1. Work and Power

2. What is Work? Work is the product of force and distance
Equation: Work = Force x distance or
W = Fd
Work REQUIRES motion!
Force and motion MUST be in the SAME direction for work to happen

3. What is Work? cont’d…. You walk forward dragging a box behind you.
What is the direction of the motion of the box?
What is the direction of the force on the box?
Has work been done on the box?

4. What is Work? cont’d…. You carry a book to your locker.
What is the direction of the motion on the book?
What is the direction of the force on the book?
Has work been done on the book?

5. What is Work? cont’d….
Work is measured in joules (J) or Newton-meters (N•m)
Ex: A weightlifter lifts a 1600 N barbell to a height of 2 meters. How much work has the weightlifter done?
W = Fd W = 1600N x 2m
W = ? W = 3200 J
F = 1600 N
d = 2m

6. What is Power? W P = t Power is the rate of doing work
To increase power….
Increase work or decrease time
To decrease power ….
Decrease work or increase time W
P = t

7. What is Power? cont’d …. Power is measured in watts (W)
Before the watt, power was measured in horsepower
746 hp = 1 W

8. Mechanical Advantage Input force - the force you apply
Output force – force which is applied by the object
Mechanical Advantage – small input force produces larger output force
Often used by simple machines

9. Ideal Mechanical Advantage (IMA)
Friction loss is not taken into consideration
Ratio of distance traveled by effort and resistance force DE DR
IMA=
DE = Distance traveled by effort force
DR = Distance traveled by resistance force

10. IMA Example IMA= Effort Length = 11.0 cm = 5.5
Suppose you need to remove a nail from a board by using a claw hammer. If the effort length for a claw hammer is 11.0 cm and the resistance length is 2.0 cm. What is the mechanical advantage?
IMA= Effort Length = 11.0 cm = 5.5
Resistance Length cm

11. Actual Mechanical Advantage (AMA)
Frictional losses are taken into consideration
Ratio of force magnitudes FR FE
AMA=
FR = Magnitude of resistance force
FE = Magnitude of effort force

12. AMA Example AMA= Resistance Force = 60 N = 3
Kisha applied 20N of force to turn an ice cream freezer crank. The crank’s resistance was 60 N. What was the mechanical advantage of the crank?
AMA= Resistance Force = 60 N = 3
Effort Force N

13. AMA vs IMA AMA is what actually happens
IMA is what could happen without friction
Since there is always some friction, AMA is less than IMA.

14. Efficiency Work Output Efficiency = X 100% Work Input
Because there is always some friction, the efficiency of any machine is always less than 100 percent.

15. Efficiency Example
Alice and Jim calculate that they must do 1800 J of work to push a piano up a ramp. However, because they must also overcome friction, they must actually do 2400 J of work. What is the efficiency of the ramp?
1800 J
2400 J
X 100 = 75%

16. The 6 Simple Machines Screw Wedge Inclined Plane Lever Pulley
Wheel and Axle