University:
North Central College
Course:
PHYS 305 | Classical Mechanics
Academic year:

2019
Views:

291

Pages:

4
Author:

SpaceCadet101

Chapter 4.1 Notes – Work and Power Define WORK and how it is calculated. W = Fd Energy is a measure of an object's ability to do work. Power is the rate at which the work is being done. Explain how work is related to energy. Both work is measured in joules; work is a form of energy. Give an example of work that increases the energy of an object. Stretching a rubber band. Give an example of work that does not increase the energy of an object. Sliding a block on a level table. While sitting in class, your body exerts a force of 600 N on your chair. How much work do you do? You do no work unless the force moves over a distance. Zero work done. If forces A, B, and C have equal strength, which one will do the most work? A: no work B: not as much work as C C: work W = travel a distance How do you calculate work done against gravity? W = mgh Work is the same whether going up stairs or a ramp. Because same distance (lifting, pulling, or pushing) is NOT carrying Define POWER and how it is calculated. Power (P) = W (work (joules)) / t (time (sec)) What is another unit of power often used to describe engine power? Horsepower 1 Horsepower = 746 watts Is your power greater when you run or walk up a flight of stairs? Why? Power is greater when you run! Same distance Power is greater when shorter time A motor does 10,000 joules of work in 50 seconds. What is the power of the motor? P = W/t P = 10,000/50 P = 200 watts **Power** In science, work is defined as the force needed to move an object a certain distance. The amount of work done per unit of time is called power. **EXAMPLE** Suppose you and a friend are helping a neighbor to reshingle the roof of his home. Each carry 10.0 bundles of shingles weighing 300. newtons apiece up to the roof which is 7.00 meters from the ground. You are able to carry the shingles to the roof in 10.0 minutes but your friend needs 20.0 minutes. Both of you did the same amount of work (force x distance) but you did the work in a shorter time. W = F x d W = 10 bundles of shingles (300 N/bundle) x 7.00 m = 21,000 joules However, you had more power than your friend. Power (watts) = Work (joules) / Time (seconds) Let's do the math to see how this is possible. **Step one: Convert minutes to seconds.** 10 minutes x 60 seconds/minute = 600 seconds (You) 20 minutes x 60 seconds/minute = 1,200 seconds (Friend) **Step two: Find power.** 21,000 joules / 600 seconds = 35 watts (You) 21,000 joules / 1,200 seconds = 17.5 watts (Friend) As you can see, the same amount of work that is done in less time produces more power. You are familiar with the word watt from a light bulb. Is it now clear to you why a 100-watt bulb is more powerful than a 40-watt bulb?

Chapter 4.1 Work and Power Notes

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