1. Ch 8 Energy Notes
ENERGY – 5.2 PPT

2. Kinetic Energy If an object is moving, it has energy.
Ch 8 Energy Notes
Kinetic Energy
If an object is moving, it has energy.
This energy is called kinetic energy - the energy of motion.

3. Kinetic Energy An object’s kinetic energy depends on:
Ch 8 Energy Notes
Kinetic Energy
An object’s kinetic energy depends on:
the object’s mass.
Kinetic energy is directly proportional to mass.
the object’s speed.
Kinetic energy is directly proportional to the square of the object’s speed.

4. Ch 8 Energy Notes
Kinetic Energy
In symbols: 1
KE =
mv2 2

5. Kinetic Energy Kinetic energy is a scalar quantity.
Ch 8 Energy Notes
Kinetic Energy
Kinetic energy is a scalar quantity.
Common units of kinetic energy: Joules
An object with mass of 1 kg, moving at 1 m/s, has a kinetic energy of 0.5 Joule.

6. Example #1
Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s.
Given: mass = 625 kg
V = 18.3 m/s
Formula: KE = ½ mass x v2

7. Answer
1.05 x 105 J

8. Example #2
If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy?
What is the relationship between KE and velocity?

9. ANSWER
4.20 x 105 J
KE =the square of an object’s velocity. The speed doubled, so the amount of KE increased by 4.
If the speed triples, then KE will increase by _____.

10. Work-Energy Theorem What is the relationship between work and energy?
The amount of work done is essentially the change in kinetic energy.

11. Ch 8 Energy Notes
Work
When the kinetic energy of an object changes, work has been done on the object.
Units of work: Joules
Work is a scalar quantity.

12. Work-Energy Theorem Wnet = Δ KE Wnet = KEf – KEi
Wnet = 1/2mv2f – 1/2mv2i
From this theorem, we see that the speed of an object increases if the net work done is positive, because the final KE is greater than the initial KE
The speed of an object decreases if the net work is negative, because the final KE is less than the initial KE.

13. Potential Energy Potential Energy is stored energy.
Stored chemically in fuel, the nucleus of atom, and in foods.
Or stored because of the work done on it:
Stretching a rubber band.
Winding a watch.
Pulling back on a bow’s arrow.
Lifting a brick high in the air.

14. Gravitational Potential Energy
Potential energy that is dependent on height is called gravitational potential energy.

15. Gravitational Potential Energy
A waterfall, a suspension bridge, and a falling snowflake all have gravitational potential energy.

16. Gravitational Potential Energy
If you stand on a 3-meter diving board, you have 3 times the G.P.E, than you had on a 1-meter diving board.

17. Gravitational Potential Energy
Objects with more mass have greater GPE
Objects higher from the ground have more GPE.
The formula to find GPE is
GPE = mgh
PEg = mgh

18. Example #1
A 4.0 kg rock is sitting on a 3.0-meter ledge. How much potential energy does the rock have?

19. Answer
PEg = mgh
PEg = 4.0kg x 9.81 m/s x 3.0m
PEg = 1.2 x102 J

20. Example #2
Which object has the most potential energy? A 5.6kg rock at the top of a mountain 35 meters high, or a 5.6kg rock at the top of a mountain 55 meters high?

21. Answer
The rock at the top of the mountain 55 meters high.

22. Elastic Potential Energy
Energy that is stored due to being stretched or compressed is called elastic potential energy.

23. Elastic Potential Energy (PEelastic)
Depends on the distance compressed or stretched from its relaxed length.
PEelastic = ½ kx2
PE elastic = ½ x spring constant x (distance stretched or compressed)
Spring constants have units of N/m