Chapter 7Energy, Part 1

• Work• Power• Mechanical Energy

– Potential Energy– Mechanical Energy– Work-Energy Theorem– Conservation of Energy– Efficiency– Comparison of Kinetic Energy and Momentum– Energy for Life– Sources of Energy

Next time

WorkSomething done by a force when the force acts on the object,

moving it through a distance.

A book falls off a table and free falls to the ground. (Gravity does work on the book)

A rocket accelerates through space. (The exhaust gases do work on the rocket)

You lift a heavy box from the floor to a table 3 ft higher. (You do work on the box)

Examples of doing work.

For work to be done, a force must be applied to an object and the object must move in the direction of the force.

hF

Work

Work IS done on an object when:

Force

Acts on it in the direction of motion

Or has a component in the direction of motion

Work IS NOT done on an object when there is no motion.

Or when the force is applied perpendicular to the motion.

Since work is a force moving over a distance …..

Work = Force x distance

W = F x dThe Unit of Work is the Joule (or N-m)

Question 1A force sets an object in motion. When the force is multiplied by the

time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force x distance?

A. Energy

B. Engine speed

C. Work

D. Displacement

Question 1 AnswerA force sets an object in motion. When the force is multiplied by the

time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force x distance?

A. Energy

B. Engine speed

C. Work

D. Displacement

PowerPower is the amount of Work done per unit time

Work donePower =

Time interval

It’s the RATE of doing Workor, the RATE of burning Energy

The Unit of Power is the “Watt” (or Joules/sec)

A 100 kW engine will do 100 thousand Joules of Work every second!100 kW = 134 Horsepower (the English units for Power)

A 60 W light bulb burns 60 Joules of energy every second.

Mechanical EnergyEnergy is the capacity for doing work.

Some things store energy in their structure or or because of their physical nature.

Examples: Gasoline, energy in food, energy stored in the atom, etc.

Sometimes energy is stored by an object due to an it’s position or because it’s in motion .. This is “Mechanical Energy”.

This kind of energy can be stored in objects by doing work on them.

Potential Energy – Energy stored by an object due to its position.

Kinetic Energy – Energy stored by an object because it is moving.

There are two most common forms of Mechanical Energy.

Potential EnergyEnergy stored in a system because of its position.

PE = mgh

When an object is lifted against gravity, it’s potential energy due to gravity is:h

It doesn’t matter what path the ball takes to get from the ground to height “h” … the gain in potential energy is the same!

In Joules

PE = Weight x heightor

Kinetic EnergyWith kinetic energy, an object or system has the ability to do

work due to its motion.

An object’s kinetic energy depends on its mass AND its speed.

The faster something is moving and the heavier it is, the more work it can do, so …

Again, in Joules

Work-Energy TheoremThe work done on an object is equal to the change in its kinetic energy.

Work = ΔKE

In this example, the work done by gravity on the block (mgh) is equal to the change in kinetic energy of the block (½ mv2)

so, mgh = ½ mv2

If you know the height of the block, you can predict it’s speed at the bottom of the ramp!

(doesn’t matter how LONG the ramp is)

Gives the same answer as above.

Question 2

In which car will you be moving the fastest at the very bottom of the incline?

1. Front car2. Middle car3. Rear car4. Other

Question 2 Answer

1. Front car2. Middle car3. Rear car4. Other

In which car will you be moving the fastest at the very bottom of the incline?

Chapter 7Energy, Part 2

• Work• Power• Mechanical Energy

– Potential Energy– Mechanical Energy– Work-Energy Theorem– Conservation of Energy– Efficiency– Comparison of Kinetic Energy and Momentum– Energy for Life– Sources of Energy

Last time

Conservation of Energy Energy can take on several forms (kinetic, potential, heat, light, etc)

Energy can’t be created or destroyed; it may be transformed from one form to another, but the total amount of energy can never change.

A common example is the pendulum:

The formula to calculate the potential energy is: PE = mgh

The mass of the ball = 10kg The height, h = 0.2m

The acceleration due to gravity, g = 9.8 m/s^2 Substitute the values into the formula and you get:

PE = 19.6J (J = Joules, unit of energy)

 The position of the blue ball is where the Potential Energy (PE) = 19.6J while the Kinetic Energy (KE) = 0.

The position of the purple ball is where the Kinetic Energy is at its maximum while the Potential Energy (PE) = 0.

The position of the pink ball is where the Potential Energy (PE) is once again at its maximum and the Kinetic Energy (KE) = 0.

Total Mechanical Energy = PE + KE

Using our common sense we know that it's impossible for the pendulum to swing higher than the height h without giving it a push yourself. If there was no friction, the pendulum would swing back and forth forever because of the law of conservation of energy.

Conservation of Energy

All Potential Energy, no Kinetic Energy

1/2 Potential Energy, 1/2 Kinetic Energy

1/4 Potential Energy, 3/4 Kinetic Energy

No Potential Energy, all Kinetic Energy

3/4 Potential Energy, 1/4 Kinetic Energy

Work done to get diver to the top of the tower!

Question 1What will be the kinetic energy of a pendulum bob when it undergoes a

10 kJ decrease in potential energy?

A. 10 MJ

B. 500 kW

C. 10kJ

D. None of the above

Machines …… multiply forces or change the direction of forces.

Work in = work outInput distance

largeOutput

distance small

Input force small

Output force Large

Lever

Can’t get more work out than you put in … conservation of energy !!!

Lever

Pulley Changes Direction of Force

PulleyMultiplies Force

Block and TackleA System of pulleys

that changes the direction and multiplies

force

EfficiencyIn a machine, there are always losses of energy, to heat, to friction, etc.

Efficiency measures how well a machine limits losses.

Efficiency = Useful energy output

Total energy input

In a lever, if you do 100 J of work and the machine puts out only 98 J, you’ve lost 2 J to thermal heating of the machine, and the lever’s efficiency is 98%.

Question 2You’re using a block and tackle to lift a heavy load. You do 50 J of

work on the rope of the machine, but the work done by the machine on the load is only 40 J. What is the efficiency of the block and tackle?

A. 100%

B. 80%

C. 120%

D. 60%

Kinetic Energy vs. MomentumBoth are properties of motion.

But they are different properties.

Kinetic Energy Momentum

Scalar Vector

Proportional to v2 Proportional to v

Can be converted to other energy forms

Cannot be converted, can only be gained or lost

Sources of commercial energy

Petroleum and coal

Solar energy – photovoltaic cells

Wind generation

Nuclear Power

Geothermal Energy

Hydrogen-based fuel technologies