Motion, forces and energy - energy

what is energy

• it is the ability to do work

what is the SI Unit of energy

joules (J)

What are some of the types of energy

mechanical energy
thermal/heat energy
sound energy
light energy
chemical energy
electrical energy etc.

mechanical energy can be split into

kinetic energy
potential energy

potential energy can be split into

gravitational potential energy
elastic potential energy

what is gravitational potential energy (g.p.e)

• is the energy possessed by a body due to its position above the ground

what are the factors of g.p.e

⬆weight ⬆g.p.e
⬆height ⬆g.p.e

formulaes for g.p.e

g.p.e = weight x height (w x h)
g.p.e = mass x gravitational pull x height (m x g x h

QUESTIONS on g.p.e

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1. An object with a mass of 15kg is at a point above the ground of 10m. Calculate the g.p.e

g.p.e = m x g x h
= 15 x 10 x 10
= 1500J

2. An object held at a point above the ground has 2352 J of g.p.e. The objects mass is 20kg. How far above is the object from the ground

g.p.e = m x g x h
h = g.p.e ÷ (m x g)
= 2352 ÷ (20 x 10)
= 2352 ÷ 200
= 11.76m

3. An object held at a point 1.5m has 1200J of g.p.e. Calculate the mass of the object.

g.p.e = m x g x h
m = g.p.e ÷ (h x g)
= 1200 ÷ (1.5 x 10)
= 1200 ÷ 15
= 80kg

4. An athlete of mass 50kg runs up a hill of height 1200m above sea level. The foot of thehill is 400m above sea level. How much did g.p.e increase

g.p.e = m x g x h
h = 1200 - 400
= 800

g.p.e = m x g x h
= 50 x 10 x 800
= 400,000J

what is kinetic energy

• is the energy possessed by moving objects

what are the factors of k.e

⬆mass ⬆k.e
⬆speed ⬆k.e

formulae for k.e

k.e = 1/2 of mass x speed²
= 1/2mv²

NOTE

k.e is a scalar quantity (only affected by size and magnitude)