1. Kinetic and Potential Energy

2. Energy and Work are closely related
Work is a transfer of Energy
SI unit for work is Joules (J)
Energy is the capacity to do Work
We recognize it by the changes it causes
We use energy to carry a backpack upstairs.
(work)

3. Kinds of Energy 1. Potential Energy(PE)
the energy that an object has because of its position, shape or condition.

4. Potential Energy of Position
PE depends on an objects mass, height, and the acceleration due to gravity.
PE= mgh

5. Kinds of Potential Energy
Potential Energy of Position (Gravitational PE)
Figure 11, page 392: the apple has potential energy that results from the gravitational attraction between the apple and earth

6. Kinds of Potential Energy
Elastic Potential Energy
The PE of an object that is stretched or compressed
Ex- stretching a rubber band or bouncing a basketball

7. Calculations with PE= mgh
Ex. calculations- page 393
Units for PE: Joules (J)
A joule is a kg-m2 / s2

8. Practice Problems
Calculate the gravitational potential energy in the following systems:
A car with a mass of 120 kg at the top of a 42 m hill
A 65 kg climber on top of Mt Everest (8800 m high)
A 0.52 kg bird flying at an altitude of 550 m

9. A car with a mass of 120 kg at the top of a 42 m hill
PE=mgh=(1200kg)(9.8m/s2)(42m)=4.9x105J

10. a 65 kg climber on top of Mount Everest (8800m high)
PE=(65kg)(9.8m/s2)(8800m)=5.6x106J

11. a 0.52kg bird flying at an altitude of 550m
PE=(0.52kg)(9.8m/s2)(550m)=2.8x103J

12. Another Problem
A science student holds a 55g egg out a window. Just before the student releases the egg, the egg has 8.0 J of gravitational potential energy with respect to the ground. How far is the student’s arm from the ground (in meters)?

13. Answer
h= PE/mg = 8.0J/(0.055kg)(9.8m/s2)=15m

14. Another Problem
A diver has 3400J of gravitational potential energy after stepping up onto a diving platform that is 6.0m above the water. What is the diver’s mass in kilograms?

15. Answer
m=PE/gh=(3400J)/(9.8m/s2)(6.0m)=58kg

16. KE= ½ mv 2 Kinetic Energy (KE)
The energy of a moving object due to the object’s motion
The KE of any moving object depends on its mass and velocity
See math skills pg 395
**Note: if you double the mass, it doubles the KE. If you double the velocity, it will QUADRUPLE the KE
KE= ½ mv 2

17. Practice Problem
Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds:
i. 29m/s
ii. 18m/s
iii. 42km/s
A 35kg child has 190J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?

18. Answers
Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds:
i. 29m/s KE=1/2mv2=(1/2)(1500kg)(29m/s)2=6.3x105J
ii. 18m/s
KE=(1/2)(1500kg)(18m/s)2=2.4x105J
ii. 42m/s
KE=(1/2)(1500kg)(42m/s)2=

19. Answer
A 35kg child has 190J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?