1. List five sources of energy that might be able to produce
This cat is not happy.
He is in need of a hair
dryer.
List five sources of energy that might be
able to produce
electricity for him.

2. Work and Energy
AP Physics
Chapter 6

3. 6.1 Work Done by a Constant Force
Work and Energy
6.1 Work Done by a Constant Force

4. 6.1 Work Done by a Constant Force
Work – the product of the magnitude of the displacement times the component of the force parallel to the displacement
Or (dot product)
Work is measure in Joules (Energy)
6-1

5. 6.1 Work Done by a Constant Force
If Force an displacement are in the same direction
Work is done, velocity increases, energy of the object increases
If Force is opposite to displacement, negative work is done, energy decreases
6-1

6. 6.1 Work Done by a Constant Force
If there is no motion, or the force is perpendicular to motion, no work is done, there is no change in velocity, there is no change in energy
6-1

7. 6.1 Work Done by a Constant Force
As long as Force and displacement are parallel, work is done
6-1

8. 6.1 Work Done by a Constant Force
Example: A person pulls a 50 kg crate 40m along a horizontal floor by a constant force of 37o. The coefficient of friction is What is the work done by each force acting on the crate?
Free body diagram N F W f
6-1

9. 6.1 Work Done by a Constant Force
Example: A person pulls a 50 kg crate 40m along a horizontal floor by a constant force of 37o. The coefficient of friction is What is the work done by each force acting on the crate?
Work done by N?
By W? N F W f
6-1

10. 6.1 Work Done by a Constant Force
Example: A person pulls a 50 kg crate 40m along a horizontal floor by a constant force of 37o. The coefficient of friction is What is the work done by each force acting on the crate?
Work done by F? N F W f
6-1

11. 6.1 Work Done by a Constant Force
Example: A person pulls a 50 kg crate 40m along a horizontal floor by a constant force of 37o. The coefficient of friction is What is the work done by each force acting on the crate?
Work done by f? N F W f
6-1

12. 6.1 Work Done by a Constant Force
Example: A person pulls a 50 kg crate 40m along a horizontal floor by a constant force of 37o. The coefficient of friction is 0.20.
What is the net work done on the object?
6-1

13. 6.2 Work Done by a Varying Force
Work and Energy
6.2 Work Done by a Varying Force

14. 6.2 Work Done by a Varying Force
Work is the area under a Force vs. displacement graph.
If force changes at a constant
rate,
Otherwise we use calculus
to calculate the area
6-2

15. 6.3 Kinetic Energy, & the Work-Energy Principle
Work and Energy
6.3 Kinetic Energy, & the Work-Energy Principle

16. 6.3 Kinetic Energy, & the Work-Energy Principle
Energy – the ability to do work
Sufficient for Mechanical Energy
Kinetic Energy (translational) – due to motion
Equation
Work-Kinetic Energy
6-3

17. 6.3 Kinetic Energy, & the Work-Energy Principle
Work-Kinetic Energy Theorem (Work-Energy Principle) – the net work done on an object is equal to the change in the object’s kinetic energy
Work-Kinetic Energy Physlet
6-3

18. 6.3 Kinetic Energy, & the Work-Energy Principle
A 1000 kg car traveling 26.7 m/s can brake to a stop in 20 m. What is the force applied by the breaks?
Free Body Diagram? N f mg
6-3

19. 6.3 Kinetic Energy, & the Work-Energy Principle
A 1000 kg car traveling 26.7 m/s can brake to a stop in 20 m. What is the force applied by the breaks?
Solve N f mg
6-3

20. 6.3 Kinetic Energy, & the Work-Energy Principle
A 1000 kg car traveling 26.7 m/s can brake to a stop in 20 m. If the car is traveling twice as fast, how long does it take to stop? N f mg
6-3

21. A rocket powered 2000kg truck can go from 0 to 27 m/s in 3.5 s.
A. What is the acceleration of the truck?
B. What is the displacement of the truck?
C. How much
work was
done on the
truck?

