Coaching Centre
Forces review
A summary of the whole Forces chapter, key words, and a mixed test covering forces and newtons, balanced and unbalanced forces, friction, car safety, gravity, mass and weight, magnets, electromagnets, static electricity and simple machines.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 8A What forces are | A force is a push or pull between an agent and a receiver. Forces can start, stop, speed up, slow down, turn or change the shape of objects. Measured in newtons (N) with a spring balance; 100 g weighs about 1 N. Contact and non-contact forces; fields. |
| 8B Balanced and unbalanced forces | Net force = all forces added, with direction. Balanced: no change in motion (still, or steady speed in a straight line). Unbalanced: speeds up, slows down or changes direction. Force diagrams: arrow direction and length. |
| 8C Friction and air resistance | Contact force against motion, caused by rough surfaces; makes heat. Depends on surfaces and how hard they press. Increase it (tread, chalk) or reduce it (lubricants, rollers, bearings). Air resistance and streamlining. |
| 8D Protection from forces | Stopping quickly needs a big force. Seat belts, airbags, crumple zones and helmets make the stop longer and gentler. |
| 8E Gravity, mass and weight | Gravity attracts masses and pulls towards the Earth's centre. Mass (kg) is the same everywhere; weight (N) = mass × gravity (9.8 N/kg on Earth). Terminal velocity, microgravity, buoyancy. |
| 8F Magnets and magnetic fields | Iron, nickel, cobalt and steel are magnetic. Like poles repel, unlike attract. Field lines from N to S, mapped with filings or compasses. The Earth's magnetic field, compasses, auroras. |
| 8G Electromagnets | Current in a coil makes a magnetic field; a soft iron core strengthens it. Stronger with more turns, more current. Can be switched on and off. Cranes, locks, speakers, motors, MRI. |
| 8H Static electricity | Rubbing moves electrons: gain = negative, lose = positive. Like charges repel, unlike attract, charged objects attract neutral ones. Insulators hold charge. Sparks and lightning. |
| 8I Simple machines | Machines change the size or direction of a force. Lever: effort × distance = load × distance. The woomera. Wheel and axle, pulleys, gears, ramps. |
Key words
agent · air resistance · attract · balanced forces · buoyancy · charge · compass · conductor · contact force · crumple zone · drag · effort · electromagnet · electron · field · force · force diagram · friction · fulcrum · gravity · insulator · lever · load · lubricant · magnetic field · mass · net force · neutral · newton · non-contact force · pole · pulley · receiver · repel · spring balance · static electricity · streamlined · terminal velocity · unbalanced forces · weight
Test yourself
Which is a non-contact force?
Charges attract and repel across a gap.
A hockey puck slides across ice at a steady speed. The forces on it are
Steady speed = zero net force.
A 2 kg bag of flour has a weight on Earth of about
2 × 9.8 ≈ 20 N.
Ball bearings are used in wheels to
Rolling has far less friction than sliding.
An airbag reduces injury because it
Longer stop, smaller force.
A north pole is brought near the north pole of another magnet. They
Like poles repel.
A balloon rubbed on wool becomes negatively charged. This means the balloon has
Only electrons move.
To lift a load with the least effort using a lever, put the fulcrum
Effort × distance = load × distance.
Label the forces on the skateboarder.
Which force is at work?
Forces are measured in . When the forces on an object cancel, they are . Friction always produces . Your mass stays the same on the Moon, but your is smaller. An electromagnet uses a coil of wire around a core of soft .
A cyclist rides off from a set of traffic lights, travels along a flat road at a steady speed, then brakes and stops at the next lights. Describe and explain the forces acting on the cyclist and bike during each stage, using the ideas of balanced and unbalanced forces, friction and air resistance.
Throughout the ride, the downward pull of gravity (weight) is balanced by the upward support force of the road, so the bike doesn't move up or down. When the cyclist rides off from the lights, the driving force from pedalling is bigger than the air resistance and friction acting backwards. The forces are unbalanced, with a net force forwards, so the bike speeds up. As the bike goes faster, air resistance increases. Once the cyclist is travelling at a steady speed on the flat road, the driving force exactly equals the air resistance and friction, so the forces are balanced and the net force is zero, which is why the speed stays the same. When the cyclist brakes, the brake pads rub on the wheel rims or discs, making a large friction force backwards. Now the forces are unbalanced backwards, so the bike slows down until it stops. The friction in the brakes turns the bike's movement energy into heat, so the brakes get warm.