Physics · Exam skills
Know the equation but still get the wrong answer?
Check units before substitution. A conversion can change an answer by a factor of a thousand.
The short answer
An equation only gives a meaningful numerical answer when its units are compatible. For an answer in joules using Ek = ½mv², use mass in kilograms and speed in metres per second.
Conceptual illustration. Colours distinguish parts of the model.
TRY A SMALL CHANGE
Predict first. Then move the control.
Where the confusion starts
The units beside the given numbers are part of the question. Substituting a mass in grams into a calculation expecting kilograms makes the result one thousand times too large. Area and volume conversions also require squaring or cubing the scale factor.
WORK IT THROUGH
Find the kinetic energy of a 500 g object moving at 4 m/s.
- Convert first: 500 g = 0.500 kg.
- Write the equation: Ek = ½mv².
- Substitute: Ek = 0.5 × 0.500 × 4² = 4 J. Keep the squared speed in brackets if using a calculator.
CHECK YOUR UNDERSTANDING
A 200 g object moves at 3 m/s. What is its kinetic energy?
Make a prediction or write an answer before opening the explanation.
Reveal the explanation
0.9 J. Convert 200 g to 0.200 kg, then calculate 0.5 × 0.200 × 3².
SEE IT. CHANGE IT. EXPLAIN IT.
Take the next step in the lesson.
Use the visual explanation in the lesson, then choose Practice to test the idea with another question.
Spotted something unclear? Send feedback. These are original teaching explanations, not predictions of future exam questions.