Chapter 9 Section 1 Notes

Work, Power, and Machines

What is Work?

Work is done only when a force causes a change in motion of an object.

Work is done by a force on an object. Work is calculated by multiplying the force

by the distance over which the force is applied.

Work = Force x distance; W = F x d

Work

If the distance something moves is zero, there has been no work done Example: If you are trying

to push a car stuck in the mud & it doesn’t go anywhere, you have done no work because the distance moved is zero. But, you have applied a force.

Work

Work is measured in Joules. All of these units are equivalent: 1 N • m; 1 J;

1 kg • m2/s2

You do 1 Joule of work when you lift an apple, which weighs 1 N, from your arm’s length down at your side to the top of your head (1 meter).

Work sample problem:

Nick lifts a 0.150 kg sandwich 0.30 m from the table to his mouth. How much work does he do?

Solution:

Power

Power measures the rate at which work is done, or, how much work is done in a certain amount of time.

Power = work/time; P = W/t SI unit for power: Watt (W) A watt is the amount of power required to do

1 J of work in 1 s.

Power sample problem:

While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts?

Solution:

Machines and Mechanical Advantage Machines multiply and

redirect forces. Machines help to do

work by redistributing the work we put into them.

Machines

Machines can: Change the direction of the input force Increase output force by changing the

distance over which the force is applied; called multiplying the force.

Machines

You can do the same amount of work while applying a different force. Why?

As force decreases, the distance increases, so you are doing the same amount of work.

Machines make work easier by increasing the distance over which force is applied.

Mechanical Advantage

Mechanical Advantage (abbreviated MA): tells us how much a machine multiplies force or increases distance.

Equation: MA = output force = input distance

input force output distance If MA >1: multiplies the input force; helps you

move or lift a heavy object, such as a car. If MA <1: increases distance and speed.

Mechanical Advantage

Example: Find the MA of a ramp that is 6.0 m long and 1.5 m tall.

Example: Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a MA of 5.2, how much force is exerted on the nail in the claw?

G Force

G Force is a measurement of an object's acceleration expressed in g's. It may also informally refer to the reaction force resulting from an acceleration, with the causing acceleration expressed in g's.

G force acts on all body parts, including organs, which are only loosely connected together

In 1954, John Paul Stapp experienced 46.2 g all for science. He was strapped into a rocket-powered sled on train tracks and decelerated from 630 mph to 0 in 1.25 seconds. This is the same as hitting a brick wall at 120mph! He survived, but his eyes filled with blood and he was temporarily blinded in what is called a “red out”.

Video of John Paul Stapp