CHAPTER 4: WORK AND ENERGY

THE BIG IDEA:Energy is transferred when a force moves an object.

SECTION 4.1: Work is the use of force to move an object.

Force is necessary to do work

     Work is the use of force to move an object some distance.

     You do work only when you exert a force on an object and move it.

     Reading a page is not work. Turning the page is.

Calculating work

        Work =Force x Distance

        W=Fd

        When you multiply a force in newtons times a distance in meters, the product is one newton-meter, or the joule (J).

 

Use the formula for work to solve the following problem:

 

How much work is being done if a person pushes a cart with a force of 70N for 2m?††††

W=140J

 

 

Objects that are moving can do work

        You do work on objects, but objects also do work .

        Read the second paragraph on page 118 of your book.

        Throughout history, people have taken advantage of the capability of objects in motion to do work.Waterwheels and Windmills are examples.

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SECTION 4.2:Energy is transferred when work is done.

Work transfers energy

Work changes potential and kinetic energy

Calculating Gravitational Potential Energy

Gravitational Potential Energy= mass x gravitational acceleration x height

GPE=mgh

If you lift a 2kg box of toys to the top shelf of your closet, which is 3m high, how much gravitational potential energy will the box of toys have?

Hint:gravitational acceleration= 9.8m/s2

GPE=58.8J

Calculating Kinetic Energy

†† 2

A grasshopper with a mass of 0.002kg jumps up at a speed of 15m/s.What is the kinetic energy of the grasshopper?

KE=0.225J

Calculating Mechanical Energy

The total amount of energy is constant

Forms of energy