Worked examples on kinetic energy
Two worked in full, then two for you — and yours run the formula backwards, which is the harder direction.
Example 1
The problem
An object has a mass of 6 kg. Calculate its kinetic energy at a moment when it is moving with a velocity of 4 m s-1.
kinetic energy = ½ m v2
= ½ × 6 × 42
= ½ × 6 × 16
= 48 J
Example 2
The problem
Calculate the kinetic energy of an object of mass 4 kg at a moment when it is moving with a velocity of 2 m s-1.
kinetic energy = ½ m v2
= ½ × 4 × 22
= ½ × 4 × 4
= 8 J
Exercise 18.2
The problem
- A dog has a mass of 10 kg. At what velocity is the dog running when its kinetic energy is 20 J?
- An object of mass 500 g is moving at a certain velocity, and at that moment its kinetic energy is 9 J. Calculate the velocity of the object at that moment.
Two things to watch in the exercise
Both questions give you the energy and ask for the velocity, so you must undo the square: find v2 first, then take its square root.
The second question gives the mass as 500 g. The formula needs kilograms, so use 0.5 kg.