1. 5.2.2 Work/Energy Principle
Work in / Work out
Work/Energy Principle

2. Definitions – Types of Energy
potential energy: STORED ENERGY
can be in a FIELD or DEVICE
gravitational PE is stored in gravitational fields
elastic PE is stored in elastic objects (springs)

4. Climber has gained gravitational potential energy PEg because he is high up

5. PEs depends on the stretch of springs
Climber has gained gravitational potential energy PEg because he is high up
PEs depends on the stretch of springs
Greatest PEs
Least PEs

6. Example #1 – Gravitational PE
A 30 kilogram object is positioned 2.0 meters above the surface of the Earth.
What is the gravitational potential energy of the object?
ΔPE = mgΔh
ΔPE = (30 kg)(9.81 m/s2)(2.0 m)
ΔPE = J
30 kg
2.0 m

7. Example #2 – Elastic PE
The length of a spring with a spring constant of 100 newtons per meter is changed by 4.0 centimeters.
What is the elastic potential energy stored in the spring?
10 cm
14 cm

8. Example #2 – Elastic PE
The length of a spring with a spring constant of 100 newtons per meter is changed by 4.0 centimeters.
What is the elastic potential energy stored in the spring?
PES = ½ kx2
(1) Convert cm to m
10 cm
14 cm

9. Example #2 – Elastic PE
The length of a spring with a spring constant of 100 newtons per meter is changed by 4.0 centimeters.
What is the elastic potential energy stored in the spring?
PES = ½ kx2
PES = ½ (100 N/m)(0.04 m)2
PES = 0.08 J
10 cm
14 cm

10. Definitions – Types of Energy
kinetic energy: ENERGY OF MOVING OBJECTS
depends on mass and velocity

11. Example #3 – Kinetic Energy
A 5.0 kilogram toy car is moving at 2.0 meters per second.
What is the kinetic energy of the toy car?

12. Example #3 – Kinetic Energy
A 5.0 kilogram toy car is moving at 2.0 meters per second.
What is the kinetic energy of the toy car?
KE = ½ mv2
KE = ½ (5.0 kg)(2.0 m/s)2
KE = 10 J

13. Definitions – Types of Energy
internal energy: TEMPERATURE/HEAT
This heating can occur due to FRICTION

14. Work/Energy Principle
When work is done ‘on a system’ there is a net gain of one or more types of energy by that system. Energy types can change within a system.
if “h” increases
Gravitational PE
if “x” increases
Elastic PE
WORK
if “v” increases
Kinetic Energy
if temp. increases
Internal Energy

15. Work/Energy Principle – Scenario #1
A car is moving along a flat surface with an increasing speed. What type(s) of energy are changing?
PEg
PES KE Q

16. Work/Energy Principle – Scenario #1
A car is moving along a flat surface with an increasing speed. What type(s) of energy are changing?
PEg
PES KE Q X X ↑ ?

17. Work/Energy Principle – Scenario #2
A car is moving up an incline with constant speed. What type(s) of energy are changing?
PEg
PES KE Q

18. ↑ Increase ↓ Decrease X Remain the Same ? Cannot determine
BEFORE viewing the next slide, attempt to determine how the types of energy is changing
↑ Increase
↓ Decrease
X Remain the Same
? Cannot determine

19. Work/Energy Principle – Scenario #2
A car is moving up an incline with constant speed. What type(s) of energy are changing?
PEg
PES KE Q ↑ X X ?

20. Work/Energy Principle – Scenario #3
A car is applying its brakes and slowing down as it moves down a slope. What type(s) of energy are changing?
PEg
PES KE Q

21. ↑ Increase ↓ Decrease X Remain the Same ? Cannot determine
BEFORE viewing the next slide, attempt to determine how the types of energy is changing
↑ Increase
↓ Decrease
X Remain the Same
? Cannot determine

22. Work/Energy Principle – Scenario #3
A car is applying its brakes and slowing down as it moves down a slope. What type(s) of energy are changing?
PEg
PES KE Q ↓ X ↓ ↑

23. Work/Energy Principle – Scenario #4
A cart is moving on a flat surface toward a wall and is stopped by a spring on its front. What type(s) of energy are changing?
PEg
PES KE Q

24. ↑ Increase ↓ Decrease X Remain the Same ? Cannot determine
BEFORE viewing the next slide, attempt to determine how the types of energy is changing
↑ Increase
↓ Decrease
X Remain the Same
? Cannot determine

25. Work/Energy Principle – Scenario #4
A cart is moving on a flat surface toward a wall and is stopped by a spring on its front. What type(s) of energy are changing?
PEg
PES KE Q X ↑ ↓ ?

26. Work/Energy Principle – Scenario #5
A rocket flies upward with an increasing velocity [ignore air resistance]. What type(s) of energy are changing?
PEg
PES KE Q

27. ↑ Increase ↓ Decrease X Remain the Same ? Cannot determine
BEFORE viewing the next slide, attempt to determine how the types of energy is changing
↑ Increase
↓ Decrease
X Remain the Same
? Cannot determine

28. Work/Energy Principle – Scenario #5
A rocket flies upward with an increasing velocity [ignore air resistance]. What type(s) of energy are changing?
PEg
PES KE Q ↑ X ↑ X

29. End of PRACTICE