a box by distance d over (d) only chemical energy.
a rough horizontal surface. Study (iv) Work done and energy are related in
Figure 9.26. that
d (a) they are both a result of
F mechanical processes.
(b) they have the same unit in which
Figure 9.26 they are measured.

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