Calculating KE & Changes in GPE (WJEC GCSE Science (Double Award): Physics): Revision Note

Calculate the kinetic energy of a vehicle of mass 1200 kg moving at a speed of 27 m/s.

Answer:

Step 1: List the known quantities

• Mass of the vehicle, m = 1200 kg

• Speed of the vehicle, v = 27 m/s

Step 2: Write down the equation for kinetic energy

$KE\hspace{0.17em}=\frac{1}{2}m{v}^2$

Step 3: Calculate the kinetic energy

$KE\hspace{0.17em}=\frac{1}{2}\times 1200\times {27}^2$

KE = 437 400 J

Step 4: Round the final answer to 2 significant figures

KE = 440 000 J

When performing calculations using the kinetic energy equation, always double-check that you have squared the speed. Forgetting to do this is the most common mistake that students make.

A man of mass 70 kg climbs a flight of stairs that is 3 m higher than the floor.

Gravitational field strength, g = 10 N/kg.

Calculate the change in the man's gravitational potential energy.

Answer:

Step 1: List the known quantities

• Mass of the man, m = 70 kg

• Gravitational field strength, g = 10 N/kg

• Change in height, Δh = 3 m

Step 2: Write down the equation for gravitational potential energy

PE = mgh

Step 3: Calculate the gravitational potential energy

PE = 70 × 10 × 3

PE = 2100 J