CK-12-Physics-Concepts - Intermediate

http://www.ck12.org Chapter 8. Work, Power, and Simple Machines

One of the units you will see for work is shown above: the Newton meter (Nm). More often, however, units of work

are given as the Joule(pronounced "jool") in honor of James Prescott Joule, a nineteenth century English physicist.

A Joule is a kg·m^2 /s^2.

Example Problem: A boy lifts a box of apples that weighs 185 N. The box is lifted a height of 0.800 m. How much

work did the boy do?

Solution:W=F d= (185 N)( 0 .800 m) =148 N m=148 Joules

Work is done only if a force is exerted in the direction of motion. If the motion is perpendicular to the force, no

work has been done. If the force is at an angle to the motion, then the component of the force in the direction of the

motion is used to determine the work done.

Example Problem: Suppose a 125 N force is applied to a lawnmower handle at an angle of 25° with the ground

and the lawnmower moves along the surface of the ground. If the lawnmower moves 56 m, how much work was

done?

Solution: The solution requires that we determine the component of the force that was in the direction of the motion

of the lawnmower because the component of the force that was pushing down on the ground does not contribute to

the work done.

Fparallel= (Force)(cos 25◦) = (125 N)( 0. 906 ) =113 N

W=Fparalleld= (113 N)(56 m) =630 J

Summary

• Work is the force exerted on an object multiplied by the distance the object moves due to that force.


• The unit for work is called the joule, which is a kg m^2 /s^2.


• If the force is at an angle to the motion, then the component of the force in the direction of the motion is used
  to determine the work done.

Practice

Questions

The following video introduces energy and work. Use this resource to answer the questions that follow.

http://wn.com/Work_physics_#/videos

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/63049