1. Work and Simple Machines

2. Work and Simple Machines

3. Work Work: Mechanical work-most familiar form of work
The force exerted over a given distance
W=Fd
This is a scalar quantity because direction is not given
Work can be positive, negative or zero

4. What is work?
In science, the word work has a different meaning than you may be familiar with.
The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

5. What’s work?
A scientist delivers a speech to an audience of his peers.
A body builder lifts 350 pounds above his head.
A mother carries her baby from room to room.
A father pushes a baby in a carriage.
A woman carries a 20 kg grocery bag to her car?

6. What’s work?
A scientist delivers a speech to an audience of his peers. No
A body builder lifts 350 pounds above his head. Yes
A mother carries her baby from room to room. No
A father pushes a baby in a carriage. Yes
A woman carries a 20 kg grocery bag to her car? No

7. Work or Not?
According to the scientific definition, what is work and what is not?
a teacher lecturing to her class
a mouse pushing a piece of cheese with its nose across the floor

8. Work or Not?
According to the scientific definition, what is work and what is not?
a teacher lecturing to her class
a mouse pushing a piece of cheese with its nose across the floor

10. Mechanical work
Positive mechanical work: this is when the force acts on a system in the same direction as the system’s motion
In other words- work is positive when the force applied on the system is greater than the force exerted by the system
If I push a box and it moves in the direction I have pushed it then I have a positive mechanical work, if it does not move work is zero
A SYSTEM MUST MOVE FOR WORK TO BE DONE (on it)

11. Formula for work Work = Force x Distance The unit of force is newtons
The unit of distance is meters
The unit of work is newton-meters
One newton-meter is equal to one joule
So, the unit of work is a joule

12. W=FD Work = Force x Distance
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

13. W=FD Work = Force x Distance
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules
(W = 20N x 10m)

14. Calculating Work
London’s Great Clock mechanism is run by a falling weight, which turns the many gears that position the hands. The 2490 N (560 lb) clock weight has to be periodically wound up by an electric winch. Assuming the weight is raised 53 m and the winch exerts a force equal to the clock weight, how much work does the winch do on the weight?
W=Fd
W=?
F= 2490 N
d= 53 m
W= (2490 N)(53m)
131, 970 N m
(2490kg m/s2)(53m)
131,970 kg m2/s2
W= 130,000 J

15. Check for Understanding
1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds.
Which student does the most work?
Which student delivers the most power?
Explain your answers.

16. Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads.
Yet, Ben is the most powerful since he does the same work in less time.
Power and time are inversely proportional.

17. Force=Mass x Acceleration
2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.
Force=Mass x Acceleration
Work=Force x Distance
Power = Work/Time

18. Force=Mass x Acceleration
2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.
Force=Mass x Acceleration
Force=10 x 2
Force=20 N
Work=Force x Distance
Work = 20 x 10
Work = 200 Joules
Power = Work/Time
Power = 200/5
Power = 40 watts

19. Power Power is the rate of doing work P=W/Δt
Watt (W)- SI unit for power
W= J/s
Newton’s 2nd law of motion states that if you exert more force you will have a greater acceleration for a given mass; therefore you have a faster speed in less time
This means that a larger engine can do more work in a shorter interval than a smaller engine= the larger engine produces more POWER

20. Power Power is the rate at which work is done. Power = Work*/Time
*(force x distance)
The unit of power is the watt.

21. Calculating Power
The electric winding mechanism in the Great Clock takes about 40 min. to rewind the clock weight cable. Refer back to the Great Clock force example, then calculate the power of the electric winch.
P=W/Δt
P=?
W= 131,000 J
Δt= 40 min
40 min(60 s/min)= 240 s
P= 131,000 J/(240 s)
545 J/s
545 kg m2/s2/s
545 W

22. Simple Machines and Efficiency
Simple machines: a basic mechanical device that either reduces effort or increases the magnitude of motion for a given input motion when doing a certain amount of work.
A tool compose of basic mechanical parts that is to reduce effort or increase amount of motion
Levers
Wheels and axles
Gears
Pulleys
Inclined planes
Wedges
Screws

23. Efficiency of Simple Machines
Because simple machines do not create energy, you cannot get more work out of something than the amount of work you put into it.
Most often you must put more work into a system than the work done by the system
Efficiency= is a percentage of how well the simple machine does its job

24. Efficiency
We said that the input force times the distance equals the output force times distance, or:
Input Force x Distance = Output Force x Distance
However, some output force is lost due to friction.
The comparison of work input to work output is called efficiency.
No machine has 100 percent efficiency due to friction.

25. Efficiency
Efficiency= Wout X 100%
Win

26. History of Work
Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.