Chapter 5 Part 2

Mechanical energy:Kinetic

Potential GravitationalPotential Elastic

Kinetic Energy

Practice

A 6.0 kg cat runs after a mouse at 10.0 m/s. What is the cat’s kinetic energy? (300J)

The 0.100 kg mouse runs as fast as the cat. What is the mouse’s kinetic energy? (5.00 J)

A 24 kg dog joins the race with the same kinetic energy as the cat. What is the dog’s speed? (5m/s)

Exploration experiment

• You have 3 balls with equal sizes, but made of different materials. If they all move at the same speed, which ball would have a higher KE?

• If all balls have equal KE, which one is moving slower?

fo hhmgW gravity

Work done by gravity

Calculate the work done by gravity on each ball if they all fall from the height of 60.0cm

• m1

• m2

• m3

• In which case the amount of work is the greatest?

• In which case is KEf the greatest?• Which ball is moving fastest?

Work-KE theorem

Positive work =increase in KENegative work = decrease in KE

When will Fg do positive work?When will Fg do negative work?When will Ffrk do positive work?When will Ffrk do negative work?

Energy…

• Energy can neither be created (by people that is??) nor destroyed.

• So, where does KE of a falling ball come from???

• Energy can be stored – then it is called POTENTIAL

fo hhmgW gravity

Work done by gravity

DEFINITION OF GRAVITATIONAL POTENTIAL ENERGY

The gravitational potential energy PE is the energy that anobject of mass m has by virtue of its position relative to thesurface of the earth. That position is measured by the heighth of the object relative to an arbitrary zero level:

mghPE

Can potential gravitational energy be negative???

1h

3h

Compare gravitational energies.a) Can they all be equal?b) Can the navy ball have more PE than the green one?

Elastic Potential EnergyElastic potential energy – energy in a stretched

or compressed elastic objectk – spring constantx – distance of deformation

A Gymnast on a Trampoline

What kind of mechanical energies are involved in this activity??

A Gymnast on a Trampoline

The 65.0kg gymnast leaves the trampoline at an initial height of 1.20 m and reaches a maximum height of 4.80 m before falling back down. What was the initial speed of the gymnast? How much potential elastic energy did the trampoline store? If the trampoline was stretched down by 30.0 cm, and the mass of the what is the k for the material?