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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.
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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”.
