Mechanical Energy

Gravitational Potential Energy

•The energy possessed by an object because of its position relative to a lower position

•The potential of an object to be pulled down by gravity

A GPE Machine

Eg = mghEg – gravitational

potential energy (J)m – mass (kg)g – gravitational field strength (N/kg or m/s2)h – height above reference

level (m)

•We always have to indicate a reference level

Ex. The gravitational potential energy is 30J relative to the Earth’s surface

Sample Problem:

A 58 kg skydiver stands in the door of a plane preparing to jump. If the earth is 2.8 km below the skydiver, what is the skydiver’s potential energy relative to the earth?

Kinetic Energy

•the energy possessed by an object due to its motion

Calculating Kinetic Energy:

Ek = ½ mv2

Ek – Kinetic Energy (J)

m – mass (kg)v – Speed (m/s)

Sample Problem:

What is the kinetic energy of a 60 g tennis ball that has a speed of 3.6 m/s?

Mechanical Energy

•The sum of the gravitational potential energy and the kinetic energy

•Think of TOTAL energy of an object due to it’s motion/position

•Mechanical energy will always stay the same unless WORK is done

•Ex. When a parachutist jumps from a plane, she initially has a large GPE and no kinetic energy. As she falls, she speeds up as her GPE converts to kinetic energy

Her Mechanical Energy will always be the same

Calculating Mechanical Energy

Em = Eg + Ek

Ek – Kinetic Energy (J)Eg – Gravitational Potential

Energy (J)Em – Mechanical Energy (J)

Applications of Mechanical Energy•South African vultures eats bones – if they are too big the bird will drop them from a greater height so they break

Applications of Mechanical Energy•A roller coaster on a track – the height of

the first hill has to be high enough giving a high potential energy – this will be converted into kinetic energy which will make it go really fast

•A roller coaster on a track – the height has to be high enough to create enough potential energy – this will be converted into kinetic energy which will allow the cart to get around the track•A roller coaster on a track – the height has to be high enough to create enough potential energy – this will be converted into kinetic energy which will allow the cart to get around the track

Top Thrill Dragster – Cedar Point, Ohio

•128 m tall, 193 km/h on the up and down