PHYSICS 1 HONORS

CHAPTER 8 ENERGY

Sections 8.4 to 8.7

KINETIC ENERGY Any object in motion is capable of doing

work. This is because a moving object has

kinetic energy Kinetic energy depends on the:

Object’s massObject’s velocity

KE = ½ mv2 or ½ ms2

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KINETIC ENERGY If you throw and object, the work you do

to put the object in motion will be equal to its kinetic energy.

Since:

W = F x d and KE = ½ mv2

F x d = ½ mv2

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KINETIC ENERGY What happens to KE if you double the

velocity? Triple the velocity?

How much more work do you have to do to double the speed of an object? Triple the speed?

How about stopping distance if you are driving twice as fast? Three times as fast?

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KINETIC ENERGY

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WORK – ENERGY THEOREM Whenever work is done, it changes the

energy of an object (in this case KE) Whenever KE changes, work must be

done. So:

W = ∆E

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CONSERVATION OF ENERGY It is important to understand how

energy transforms. If Joe climbs on a lab table, he does

work, which is transformed into PE. If he jumps off the table, the PE is

transformed to KE. If Joe jumps onto a see-saw and Aslan is

standing on the other end, Joe’s KE is transformed to Aslan’s KE.

If Aslan ends up on top of another lab table, his KE is transformed to PE

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CONSERVATION OF ENERGY Energy may be transformed, but this is

done with no net loss or net gain. However, it may be transformed into

different forms of energy.

If Joe’s KE had been transformed to heat in the see saw and heat in his shoes in addition to KE for Aslan, is it likely that Aslan will have enough KE to reach the top of the lab table?

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CONSERVATION OF ENERGY The Law of Conservation of Energy

states:

Energy cannot be created nor destroyed. It can be

transformed from one form to another, but the total amount of energy never

changes.

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CONSERVATION OF ENERGY

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MACHINES A machine is device used to muliply

forces or change the direction of forces.

One of the simplest machines is a lever.We do work on one end of a lever.The other end of the lever does work on the

object. If we push down on the lever, the object will

move up.

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MACHINES

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MACHINESWork input = Work output

So

(Force x distance)input = (Force x distance)output

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MACHINES If the pivot point of the lever is close to

the object, then:

A small input force exerted over a long distance will result in:

A large output force exerted over a short distance.

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MACHINES

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MACHINES

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MACHINES For a machine like a lever:

If your input force is 15 N and your output force is 60 N your mechanical advantage is:

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ForceInput

ForceOutput Advantage Mechanical

415

60

I.F.

O.F M.A.

N

N

MACHINES There are 3 ways to set up a lever:

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MACHINES – TYPE 1 LEVER

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MACHINES – TYPE 2 LEVER

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MACHINES – TYPE 3 LEVER

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MACHINES A pulley is a lever that changes the

direction of the force. Pulley systems can multiply forces

A single pulley is a Type 1 Lever.

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MACHINES – SINGLE PULLEY

Changes direction of the force

Fulcrum is the axis (axle) of the pulley

Mechanical advantage = 1

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MACHINES In this case the

pulley is acting as a Type 2 Lever

The type of system has a mechanical advantage of 2

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Input

fulcrum

output

MACHINES For simple pulley systems, the

mechanical advantage is equal to the number of strands that actually support the load.

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MACHINES

1 stands support the load – M.A. =1 2 stands support the load – M.A. =2

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MACHINES

Load moves half as much – effort halved.

Again in this simple pulley system there are 2 strands holding the load, so the M.A. = 2

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MACHINES

When you pull 4 m the load moves 1m – M.A. = 4

Applied force x input distance = output force x output distance

You can also find M.A. in this situation by taking input distance/output distance

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MACHINES No machine can create energy.

Machines can only transfer energy or transform it from one form to another.

Machines can give you a mechanical advantage.

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