1. GIG
Read the passage and mark your answers on your whiteboard. NOT ON THE PAPER.
Question or

2. Potential and Kinetic Energy

3. Work at an Angle
Sometimes the direction in which a force is applied is not the same as the object’s movement. In this case, only the component of the force in the direction of movement does work.

4. Work at an Angle

5. Work at an Angle Formula for work with displacement at an angle
W = Fdcosθ
Work = force ✕ distance ✕ cosineθ

6. Force Perpendicular to Motion Does No Work
You push a 10kg box 1.0m across the floor, using 100N of force. Then you push a 0.1kg box 1.0m across the floor, again using 100N of force.
How much work did you do each time?

7. Force Perpendicular to Motion Does No Work
You push a 10kg box 1.0m across the floor, using 100N of force. Then you push a 0.1kg box 1.0m across the floor, again using 100N of force.
How much work did you do each time?
W = Fd = 100N ✖ 1.0m = 100J
Gravity and normal force do not change this calculation.

8. Negative Work
If the object moves in the opposite direction of the force, negative work has been done, equal to:
W= -Fd

9. Types of Energy
Energy exists in several different states, such as the chemical energy in covalent bonds and nuclear energy holding atoms together.
In physics, we are especially concerned with kinetic energy and potential energy.

10. Kinetic Energy
Kinetic energy is the energy an object has because of its motion.
The formula for kinetic energy is:
KE = .5m(v2)
Kinetic energy = ½ mass times velocity2

11. Sample Problem
A cat weighing 5.3kg runs across the floor at 8.7m/s. What is its kinetic energy?

12. Sample Problem
A cat weighing 5.3kg runs across the floor at 8.7m/s. What is its kinetic energy?
KE = .5mv2 = .5(5.3kg)(8.7m/s)2 = 200J

13. The Work-Energy Theorem
Work and kinetic energy are related. The total work done on an object is equal to its change in kinetic energy.
Wtotal = ΔKE = .5mvf2 - .5mvi2
Total work = ½ mass times final velocity2
- ½ mass times initial velocity2

14. Sample Problem
A 78kg parachutist jumps from an airplane. How much work has gravity done when the parachutist’s velocity is 55m/s?

15. Sample Problem
A 78kg parachutist jumps from an airplane. How much work has gravity done when the parachutist’s velocity is 55m/s?
Wtotal = .5(78kg)(55m/s)2 - .5(78kg)(0m/s)2
= 120,000J or 120kJ

16. Potential Energy
Potential energy is energy that is stored for later use.
One of the most common types of potential energy is gravitational potential energy, that is, energy available if an object above the ground falls down.

17. Potential Energy Gravitational potential energy formula
PEgravity = mgh
Gravitational potential energy = mass ✕ gravity ✕ height

18. Sample Problem
A 3.5kg bowling ball rests precariously on the edge of a shelf 1.4m above the ground. How much gravitational potential energy does it have?

19. Sample Problem
A 3.5kg bowling ball rests precariously on the edge of a shelf 1.4m above the ground. How much gravitational potential energy does it have?
PEgravity = mgh = 3.5kg ✕ 9.81m/s2 ✕ 1.4m = 48J

20. Potential and Kinetic Energy Lab
Follow the instructions on your lab report sheet.

21. Spring Potential Energy
A spring and similar objects which have the ability to deform and return to their original state have spring potential energy, also known as elastic potential energy.

22. Spring Potential Energy
Formula
PEspring = .5kx2
Spring Potential Energy = ½k ✕ distance deformed2

23. Practice Problem
A spring with a k value of 120N/m is stretched 2.3cm. What is its potential energy?

24. Practice Problem
A spring with a k value of 120N is stretched 2.3cm. What is its potential energy?
PEspring = .5kx2 = .5(120N/m) ✕ (.023m)2 = .032J

25. Homework
None.

26. Closure
A 50.0kg diver stands on a high dive 3.00m above the water, then dives. How much has his potential energy changed when he is 2.00m above the water? At 1.00m? At 0.00m?
Does his kinetic energy change in the same linear fashion?

27. Closure
A 50.0kg diver stands on a high dive 3.00m above the water, then dives. How much has his potential energy changed when he is 2.00m above the water? At 1.00m? At 0.00m?
When he is ⅓ of the way down, he has lost ⅓ of his PE. ⅔, he has lost ⅔. When he reaches the water, he has 0 PE.
Does his kinetic energy change in the same linear fashion?
No, because it depends on velocity2.