Energy

Unit 4

Energy is the most central concept underlying all science.

Energy comes to us from the sun in the form of sunlight, it is in the food

we eat, & it sustains life.Persons, places, & things have energy,

but we observe only the effects of energy when something is happening- only when energy is being transformed

from one form to another.

Work= Force x distance

We do work when we lift a load against Earth’s gravity. The heavier the load or the higher we lift it, the more work we do. Two things enter into every case where work is done: (1)the application of the force, and (2) the movement of something by that force.

If we lift two loads up one story, we do twice as much work as we would in lifting one load the same distance,

because the force needed to lift twice the weight is twice as great.

Similarly, if we lift one load two stories instead of one story, we do twice as much work because the distance is

twice as great.

Notice that the definition of work involves both a force and a distance.

The weightlifter holding a barbell weighing 1000N over his head does

no work on the barbell. When the weightlifter raises the

barbell from the floor, he is doing work on it.

The resulting unit of work is the Newton-meter, also called the joule.

Power is the rate at which work is done.

Power= work done time interval

Twice the power means the engine can do twice the work in the same amount of time or the same amount of work in half the

time.

The unit of power is the joule per second, or the watt.

The “something” that enables an object to do work is energy. Energy is

measured in joules.

Mechanical energy is the energy due to the position of something, or the

movement of something. Mechanical energy can be in the form of either

potential or kinetic energy.

Potential Energy

• Energy that is stored and held in readiness

• Work is required to elevate objects against Earth’s gravity

• The potential energy due to elevated positions is called gravitational potential energy.

• Gravitational PE= weight x height

Kinetic Energy

• Energy of motion• KE= 1/2mv2

• The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest, or the work the object can do while being brought to rest.

• Fd= 1/2mv2

The law of conservation of energy:

Energy cannot be created or destroyed.

It can be transformed from one form into another, but the total

amount of energy never changes.

When you consider any system in its entirety, whether it is as

simply as a swinging pendulum or as complex as an exploding galaxy, there is one quantity

that does not change: energy. It may change form or it may be transferred from one place to another, but the total energy

score stays the same.

Machines

• A device used to multiply forces or simply change the direction of forces

• Follows the concept of the conservation of energy

Work input= work outputThe ratio of output force to input force for

a machine is called the mechanical advantage.

No machine can put out more energy than is put into it. No

machine can create energy. A machine can only transfer energy from one place to

another or transform it from one form to another.

An ideal machine would have 100% efficiency. In practice, 100%

efficiency never happens. In any machine, some energy is

transformed into atomic or molecular kinetic energy- making the machine warmer. This wasted

energy is heat.

Efficiency= useful work output total work input

Bud, a very large man of mass 130kg, stands on a pogo stick. How much work is done as Bud compresses the spring of the

pogo stick 0.50m?

After finishing her physics homework, Sherita pulls her

50kg body out of the living room chair and climbs up the 5m high flight of stairs to her bedroom.

How much work does Sherita do in ascending the stairs?

In the previous example, Sherita slowly ascends the stairs, taking 10s to go from bottom to top. The next

evening, in a rush to catch her favorite TV show, she runs up the

stairs in 3s.

On which night does Sherita do more work?

On which night does Sherita generate more power?

Legend has it that Isaac Newton “discovered” gravity when an

apple fell from a tree and hit him on the head. If a 0.2kg apple fell 7m before hitting Newton, what was its change in PE during the

fall?

A greyhound at a race track can run at a speed of 16m/s. What is the KE of a 20kg greyhound as it

crosses the finish line?

In a wild shot, Bo flings a pool ball of mass m off a 0.68m high pool table, and the ball hits the floor with a speed of 6m/s. How fast was the ball moving when it left the pool table? (Use the law

of conservation of energy.)

Frank, a San Francisco hot dog vender, has fallen asleep on the job. When an earthquake strikes,

his 300kg hot dog cart rolls down Nob Hill and reaches point A which is 50m high at a speed of 8m/s. How fast is the hot dog

cart going at point B which is 30m high when Frank finally

wakes up and starts to run after it?