Work and Simple Machines

Physical Science Work and Simple Machines Chapter 8 Study Guide

1. What is the scientific meaning of the word “work”?

Work happens when a force causes an object to move in the same direction of that force.

2. If you throw a ball, are you doing work? Explain your answer. Be sure to include information about when throwing a ball is work and when it is not.

You are only doing work on the ball as long as you are directly putting a force on it, in other words, when you are touching it.

3. If you are pushing against a wall, are you doing work? Explain your answer.

No. Work is only done when the object moves. If the object doesn’t move, no matter how much force you put on it, it isn’t work.

4. If you are carrying a suitcase, are you doing work on the suitcase? Why or why not?

No. You are doing work on it when you pick it up, because when you pick it up, the direction of the movement of the object is the same as the direction of the force you are putting on it. But, when you are carrying it, the force you put on it is going up, but the suitcase is going forward. 5. Complete your own version of the chart

Example Direction of force Direction of motion Doing work? Pushing a stroller Yes

Walking with backpack No

Lift weights over head Yes

Carrying suitcases No 6. What two things do we need to know to calculate work? We need to know the size of the force that was put on the object, and how far the object moved.

7. What is the formula for calculating work? (Be sure to review your calculation practice sheets from class).

W = F times d (work is equal to force times distance). The standard unit of measurement for work is the Joule (abbreviated “J”).

8. What is “power”, scientifically speaking? Power is the rate at which work is done. The faster someone or something can do a certain amount of work, the more powerful they/it is. An electric toothbrush is more powerful than a regular one, because it can do 100 strokes in less time.

9. What is the formula for calculating power? (Be sure to review your calculation practice sheets from class).

P = w/t (power is equal to the amount of work done divided by the amount of time it took to do the work). The standard unit of measurement for power is the watt. (make sure when you calculate power, the “time” is in seconds. Sometimes you may need to convert minutes or hours to seconds!)

10. What is a machine? A machine is a device that helps make work easier by changing the size or direction of a force.

11. Draw an example of a machine that you use in your day-to-day life, and explain it. A knife is an example of a machine we use daily. It is an example of a wedge, used for cutting.

12. List the 6 simple machines. pulley, inclined plane, wheel & axle, wedge, screw, and lever 13. Fill in the chart below.

Simple Machine Definition Example Name

Straight slanted surface; also called a ramp INCLINED PLANE

Double inclined plane that moves; used for WEDGE cutting

Inclined plane wrapped around a cylinder SCREW

Consists of 2 circular objects of different sizes WHEEL & AXLE

Made of a rod that pivots on a point called a LEVER fulcrum

Made of a wheel with a groove in it, and a rope PULLEY that runs inside the groove 14. What is the fulcrum of a lever? The fulcrum is the part of a lever that stays still, like the middle of a seesaw.

15. What are compound machines? Draw and describe an example. Name two other compound machines. A compound machine is a machine made of two or more simple machines. Examples include scissors, can openers, and tweezers.

These scissors are made of several different types of machines. The handles are levers. The blades are wedges. There is a screw in the middle to hold them the two blades together (and serves as the levers’ fulcrum).

16. Does a machine reduce the amount of work that is being done? Explain your answer.

No, the same amount of work is done, whether you do it yourself, or use a machine. For instance, if you need to lift a 100N box 4 meters, it requires 400 J of work. Using an eight meter ramp will make the force go over a longer distance, but would only require 50 N of force (50 times 8 would still equal), Either way, you still do 400J of work.

17. If machines do not reduce the amount of work being done, how do they help us?

Machines can change the size of the input force necessary. In the example above, we’d only have to use half of the force that we’d have to use if we lifted the box without a ramp. Also, they can change the direction of the force, which is especially helpful when we are trying to lift something. 18. What does “force/distance trade-off” mean?

You can use a machine to allow you to use less force, but you have to make up for it by applying that force through a greater distance. Increase distance, decrease force.

19. What is a machine’s mechanical advantage?

Mechanical advantage tells us how many times the machine multiples the force you put into it.

20. If I say a machine has a mechanical advantage of 20, what does that mean?

It means that the machine multiplies the force you put into it twenty times. So, if the machine multiplies the force by 20, you would only have to put in a small amount of force to do a large amount of work.

21. You apply 200 N to a machine, and the machine applies 2000 N to an object. What is the mechanical advantage of the machine?

MA = output force MA = 2000 N MA = 10 N input force 200 N

22. What is mechanical efficiency?

Mechanical efficiency is a measure of how much of a machine’s energy is wasted, overcoming friction. Mechanical efficiency is always a percentage. A very low percentage (like 25%) means that only about 25% of the energy actually gets used for the purpose of the machine, the rest is wasted overcoming friction. A very high percentage, like 85%, means that all but 15% of the energy gets used for the work. No machine is 100% efficient, because there are always surfaces rubbing against each other, and, therefore, friction to overcome.

23. What is the mechanical efficiency of a lever with a work input of 60 J, and a work output of 43 J?

ME = work output X 100 ME = 43 J X 100 ME = 71.7% work input 60 J

24. Can there be a machine that is 100% efficient? Why or why not?

No machine can be 100% efficient due to friction. Machines have moving parts that rub together, so there will always be some friction. ______Students in honors classes need to be able to calculate IMA of a wedge, inclined plane, and wheel & axle. Review the mixed review practice sheets from class.

Magic Triangles for MA and ME