Chapter 10 & 11

Energy & Work

Energy

• The capacity of a physical system to perform work.

• Can be heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms.

• Energy is transformed from one form to another. It is not created or destroyed.

Work• Work = (force) x (distance)• To have work, there must be:– A force– Movement of something by that force

Is work being done when you push against a wall?

NO! There is no movement!

Is work being done when a weightlifter lifts a weight?

Yes – The weight is moving….

What is the weightlifter holds the weight in place, is work being done?

NO! No movement!

Work

• Work = (Force) x (Distance)Units: Force = Newton (N)

Distance = Meter (m)Work = Nm = joule (J)

1 Joule of work is equal to one Newton of force applied over the distance of 1 meter.

Potential Energy (PE)• The stored energy an object has because of its

state, composition, or position.– Ex: A compressed spring• The spring is able to bounce back due to PE

– Ex: Food, Fuels• When atoms are rearranged, energy is released

Potential Energy (PE)– Ex: When objects are lifted against gravity• Gravitational Potential Energy = weight x height

= mgh• Depends only on the weight and the vertical

displacement. – Does not depend on the path taken!

Using the steps, ramp, or lifting the ball directly up all produce the same PE!

Kinetic Energy (KE)• The energy associated with a moving object due

to its motion– Gravitational potential energy can be transformed into

kinetic energy

Kinetic Energy = ½ mass x speed2

KE = ½ mv2

Since Speed is squared, a small change in speed can cause a large change in KE.

KE can be positive or zero, never negative.

The Work-Energy Theorem• The change in kinetic energy is equal to work

done.Work = ΔKE

• If there is no change in an object’s energy, then no work has been done.

• Energy is required to reduce the speed on an object.– A car is able to stop due to the work done by the

brakes.

Comparison of Kinetic Energy and Momentum

• Momentum– Vector Quantity• Directional• Able to be canceled

– Depends on velocity• M = mv

• Kinetic Energy– Scalar Quantity

• Can’t be canceled– Transform from one

form to another

– Depends on velocity2

• KE = ½ mv2

Conservation of Energy• In the absence of external work input or output,

the energy of a system remains unchanged. Energy cannot be created or destroyed

• Energy is transformed from one form to another

When the demolition ball or arrow is released, then the potential energy is transformed into kinetic energy

The water behind a dam has potential energy. When it flows through the dam, the energy of its movement can be used to power a generator, which can transform the energy into electircity

Power• Power: the rate at which work is done

Remember: Work = (Force) x (Distance)

So it takes the same amount of WORK walking up stairs as it does running….

But the POWER changes because the TIME changes. Power is greater running up the stairs than walking.

Power

• The rate at which energy is changed from one form to another.

• Unit:Work = Joule (J)Time = second (s)Power = J/s = watt (W)

1 Watt of power is equal to 1 joule of work done in 1 second.

Machines• Machine: A device used to multiply a force or to

change the direction of a force– Ex: Lever• Changes the direction of the force

– When we push down, it pushes the object up

• (Force x Distance)input = (Force x Distance)output

– If the pivot point (fulcrum) is close to the load, then a small input force produces a large output force

Machines– Ex: Pulley• Changes the direction of the force• Does not multiply force

– Ex: Block and Tackle• System of pulleys• Multiplies force at the expense of distance

EfficiencyEfficiency = Work Done

Energy Used

• Using the same energy input, some machines have a greater output

– They are more efficient!– Energy is wasted as heat• Typically a lever is more efficient than a pulley because

more energy is lost as heat in a pulley system