Mechanical Energy

Mechanical Energy

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Essential Question

What factors determine the kinetic and potential energy of an object?

What is Energy?

• Energy is the ability to do work (cause something to move)

• measured Joules (J)

• Mechanical Energy

  • total energy an object has based on position and motion

  • 2 types of ME

  • Kinetic Energy - energy of motion

  • Potential Energy - stored energy

• ME = KE + PE

Multiple Choice

An object has 10 J of KE and 15 J of PE. How much mechanical energy does the object have?

• 5 J

• 30 J
• 20 J
• 25 J

Multiple Select

Choose all of the examples of kinetic energy

• A rolling ball

• A person running

• A person sitting still
• A book on a shelf

Multiple Select

Choose all of the examples of potential energy

• A stretched rubber band

• Romeo falling off from Juliet's balcony

• A roller coaster cart at the top of a drop

• Romeo standing at the top of Juliet's balcony

Kinetic Energy

• Kinetic Energy is the energy of motion that depends on speed and mass

• Can be transformed into other types of energy

• KE and Speed

  • increase speed = increase KE

  • doubling in speed = increase in KE by factor of 4

• KE and Mass

  • increase mass = increase KE

  • doubling mass = doubling in KE

Multiple Choice

Which object has more KE if traveling the same speed?

• Truck

• Motorcycle

Solving for Kinetic Energy

• KE = 1/2 m * v²

• Example: A 10 kg ball rolls down a hill at 5 m/s

• Solution:

  • KE = 1/2 m * v²

    • m = 10 kg

    • v = 5 m/s

  • KE = 1/2 (10 kg) * (5 m/s)²

  • KE = 5 kg * 25 m²/s²

  • KE = 125 J

Multiple Choice

A 6 kg ball is thrown at 2 m/s. What is the KE of the ball?

• 8 J
• 4 J
• 12 J
• 24 J

Potential Energy

• Potential Energy is an object's stored energy dependent on position and mass

• Can be transformed into other types of energy

• Gravitational Potential Energy

  • PE dependent on an object's mass and height

• Elastic Potential Energy

  • PE stored due to an object being stretched or compressed

    • Ex: Rubber bands, springs

Multiple Choice

At which point does the cart have the most PE?

• 1

• 5

• 4

• 3

Solving for Gravitational Potential Energy

• PE = m * g * h

• Example: A 200 kg roller coaster cart is at the top of the 100 m drop. What is the PE of the rider in the cart?

• Solution:

  • PE = m * g * h

    • m = 200kg

    • g = 10 m/s² (9.81 m/s² )

    • h = 100 m

  • PE = 200kg * 10m/s² * 100 m

  • PE = 200,000 J

Multiple Choice

A 2 kg ball sits on top of a 5 m table. What is the PE of the ball?

• 50 J
• 100 J

• 25 J
• 10 J