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? A bow is drawn back. An arrow is shot from the bow. An airplane flying A boulder at rest at the top of a hill. 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 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 Kinetic 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 Force The work done by a constant force is defined as the distance moved multiplied by the component of the force in the direction of displacement:

Using WORK to verify mgh: Θ: 36.9 5 meters. 5 N 3 meters. 4 meters. 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. 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.

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.

Energy and The Law of Conservation of Energy

Law of Conservation of Energy Energy can never be created or destroyed. Energy can be converted from one form into another Kinetic to potential Potential to kinetic Kinetic to heat (from friction) Total Energybefore = Total Energyafter

The total energy is the same at all points of an object’s path

A falling rock has a potential energy of 750 J and kinetic energy of 450 J. A few seconds later, it has a potential energy of 400 J. Determine its new kinetic energy.

A heavy weight that has a mass of 50 kg is lifted using a rope and pulley to a height of 8 meters above a wood post. The rope is cut and the weight falls to hit the post. What is the speed of the weight on impact? Hint: List the Ep and Ek for each position of the weight.

A 2,000 kg car moving with a speed of 5 m/sec starts up a hill, and the accelerator and brakes are not working! How high does the car roll before it stops?

A 0. 057 kg car starts from rest at the top of a 0. 133-meter hill A 0.057 kg car starts from rest at the top of a 0.133-meter hill. Find the speed when it reaches the bottom of the hill.

A 500 kg roller coaster car starts from rest at the top of a 60-meter hill. Find the speed when it is halfway to the bottom.

What is power? Power – the rate at which work is done More power means… More work is done in the same amount of time The same amount of work is done in less time EX. A person that is a more powerful runner is faster and can run farther in the same amount of time as a less powerful runner (more work in equal time) EX. A car with a more powerful engine can accelerate to 60mph faster than a car with a less powerful engine (equal work in less time)

Calculating Power Power is measure in watts (W) We need to know 2 things in order to measure power: The amount of work being done The amount of time it takes to do the work We can calculate power using the following formula: Power = Work / Time (P = W / t) 1 watt equals 1 joule divided by 1 second

Calculating Power – Sample Problem 1 If an engine does 100,000 joules of work in 10 seconds, how much power did it use? P = W / t P = 100,000J / 10s P = 10,000W The engine used 10,000 watts of power

Calculating Power – Sample Problem 2 Because work equals force multiplied by distance, another way to write the power formula is: Power = (Force x Distance) / Time P = (F x d) / t P = W / t is the same as P = (F x d) / t

Calculating Power – Sample Problem 2 If an engine exerts 3500 newtons of force to move a car 50 meters in 10 seconds, how much power did it use? P = (F x d) / t P = (3500N x 50m) / 10s P = 175,000J / 10s P = 17,500W The engine used 17,500 watts of power

A Watt Measures Work Done and Energy Used In an Amount of Time A Watt equals 1 Joule per second The more watts, the more work is done each second Joules also measure energy, so a watt also measures energy use per second EX. A 100W light bulb uses 100J of energy each second that it is on