22. Work and Energy
6.4 Potential Energy

23. Potential Energy – due to position or configuration
Gravitational Potential Energy – (Ug) due to position above the earths surface
6-4

24. Elastic Potential Energy
Elastic Potential Energy – due to the position of a spring
Hooke’s Law
Equation for Elastic Potential Energy
k=spring constant
Determining a Spring
Constant
Elastic Potential Energy
6-4

25. 6.5 Conservative and Nonconservative Forces
Work and Energy
6.5 Conservative and Nonconservative Forces

26. 6.5 Conservative and Nonconservative Forces
Conservative Forces – independent of pathway (gravity)
Energy can be returned (conserved)
Nonconservative Forces – depends on pathways (friction)
Energy can not be returned
Physlet
6-5

27. 6.6 Mechanical Energy and its Conservation
Work and Energy
6.6 Mechanical Energy and its Conservation

28. 6.6 Mechanical Energy and its Conservation
If no energy is lost to nonconservative forces
We can expand that to include the types of energy we have
Principle of Conservation of Mechanical Energy-energy just switches forms
6-6

29. 6.6 Mechanical Energy and its Conservation
If energy is lost to nonconservative forces
For example if the energy was lost to friction
6-6

30. 6.7 Problem Solving Using Conservation of ME
Work and Energy
6.7 Problem Solving Using Conservation of ME

31. 6.7 Problem Solving Using Conservation of ME
List the types of energy before the reaction
List the types of energy after the reaction
Consider any non conservative forces
6-7

32. jumps off a cliff that is 102 m high. Assuming that the
The 75 kg Henry (French)
jumps off a cliff that is 102
m high. Assuming that the
bungee has a resting length
of 40 m, follows Hooke’s
Law, and stops the guy 3 m before he hits the surface, what is the elastic constant of the bungee cord?

33. S-32
An unfortunate 45 kg child never learned to slide on anything but his face. If his face and the dirt have a coefficient m=0.4, and he is running at 11 m/s when he starts his slide how much work is done by friction by the time he comes to a stop?

34. Work and Energy
6.8 Other Forms of Energy

35. Electric Nuclear Thermal Chemical
6.8 Other Forms of Energy
Electric
Nuclear
Thermal
Chemical
6-8

36. Energy can be transformed from one form to another.
6.8 Other Forms of Energy
Law of Conservation of Energy – The total energy is neither increased nor decreased in any process.
Energy can be transformed from one form to another.
6-8

37. Work and Energy
6.10 Power

38. Power – the rate at which work is done
Measured in watts
Often convenient to write in terms of force
6-9

39. 6.10 Power
6-9

40. The worlds strongest woman lifts 186 kg
upward a distance of 0.75 m. Assuming that
the mass accelerated upward from rest the
whole distance in
0.44s,
What is the work
done by the dainty
lady?
How much power
did she generate?

41. Mike is not a very impressive driver. He
drives his 1500 kg minivan into the living room
of his mom’s house. If the van was traveling
at 20 m/s and came to a stop in 2 m, what is
the average force on
Mike and the van?

42. Tarzan (and his very 50’s family) are out
S-35 I can use the conservation of energy to calculate changes in position or speed
Tarzan (and his very 50’s family) are out
swinging on their vine. The vine is 45 m long
and makes and angle of 10o
to the vertical. If Tarzan
(m=105 kg) runs at 15 m/s
and jumps on the vine, what
will be the vertical angle at
the highest point the vine
reaches?

43. Sven likes to ride his pogo stick really
S-36 I can use the conservation of energy to calculate changes in position or speed
Sven likes to ride his pogo stick really
high. If he has a mass of 115 kg,
and manages to reach a maximum
height of m when the
spring is compressed 0.4 m,
what is the constant of the spring?

44. jump off the cliff, and the rope was 89 m long,
S-37 I can use the conservation of energy to calculate changes in position or speed
The worlds biggest
swing drops 19
stories (57 m). If our
150 kg chubby
champion ran at 11 m/s to
jump off the cliff, and the
rope was 89 m long,
what is his velocity at
the bottom?

45. S-38 I can use the conservation of energy to calculate changes in position or speed
Big Mouse Trap!

46. Don’t mess with this dog. If he has a mass of 25 kg (all muscle)
and hits the 5 kg
pendulum going
7.2 m/s, what will be
the maximum vertical
angle the rope makes.
The string is 8 m long.

47. S-40 I can relate transformations between kinetic and potential energy
Using your brilliant knowledge of energy,
why has the style of the high jumping
changed over the years from
A. Scissor B. Straddle C. Flop

48. S-41
A 112 kg weasel running at 32 m/s trips and rolls into a ball. He rolls up a 45m long frictionless hill that makes an angle of 22o to the horizontal. At the top of the hill, falls off a cliff that is 120 below his starting point. He falls on a spring that compresses 1.5 m before shooting him back into the air. He passes his girlfriend who is sitting in a tree that is 81 m tall. What is his velocity as he passes his girlfriend?

49. This is a ridiculously huge rabbit May your brain be as
large while you take
your test!