Chapter 5 Part 2 Mechanical energy: Kinetic Potential Gravitational Potential 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 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?

Work done by gravity

Calculate the work done by gravity on each ball if they all fall from the height of 60.0cm m 1 m 2 m 3 In which case the amount of work is the greatest? In which case is KE f the greatest? Which ball is moving fastest?

Work-KE theorem Positive work =increase in KE Negative work = decrease in KE When will F g do positive work? When will F g do negative work? When will F frk do positive work? When will F frk 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

Work done by gravity

DEFINITION OF GRAVITATIONAL POTENTIAL ENERGY The gravitational potential energy PE is the energy that an object of mass m has by virtue of its position relative to the surface of the earth. That position is measured by the height h of the object relative to an arbitrary zero level: Can potential gravitational energy be negative???

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

Elastic Potential Energy Elastic potential energy – energy in a stretched or compressed elastic object k – spring constant x – 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?