Forces and energy
Clear notes with worked examples, then 4 practice questions with answers. Written by Beyond Tutors tutors for the KS3 National Curriculum.
Forces
A force is a push or a pull, measured in newtons (N).
Contact forces: friction, air resistance, tension, normal (support) force
Non-contact forces: gravity (weight), magnetism, static electricity
Resultant force
- Same direction: add. Opposite directions: subtract.
10 N right and 4 N left → resultant 6 N to the right.
- Balanced forces (resultant 0): the object stays still or keeps moving at the same speed.
Mass and weight
Mass is the amount of matter (kg) and does not change.
Weight is the force of gravity (N): weight = mass × gravitational field strength
On Earth N/kg, so a 50 kg pupil weighs 500 N. On the Moon they weigh about one sixth as much.
Energy stores
kinetic (moving), gravitational (high up), elastic (stretched), chemical (food, fuel, batteries), thermal (hot), nuclear
Energy is never used up; it is transferred between stores.
RENEWABLE vs NON-RENEWABLE
Renewable: wind, solar, tidal, hydroelectric, geothermal
Non-renewable: coal, oil, natural gas, nuclear
Practice questions
Try each one, then tap to see the answer and why.
- Question 1 · Resultant force
Two people push a box the same way with 30 N and 45 N. What is the resultant force?
- A75 N
- B45 N
- C15 N
- D1350 N
Show the answer
A: 75 N
Forces in the same direction add: N.
- Question 2 · Resultant force
A car's engine pushes it forward with 900 N. Friction and air resistance total 900 N. What happens?
- AIt speeds up
- BIt keeps moving at a steady speed
- CIt slows down
- DIt goes backwards
Show the answer
B: It keeps moving at a steady speed
The forces are balanced (resultant 0 N), so the speed does not change.
- Question 3 · Forces
Which of these is a non-contact force?
- AFriction
- BGravity
- CAir resistance
- DTension
Show the answer
B: Gravity
Gravity acts without touching. Friction, air resistance and tension are contact forces.
- Question 4 · Weight
A bag has a mass of 4 kg. Taking N/kg, what is its weight?
- A0.4 N
- B40 N
- C14 N
- D4 N
Show the answer
B: 40 N
Weight = mass × = N.