Energy and Work

Energy

• Energy is the ability to change or cause change.

• If something has no energy, there can be no change.

Potential and Kinetic Energy

• Potential Energy– Energy of position– Has potential to

cause change– Ex. Compressing a

spring, a ball lifted above the floor, etc.

Kinetic Energy— Energy of motion— Ex. A car racing, a ball

falling, etc.

— Unit is a Joule (J)

Which Have Potential or Kinetic Energy?

1. A bow is drawn back.2. An arrow is shot from the bow.3. An airplane flying4. A boulder at rest at the top of a hill.5. A boulder rolling at the bottom of a hill.

How Much Energy?

•What is the potential energy of a 2-kilogram potted plant that is on a 2.2 meter-high plant stand?

•What is the kinetic energy of a 3-kilogram ball that is rolling at 2 m/s to the left?

• A biker with a mass of 74 kg is traveling at 8 m/s to the right. What is her kinetic energy?

• If she bikes up a hill that is 40 m high, what will her potential energy be at the top of the hill?

Potential vs Kinetic

• Potential Energy– Increases when mass,

gravity or height increases.

– If a ball is raised twice as high, the Ep doubles

Kinetic Energy— Increases when mass

increases— If the mass doubles, Ek

doubles— If the velocity

doubles, Ek quadruples (squared)

— What if velocity were tripled?

Work

• Work is a transfer in energy that results by applying a force over a distance

• Work can change potential energy or kinetic energy

Work Done by a Constant Force

In the SI system, the units of work are joules:

As long as this person does not lift or lower the bag of groceries, he is doing no work on it. The force he exerts has no component in the direction of motion.

No Work

• If there is no change in kinetic energy and potential energy, no work is done– Holding weights above your head– Moving at a constant speed

Work Done by a Constant ForceThe work done by a constant force is defined as the distance moved multiplied by the component of the force in the direction of displacement:

You’re not pushing against the entire weight of the object, you need to figure out the component of the weight you ARE pushing against.

Using WORK to verify mgh:

3 meters.

Θ: 36.9

4 meters.

5 meters.

Assuming you are using a constant force and knowing the distance up the ramp, figure out the energy supplied by doing WORK on the box.

5 N

• How much work is done by pushing a car 200-meters to the next gas (on top of a small hill) station with a force of 420-Newtons?

• A waiter lifts platter of food 1.8 meters above the ground with a force of 7 N.

• A waiter holds the same platter of food over his head while he walks to the customer’s table 10 meters away.