Chapter 8: Work & Machines

By:Mr. Manning

What is Work? (2:18)

What is the scientific meaning of work?

Work is the action that results when a force causes an object to move in the direction of the force.

If you had to read an entire chapter by tomorrow that would appear to be a lot of work, but scientifically it is not.

If we cancelled class right now and went bowling then we could do some real work.

True work is done because a force is applied to the bowling ball causing it to move in the same direction.

Work: cont’d Applying a force does not always mean that

work is done. Ex: Pushing a car that has the brakes on. Remember, in order for work to take place the

object must move. If you were running to catch a flight carrying a

suitcase, is work being done? No, the force & direction of movement are in

different directions. Work only takes place when the movement is in

the same direction of the force. (ex: when picking up the suitcase work is done.)

Work (1:21)

Calculating Work: The amount of work (W) done in moving an

object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied. W = F x d

Simply, the greater the distance you apply a force, the more work done. Also, the greater the force you apply over a distance, the more work done.

Force is expressed in newtons, and meters is the SI unit for distance or length.

The unit used to express work is the newton-meter (N●m), simply called the joule (J).

Power: Like work, the term power is used a lot in

everyday language but scientifically power is the rate at which work is done.

To calculate power (P), you divide the amount of work done (W) by the time it takes to do that work. P = W/t

Remember the unit for work is joule (J), and the unit for time is the second (s).

The unit used to express power is joules per second (J/s), more simply the watt (W).

Corny Science Joke…. Q: What is the unit of power?

A: Watt.

Q: I said, What is the unit of power?

A: Watt.

Q: I SAID….

Power: cont’d Power is how fast work happens. Power is increased when more work is done

in a given amount of time. Power is also increased when the time it

takes to do a certain amount of work is decreased.

Ex: No matter how fast you can cut wood with a handsaw, an electric saw can do the same amount of work faster.

Therefore, the electric saw has more power.

Power: (1:44)

What Is a Machine?

Imagine you steal your parent’s car and drive off to a party at your friend’s house.

On the way there…Oh no….you get a flat tire. You open the trunk, take out the tools, and then put on the spare tire.

Now imagine changing the tire without the tools. ( jack, lug nuts, tire iron, etc.)

The tools used are considered machines, a device that helps make work easier by changing the size and direction of a force.

What is a Machine? (0:46)

Examples of Machines:

Machines: (2:19)

How machines work… When you use a machine, you do work on the

machine, and the machine does work on something else.

The work you do on a machine is called work input. (ex: squeezing a pair of pliers)

The work done by the machine is called work output. (ex: the pliers loosening a screw)

The output force opposes the forces you and the machine are working against- in this case the weight of the screw and the friction between the screw and a nut.

Life + Physical Science Connection:

Humans aren’t the only animals that use tools.

Chimpanzees fashion specialized twigs to snare

termites from inside their mounds, and some

otters use carefully selected rocks to crack open

shellfish.

Mechanical Advantage & Efficiency: Do some machines make work easier than others? Yes, because some machines have more mechanical

advantage, which tells you how many times the machine multiplies force.

Why is the work output of a machine always less than the work input?

Because some of the work done by the machine is used to overcome the friction created by the use of the machine.

The less work a machine has to do to overcome friction, the more efficient it is. Mechanical energy is a comparison of a machine’s work output with the work input.

Mechanical Advantage: (2:19)