Work, Machines, and EnergyHS Physical Science

Hull Middle School

Work and Power

Work-The transfer of an energy to an object by using a force that causes the object to move in the direction of the force.

W=F x d

Work (in joules)= Force (in newtons) times distance (in meters)

Work involves a transfer of energy.

Force and work are not the same thing!

Applying a force does not always mean work. For example, you can push on a large boulder. If the boulder does not move, then work has not been done.

If the boulder were to move, then work has been done.

Work, defined…

In order for work to have been done, the object has to move in the same direction as the force applied.

See p. 211 in textbook. Discuss the examples.

Different amounts of force might be needed to do the same amount of work. See diagram on p. 212 (discuss).

Vocabulary

Joule- Unit used to express energy; equivalent to the amount of wonk done by a force of 1 N acting through a distance of 1 m in the direction of the force (symbol, J)

Vocabulary

Power-rate at which work is done, or energy is transferred.

Watt-unit used to express power; equivalent to joules per second (W).

See p. 214. Solve the power problem and discuss.

Calculating Power

P=W/t

Power is measured in watts. One watt is 1J/s.

Work is measured in Joules.

Time is measured in seconds.

Summarize

1. In scientific terms, work is done when a force causes an object to move in the _____________ of the force.

2. Work is calculated as _________ times __________.

3. The unit of work is the newton-meter, or _______.

4. _______ is a measure of how fast work is done.

5. Power is calculated as ______ divided by ______.

PRACTICE

Use W=fd

1. A crane uses an average force of 5,200 N to lift a girder 25 m. How much work does the crane do on the girder?

2. An apple weighing 1 N falls a distance of 1 m. How much work is done on the apple by the force of gravity?

3. A bicycle’s brakes apply 125 N of frictional force to the wheels as the bike moves 14.0 m. How much work do the brakes do?

4. A mechanic uses a hydraulic lift to raise a 1,200 kg car 0.50 m off the ground. How much work does the lift do on the car?

Answers

1. 1.3 x 10 (fifth) J

2. 1 J

3. 1,750 J

4. 5,900 J

Machines

Machine- A device that makes work easier by changing the size or direction of the force.

Work Input-work done on a machine.

Work Output-work done by a machine.

Simple Machines Video

http://www.youtube.com/watch?v=jGkQ64zn1j0

Calculating Work Input and Output

Work Input-The product of the input force and the distance through which force is exerted.

Work Output-The product of the output force and the distance through which force is exerted.

W=F x d

What does a machine do to forces?

Machines can either change the size or direction of the force, but not both…

A MACHINE DOES NOT CHANGE THE AMOUNT OF WORK THAT YOU DO!

FORCE AND DISTANCE TRADE-OFF

If a machine changes the size of the force, it also has to change the distance through which the force is exerted.

Force or distance will increase, but not both.

Mechanical Advantage

Mechanical Advantage-a number that tells how many times a machine multiplies force.

MA= output force/input force.

Note that this is force, not work.

What is the mechanical advantage of a machine that puts out 25N of force, when you only put in 5N of force?

Look at the illustration on p. 219.

How does a nutcracker work?

How does a hammer work?

How does a pulley work?

How does a screwdriver work?

Mechanical Efficiency

Mechanical Efficiency-A percentage, that measures work output to work input.

ME= (work output/work input) x 100

An ideal machine would have 100% efficiency, but this just doesn’t happen…

Output will always be less than input, because some friction must be overcome.

Summarize!

1. A machine makes work easier by changing the _____ or ______ of the force.

2. A machine can increase _____ or _______ , but not both.

3. ________ _________ tells how many times a machine multiplies force.

4. ______ _______ is the comparison of a machine’s work output compared to work input.

5. Why are machines never 100% efficient?