Year 7 Science · Unit 3 · Lesson 1
Challenge Worksheet
Learning Goals
1. The lesson gives two everyday force sizes in newtons: one for holding up a small apple and one for carrying a school bag full of books. State both, and explain why they are worth remembering.
Find the error
Sam’s revision notes
Line 1: Water drag on a swimmer is a non-contact force, because water is not a solid.
Line 2: A tow rope pulling a car is a contact force called tension.
Line 3: A magnet can only pull on a paper clip when the two are touching.
Line 4: A book resting on a table has no forces acting on it, because it is not moving.
2. Three of Sam’s four lines contain an error. Name each wrong line and rewrite it correctly.
3. Two force diagrams of the same crate are drawn to the scale 1 cm = 5 N. In diagram P, one 4 cm arrow points left and another 4 cm arrow points right. In diagram Q, a 6 cm arrow points right and a 2 cm arrow points left. (a) Give the size of every force in newtons. (b) In which diagram can you see at a glance that one force is bigger than the other? Which way does the bigger force act?
Read the data: the balloon and the wall
Investigation
Jess rubbed a balloon on her jumper and held it against a wall. For each test she slid a number of paper sheets between the balloon and the wall, let go, and recorded whether the balloon stayed up for 10 seconds. She rubbed the balloon again before every trial.
Tom claims: “The balloon only sticks because it touches the wall, so this must be a contact force.”
| Sheets of paper between balloon and wall | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| 0 | Yes | Yes | Yes |
| 2 | Yes | Yes | Yes |
| 4 | Yes | No | Yes |
| 6 | No | Yes | No |
| 8 | No | No | No |
Source: sample class results.
4. Use Table 1 to evaluate Tom’s claim. Name the force that attracts the balloon to the wall across the gap.
5. Describe the pattern in Table 1 as the number of sheets increases. Suggest what it tells you about the size of the force as the gap gets bigger.
6. Ella says: “When a trampolinist is up in the air, every force on her is non-contact. When she lands on the mat, every force on her is a contact force.” Evaluate Ella’s statement. Name the forces on the trampolinist at each moment and classify each one.
Question 1
A small apple: about 1 N; a full school bag: about 50 to 80 N (1). They give a quick check on whether a force or a calculated answer is sensible, e.g. 2 N to move a loaded wheelie bin is far too small (1). [2 marks]
Find the error
2. Line 1: water drag is a contact force; water is matter and touches the swimmer (1). Line 3: a magnet can pull a paper clip from a distance; magnetism is non-contact (1). Line 4: gravity pulls the book down and the table pushes it up (normal force); the two forces are equal and opposite, so they cancel (1). Line 2 is correct. [3 marks]
Question 3
(a) P: 20 N left and 20 N right (1). Q: 30 N right and 10 N left (1). (b) Q; the bigger force (30 N) acts to the right (1). [3 marks]
Read the data: the balloon and the wall
4. Tom is wrong (1). With 2 sheets of paper in the gap, the balloon was not touching the wall but stayed up in all three trials (with 4 sheets, in two of three) (1). So the force acts across a gap: it is the electrostatic force, a non-contact force (1). [3 marks]
5. As the sheets increase, the balloon stays up in fewer trials: 3 of 3 up to 2 sheets, 2 of 3 at 4, 1 of 3 at 6, none at 8 (1). The force seems to get weaker as the gap gets bigger, until it cannot hold the balloon (1). [2 marks]
Question 6
Marking guide: 1 both halves of Ella’s statement are wrong; 1 in the air, gravity pulls her down and is non-contact; 1 in the air, air resistance also acts on her and is a contact force, because air is matter touching her; 1 on the mat, the mat pushes her up (normal force), a contact force; 1 on the mat, gravity still pulls her down and is still non-contact, so not every force is contact; 1 the rule: classify one force at a time by asking “is anything touching at all?”, never the whole scene. [6 marks]
Model answer: Both halves are wrong. In the air, gravity pulls her down (non-contact), but air resistance also acts on her, and it is a contact force because air particles hit her. On the mat, the mat pushes her up (normal force, contact), yet gravity still pulls her down without touching her. So each moment has both kinds. Ella should classify each force separately by asking “is anything touching at all?”.
Common error: labelling a whole situation contact or non-contact instead of one force at a time.