CK-12-Physics-Concepts - Intermediate

9.1. Kinetic Energy http://www.ck12.org

Example Problem:A farmer heaves a 7.56 kg bale of hay with a final velocity of 4.75 m/s.

(a) What is the kinetic energy of the bale?

(b) The bale was originally at rest. How much work was done on the bale to give it this kinetic energy?

Solution:

(a)KE=^12 mv^2 =

( 1

2

)

( 7 .56 kg)( 4. 75 )^2 = 85 .3 Joules

(b) Work done=∆KE= 85 .3 Joules

Example Problem:What is the kinetic energy of a 750. kg car moving at 50.0 km/h?

Solution:

(

50. 0 km

h

)(

1000 m

km

)(

1 h

3600 s

)

= 13. 9 m/s

KE=^12 mv^2 =

( 1

2

)

( 750 .kg)( 13 .9 m/s)^2 = 72 ,300 Joules

Example Problem:How much work must be done on a 750. kg car to slow it from 100. km/h to 50.0 km/h?

Solution:From the previous example problem, we know that theKEof this car when it is moving at 50.0 km/h
is 72,300 Joules. If the same car is going twice as fast, itsKEwill be four times as great becauseKEis
proportional to the square of the velocity. Therefore, when this same car is moving at 100. km/h, itsKEis
289,200 Joules. Therefore, the work done to slow the car from 100. km/h to 50.0 km/h is( 289 ,200 Joules)−
( 72 ,300 Joules) = 217 ,000 Joules.

Summary

• Energy is the ability to change an object’s motion or position.


• The energy of motion is called kinetic energy.


• The formula for kinetic energy isKE=^12 mv^2.


• The work done on an object equals the kinetic energy gain or loss by the object,WNET=∆KE.

Practice

Questions

The following video discusses kinetic energy. Use this resource to answer the questions that follow.

http://www.youtube.com/watch?v=g157qwT1918

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/63874

1. Potential energy is present in objects that are __.


2. Kinetic energy is present in objects that are __.


3. What formula is given for kinetic energy?

Practice problems involving kinetic energy:

http://www.physicsclassroom.com/Class/energy/u5l1c.cfm