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

• Energy is the ability of a person or an object to do work or to cause a change.
• When you do work on an object, some of your energy is transferred to the object.
• You can think of work as the transfer of energy. In fact, both work and energy are measured in the same unit, the joule.

Work changes potential and kinetic energy

• Kinetic energy is the energy of motion.
• Any moving object has some kinetic energy. The faster an object moves, the more kinetic energy it has.
• Potential energy is stored energy, or the energy an object has due to its position or shape.
• If you are holding a ball above the ground, the ball has potential energy. The higher you hold the ball, the more potential energy it has.

Calculating Gravitational Potential Energy

• Potential energy caused by gravity is called gravitational potential energy.
• This must be taken into account when spacecraft are launched and when roller coasters are designed.

Gravitational Potential Energy= mass x gravitational acceleration x height

GPE=mgh

• Calculate the GPE:

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

• Kinetic Energy= mass x velocity²

2

• Notice that velocity is squared while mass is not. That is because increasing the velocity of an object has a greater effect on the object’s kinetic energy than increasing the mass.
• Calculate the kinetic energy:

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

• Mechanical energy is the energy possessed by an object due to its motion or position—in other words it is the object’s combined potential energy and kinetic energy.
• Mechanical Energy = Potential Energy + Kinetic Energy
• ME= PE+KE
• For example, a skateboarder has a potential energy of 200J due to his position at the top of a hill and a kinetic energy of 100J due to his motion. His total mechanical energy is 300J.

The total amount of energy is constant

• Energy is transferred when work is done. No matter how energy is transferred or transformed, all of the energy is still present somewhere in one form or another. This is known as the conservation of energy.
• See the example of the in-line skater on page 126.

Forms of energy

• Each of these terms is also a combination of potential and kinetic energy.
• Thermal energy- Energy an object has due to the motion of its molecules.
• Chemical energy- The energy stored in chemical bonds that hold chemical compounds together. If these bonds are broken, then energy is released or absorbed (fireworks).
• Nuclear energy- The potential energy stored in the nucleus of an atom. The source of the Sun’s energy is nuclear energy.
• Electromagnetic energy- the energy associated with electrical and magnetic interactions. Examples are electrical energy and radiant energy (the energy carried by light).