Year 7 Science · Unit 3 · Lesson 11
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Learning Goals
Compare contact and non-contact forces
Complete every empty cell.
| Feature | Contact forces | Non-contact forces |
|---|---|---|
| Must the two objects meet? | ||
| Three examples | ||
| Can it act both ways (push and pull)? | ||
| One example from home or sport |
Cause and effect: the stuck trolley
A loaded trolley is parked on a flat floor. Fill each empty box. Each empty box is caused by the box just before it. Use the words friction, balanced, unbalanced and net force.
1. A crate slides across a warehouse floor. Its force diagram shows: weight 500 N down, floor pushing up 500 N, a push of 200 N to the right, and friction of 200 N to the left. Are the forces balanced? Describe the crate’s motion.
Read the data: the throwing stick
Scenario: a woomera-style launcher
Many Aboriginal Peoples use a woomera (spear-thrower), which fits on the end of a spear and acts as an extension of the thrower’s arm. A class tests the same idea with a plastic ball-launcher stick. Each student throws the same tennis ball once by hand and once with the launcher.
| Student | By hand | With launcher |
|---|---|---|
| Amira | 18 | 31 |
| Ben | 22 | 35 |
| Chloe | 17 | 33 |
Source: sample class results.
2. Calculate the average distance by hand and the average distance with the launcher.
By hand m Launcher m
3. Use the idea of a lever to explain why the launcher, like a woomera, sends the ball further.
4. Suggest one way to make this test more reliable or fairer, and explain why it helps.
5. A scrapyard crane uses an electromagnet to lift a 1000 kg car, hold it still in the air, and then drop it onto a pile. Explain the forces and the energy at each stage. Use g = 10 N/kg.
Compare contact and non-contact forces
Touch: contact yes; non-contact no (1). Examples: friction, tension, applied push; gravitational, electrostatic, magnetic (1). Push as well as pull: contact: applied forces can push or pull; tension only pulls; friction only opposes motion (accept ‘yes, e.g. applied push/pull’); non-contact depends: electrostatic and magnetic can push or pull, gravity only pulls (1). Everyday example each (1), e.g. boot on grass; fridge magnet. [4 marks]
Cause and effect: the stuck trolley
Box 1: friction grows to match your push, so the forces are balanced and the net force is zero (1). Box 2: the forces are now unbalanced, with a net force forward (1). Box 3: the trolley starts to move and speeds up (1). [3 marks]
Question 1
Balanced: up and down cancel, and 200 N right cancels 200 N left, so the net force is zero (1). The crate keeps sliding at a steady speed in a straight line (1). [2 marks]
Read the data: the throwing stick
2. Hand: (18 + 22 + 17) ÷ 3 = 19 m (1). Launcher: (31 + 35 + 33) ÷ 3 = 33 m (1). [2 marks]
3. It makes the arm effectively longer, like a lever (1). The far end of a longer lever moves faster for the same arm movement (1), so the ball is released faster and goes further (1). [3 marks]
4. One improvement (1) with its reason (1), e.g. several throws each, averaged, so one bad throw does not decide it; same angle and spot; a still day. [2 marks]
Question 5
Marking guide: 1 weight W = 1000 × 10 = 10 000 N; 1 forces named: magnetic force up, weight (gravity) down, both non-contact; 1 held still: the magnetic pull equals the weight, so the forces are balanced; 1 switching off the current turns the electromagnet off; 1 only the weight acts, so the forces are unbalanced and the car speeds up downward; 1 energy: lifting stored gravitational potential energy, which becomes movement energy as it falls, then heat and sound. [6 marks]
Model answer: The car’s weight is 1000 × 10 = 10 000 N. Held still, the upward magnetic force equals the weight, so the (non-contact) forces are balanced. Lifting it stored gravitational potential energy. Switching off the current removes the magnetism, so only the weight acts: the forces are unbalanced and the car speeds up downward. Its stored energy becomes movement energy, then heat and sound on landing.
Common error: “balanced means not moving”; the crate moves at a steady speed.