1. Work, Power, and Simple Machines

2. Work = Force x Distance W = Fd
Work: the amount of force applied to an object to move it a distance
Units of work is the Joule (J)
1 Joule ( J ) = 1 Nm
W = Fd

3. Work
Work is done on an object when the object moves in the same direction as the applied force.
-No work without motion.
-Work must be in the same direction as the
motion of the object.

4. How much work does it take to move a 500 N car a distance of 20 m?
Example1
How much work does it take to move a 500 N car a distance of 20 m?
F = 500 N
d = 20 m
W = Fd
W = (500 N)(20 m) = 10,000 J

5. Power Power = = Units of power is the Watt ( W ) = 1 Work Time
Power: the rate at which work is done.
Power = =
P = =
Units of power is the Watt ( W ) = 1
*1 horsepower = 746 watts
Work
Time
Force x Distance
Time W t Fd t
Joule
Sec

7. Example 2
How much power does a person weighing 750 Newtons need if he takes 20 seconds to walk up a staircase 10 meters high?
F = 750 N
d = 10 m
t = 20 s
P = Fd t
P = (750 N)(10 m) = 375 W
20 s

8. Machines Changes at least one of three factors
Forces – Input or Output
Distance the force acts
Direction the force acts
Simple machines change input forces, they do not create force.
A motor produces kinetic energy output from a fuel source.

9. Mechanical Advantage
The number of times a machine increases a input force.
Mechanical Advantage =
Work (Force x Distance) is not changed.
Output force
Input force

10. Ramp A flat sloped surface Less input force over a longer distance
Inclined Plane
Ramp
A flat sloped surface
Less input force over a longer distance
Ramp

11. Wedge Starts as a thin edge and tapers to a thick end
Two inclined planes back to back
Changes the direction of the force and uses less input force over a longer distance
knife

12. Screw An inclined plane wrapped around a cylinder
Changes the direction of the force and uses less input force over a longer distance

13. Lever - First Class A rigid bar that can rotate around a fulcrum.
- Changes direction
- Less Force over a longer distance
Fulcrum is between forces
4:1 ratio Crowbar, pliers Opposite is light switch, jaw

14. Lever - Second Class A rigid bar that can rotate around a fulcrum.
- Less Force over a longer distance (as shown)
5:1 ratio can opener, nut cracker, wheelbarrow, ball of your foot muscle in calf

15. Lever - Third Class A rigid bar that can rotate around a fulcrum.
- More Force over a shorter distance
1:5 ratio bat, tennis racket, ax, shovel, fore arm elbow is the fulcrum

16. Wheel and Axle Two cylinders or circular objects
attached to each other
- The wheel has the larger radius
- The axle has the smaller radius
A lever that can turn 360o, always working
Changes distance and force
Screw driver Mechanical advantage = output radius/input radius

17. Pulley A rope over a grooved wheel - A single pulley changes direction
A pulley system (block and tackle)
- Can change direction of force
- Changes force and direction
Fixed pulleys are attached to a structure, Moveable is attached to the object that is being moved

18. A combination of two or more machines
Compound Machines
A combination of two or more machines

19. Efficiency of Machines
Efficiency = x 100%
If there were no friction, the efficiency would be 100%, but as a machine becomes more complex or moves further, friction is increased.
Output work
Input work

20. Machines Changes at least one of three factors
Forces – Input or Output
Distance the force acts
Direction the force acts