2. Conservative and Nonconservative Forces

3. Conservative and Nonconservative Forces
Gravity is a Conservative Force

4. K Conservative and Nonconservative Forces person gravity
Gravity is a Conservative Force
person K
gravity

5. K Conservative and Nonconservative Forces gravity
Gravity is a Conservative Force K
gravity
When the box is back where it started, the net change in Kinetic Energy is zero

6. Conservative and Nonconservative Forces
Is friction a Conservative Force?

7. K Conservative and Nonconservative Forces heat
Is friction a Conservative Force? K
heat
Can we ever get this energy back?

8. Conservative and Nonconservative Forces
First definition of “Conservative Force”
A conservative force does zero total work on a closed path.

9. Conservative and Nonconservative Forces
First definition of “Conservative Force”
A nonconservative force does NON-zero total work on a closed path.

10. Is the force exerted by a spring on a mass conservative?
A) Yes
B) No
C) We need more information

11. Is the force exerted by muscles conservative (assume a frictionless surface)?
A) Yes
B) No
C) We need more information

12. Conservative and Nonconservative Forces
Second definition of “Conservative Force”
The work done by a conservative force in going from point A to point B is
independent of the path taken from A to B

13. Is the force exerted by the engine of your car conservative?
A) Yes
B) No
C) We need more
information

14. Is the force exerted by the engine of your car conservative
Is the force exerted by the engine of your car conservative? Does the amount of work done by the engine in going from Alamosa to Walsenburg depend on the path?
A) Yes
B) No
C) We need more
information

15. K Potential Energy gravity +Work –Work
(Conservative Forces have potential energy functions)
+Work K
gravity
–Work
Conservative forces give back, in positive work, the energy they take away in negative work
It must “store it” somewhere…

16. (Conservative Forces have potential energy functions)
We define the change in potential energy to be the opposite of the work done by a conservative force:

17. (Conservative Forces have potential energy functions)
We define the change in potential energy to be the opposite of the work done by a conservative force:

18. K Ug Potential Energy gravitational potential energy
(Conservative Forces have potential energy functions)
We define the change in potential energy to be the opposite of the work done by a conservative force:
+Work K
Ug goes down
gravitational potential energy Ug
–Work
Ug goes up

19. A 7. 25 kg bowling ball is held 1. 75 m above the ground
A 7.25 kg bowling ball is held 1.75 m above the ground. What is its gravitational potential energy with respect to the ground?

20. A 7. 25 kg bowling ball is held 1. 75 m above the ground
A 7.25 kg bowling ball is held 1.75 m above the ground. What is its gravitational potential energy with respect to the ground?
A 7.25 kg bowling ball is held 1.75 m above the ground, directly above a 20 m deep well. What is its gravitational potential energy with respect to the bottom of the well?

21. A 7. 25 kg bowling ball is held 1. 75 m above the ground
A 7.25 kg bowling ball is held 1.75 m above the ground. What is its gravitational potential energy with respect to the ground?
A 7.25 kg bowling ball is held 1.75 m above the ground, directly above a 20 m deep well. What is its gravitational potential energy with respect to the bottom of the well?
A 7.25 kg bowling ball is held 1.75 m above the ground, directly over a 1.5 m high table top. What is its gravitational potential energy with respect to the top of the table?

22. A 7. 25 kg bowling ball is held 1
A 7.25 kg bowling ball is held 1.75 m above the ground, directly above a 20 m deep well. Define the ground to be y = 0 m. What is the change in its gravitational potential energy when you drop it into the well?

23. A 7. 25 kg bowling ball is held 1
A 7.25 kg bowling ball is held 1.75 m above the ground, directly above a 20 m deep well. Define the ground to be y = 0 m. What is the change in its gravitational potential energy when you drop it into the well?
A 7.25 kg bowling ball is held 1.75 m above the ground, directly above a 20 m deep well. Define the bottom of the well to be y = 0 m. What is the change in its gravitational potential energy when you drop it into the well?

24. E = U + K Conservation of Mechanical Energy
“Mechanical Energy” is defined to be Kinetic and Potential Energy:
E = U + K

25. E = U + K Conservation of Mechanical Energy E = constant
“Mechanical Energy” is defined to be Kinetic and Potential Energy:
E = U + K
If only conservative forces act on an object, its mechanical energy is conserved.
E = constant

26. A 7.25 kg bowling ball is held out a 3rd story window and dropped onto the sidewalk 8.15 m below. How fast is it moving when it hits the sidewalk?