1. Outcomes – Chapter 8
1. Determine the amount of work done, given the force and the distance moved.
2. Determine the amount of power required, given the work done and the time required, or the force and velocity.
3. Describe the function of a lever, pulley, and inclined plane.
4. Explain why no machine can have an efficiency of 100%.
5. Determine the IMA, AMA, work in, work out, and efficiency of a system of pulleys.
6. Define energy in terms of work.
7. Distinguish among mechanical energy, potential energy, and kinetic energy.
8. Determine the potential energy of an object given its mass or weight and its height.
9. Determine the kinetic energy of an object given mass and speed.
10. State the law of conservation of energy and solve problems using it.

2. Work, Power, Machines
Chapter 8 Part One

3. Work
In physics, work is done when a force moves an object through a distance.
When you push a box up an incline, you “do work” on the box.

4. Work = Force x Distance

5. Units Force is measured in Newtons. Distance is measured in meters.
N x m = Nm = Joules
Joules and Nm are the same things.

6. Force and Distance must be in the same direction!!
If you pick an object up, the force must be the upward force.
If you slide an object sideways on a table, the force must be the sideways force.
If you measure the distance up an incline, the force must be the one exerted in a direction up the incline.

7. Is work vector or scalar.
Force and displacement are both vector.
But, due to complicated math
(vector dot products), work is scalar.
Power and energy are scalar as well.
It does make the math easier.
Use arithmetic instead of vector addition.
Joules and watts don’t have direction.

8. How much work do you do to lift a 15 N object upward 4 meters?
When you pick an object up, you have to apply a force equal to its weight (15 N). (why?, CVPM? UAPM?)
W = F x d
W = 15 N x 4 m = 60 J

9. How much work do you do to lift a 5-kg object upward 4 meters?
When you pick an object up, you have to apply a force equal to its weight (5kgx10m/s2=50 N).
W = F x d
W = 50 N x 4 m = 200 J

10. How much work do you do to slide a 15 N object 4 meters sideways if the force of friction is 3 N?
When you slide an object sideways, you have to apply a force equal to friction (3 N). (why? CVPM? UAPM?)
W = F x d
W = 3 N x 4 m = 12 J

11. How much work do you do to slide a 15 N object up an incline 4 meters long if the force the person must push with is 7 N?
When you slide an object up an incline, you have to use the force up the incline (7N).
W = F x d
W = 7 N x 4 m = 28 J

12. How much work do you do to slide a 5-kg object sideways 4 meters if the acceleration is 6 m/s2?
F=ma
F = 5-kg x 6 m/s2
F = 30-N
W = F x d
W = 30 N x 4 m = 120 J

13. Sometimes work can be negative!
If the force and displacement are exactly opposite, then work is negative.
When you lift a box, you pull upward and do positive work.
But gravity pulls downward and does negative work!

14. How much work is done by…
CVPM tells you what about the forces? Fa
(10)(15)= 150J FF
(-10)(15)= -150J FN
(100)(0)= 0J Fg
WHY?!

15. How much work is done by…FT
(30)(6)= 180J Fg
(-20)(6)= -120J
WHY?!

16. Special Case – No Work Done!
The force (weight) is downward. There is virtually no sideways force.
The distance is sideways. There is no downward motion.
W = 14-N x 0-m = 0J
W = 0-N x 10-m = 0J

17. Assignment 8a
Due Tomorrow
Start the Pulley Lab Today

18. Start with arrangement (b)
Pulley Lab
Start with arrangement (b)

21. Today Collect Data for Part B (both weights)
Complete the calculations for this part and have them reviewed by the teacher before you collect the rest of your data tomorrow.