Year 7 Science · Unit 3 · Lesson 5
Extension · Foundation Worksheet
Learning Goals
Extension · optional
Newton's First Law and inertia sit in Stage 5 (SC5-WAM-01), beyond the Stage 4 Forces dot points this unit is built on. Nothing on this sheet is assessed as part of Year 7 Forces, so it is enrichment rather than required practice. Do it if you are interested, and skip it without losing any marks if you are not.
Match each term to its meaning
Write the letter of the correct meaning next to each term. Each letter is used once.
| Term | Letter | Meaning |
|---|---|---|
| Newton's First Law | A. A non-zero net force, which changes an object’s motion | |
| Inertia | B. Moving at the same speed in the same direction | |
| Balanced forces | C. The tendency of an object to resist any change in its motion | |
| Unbalanced force | D. Forces that cancel out, so the net force is zero | |
| Net force | E. An object stays at rest or at constant velocity unless an unbalanced force acts on it | |
| Constant velocity | F. The total force once all the forces are added, taking direction into account |
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Inertia is a force, so it can be drawn as an arrow on a force diagram.
Correct it:
2. An object that is standing still has no inertia.
Correct it:
3. The more mass an object has, the more inertia it has.
Correct it:
4. A spacecraft far from any planet or star, with its engines off, keeps moving in a straight line at a steady speed.
Correct it:
5. Given the same small flick, a 5 kg iron pot changes its motion just as much as a five-cent coin does.
Correct it:
Odd one out
Each row has a rule. Circle the one item that does not fit the rule, then say why.
| Rule | Group | Odd one out, and why |
|---|---|---|
| Gives a person’s body the force needed to change its motion | seatbelt · headrest · speedometer · bus hand-rail | |
| Keeps moving with no force driving it forward | space probe coasting far from any star · cricket ball after it leaves the bowler’s hand · car rolling with its engine off · rocket at full thrust | |
| Has far more inertia than a family car | fully loaded B-train truck · loaded freight train · container ship · empty bicycle |
1. A ball rolls across a smooth floor and gradually slows down. Ruby says: “The ball slows down because it has run out of force.” Use Newton's First Law to explain what is actually happening.
Read the data
Scenario: the coin-and-card trick
A class puts a small coin on a card that rests on top of a cup. They remove the card in three different ways, ten times each. They try a smooth plastic card and a rough sandpaper card.
| How the card was removed | Smooth plastic card | Sandpaper card |
|---|---|---|
| A fast flick | 10 | 8 |
| A quick push | 7 | 3 |
| A slow pull | 0 | 0 |
Source: sample class results.
2. Which way of removing the card worked best? Use the word inertia to explain why it worked.
3. With the same way of removing it, the coin dropped in less often with the sandpaper card. Suggest why, using the idea of an unbalanced force.
4. A fully loaded B-train truck has about 45 times the mass of a family car. Use inertia to explain why the truck needs a much longer distance to stop.
5. Why do you need to push a shopping trolley to get it moving, and then hold it back to stop it? Explain using Newton's First Law and inertia. Then explain how this changes when the trolley is full of groceries.
Match each term to its meaning
Newton's First Law → E; Inertia → C; Balanced forces → D; Unbalanced force → A; Net force → F; Constant velocity → B.
True or false
1. False: inertia is a property, not a force; it is never an arrow. 2. False: every object with mass has inertia. 3. True. 4. True. 5. False: the pot has more mass, so more inertia.
Odd one out
1 each: speedometer (no force on the body); rocket at full thrust (its engine drives it); empty bicycle (small mass, less inertia). [3 marks]
Question 1
Moving things need no force to keep moving (1). Friction and air resistance are an unbalanced backward force that slows it; without them it would roll on (1). [2 marks]
Read the data: Question 2
A fast flick (10 of 10, smooth card) (1). Inertia keeps the coin still; the card pulls so briefly its motion hardly changes, so it drops in (1). [2 marks]
Question 3
Sandpaper grips more, so the friction force on the coin is bigger (1); this unbalanced force changes its motion and drags it off (1). [2 marks]
Question 4
Inertia grows with mass: about 45 times the car’s (1). Its motion is much harder to change, so braking takes far more distance (1). [2 marks]
Question 5
Marking guide: 1 states the law; 1 a still trolley stays still until a push (unbalanced force) acts; 1 a moving trolley rolls on until a backward force stops it; 1 inertia resists change in motion and grows with mass; 1 full: more inertia, harder to start and stop. [5 marks]
Model answer: An object stays at rest or at constant velocity unless an unbalanced force acts (First Law). A still trolley stays put until I push; once rolling it keeps going until I pull back. Inertia is the trolley’s resistance to any change in its motion, and it grows with mass, so a full trolley is harder to start and to stop.
Common error: calling inertia a force that “pushes” things forward.