Year 7 Science · Unit 3 · Lesson 4
Challenge Worksheet
Learning Goals
1. State the rule that decides which forces belong on a free-body diagram. Give one example of a force that must be left off a free-body diagram of a book resting on a table.
Find the mistakes
A student wrote this about a book resting on a table
“I put the weight arrow pointing upward, because weight is what holds the book up on the table. I drew the normal force pointing downward, because the table pushes down on the book to keep it in place. Both forces point in the same direction, so the net force is zero and the book is balanced.”
2. The student’s answer contains three errors. Quote each one and give the correct science.
3. Redraw the diagram as the student should have drawn it, then give the reason the book remains at rest.
Scenario: riding a wave
A surfer is riding a wave toward the shore. Vertically, her weight of 500 N down is matched by the water pushing up on her board with 500 N. Horizontally, the wave pushes her forward with 120 N, and water drag acts backward on her with 80 N.
4. (a) Find the size and direction of the net force on the surfer. (b) Is she in equilibrium? (c) What is happening to her speed?
(a) Surfer: N
5. Suppose the drag grows until it is 120 N, while the wave’s push stays at 120 N. Predict her motion from then on, and explain.
Read the data
Scenario: a sensor on a lift cable
A sensor measures the upward pull of the cable (the tension) on a building’s lift during one trip up from the ground floor. The lift and its passengers weigh 8000 N the whole time.
| Moment | What the lift is doing | Cable tension, up (N) | Weight, down (N) |
|---|---|---|---|
| 1 | Waiting at the ground floor | 8000 | 8000 |
| 2 | Starting to move upward | 9100 | 8000 |
| 3 | Moving up at a steady speed | 8000 | 8000 |
| 4 | Slowing down near the top floor | 7200 | 8000 |
| 5 | Stopped at the top floor | 8000 | 8000 |
Source: made-up values for this worksheet.
6. Find the net force on the lift, with its direction, at moment 2 and at moment 4.
7. Moments 1, 3 and 5 have identical forces. Sam concludes: “The lift must be stopped at all three moments.” Use Table 1 to evaluate Sam’s conclusion.
8. At moment 4 the lift is still going up, but the net force points down. Explain why this is not a contradiction.
9. Ella says: “If the net force on something is zero, then no forces are acting on it.” Evaluate Ella’s claim. Use two examples, one of an object that is still and one of an object that is moving.
Question 1
Only forces on the chosen object (1); e.g. leave off the book pushing on the table (1). [2 marks]
Find the mistakes: Question 2
1 each: weight “upward” (it acts down); normal force “downward” (the table pushes up); “same direction ... zero” (such forces add; only opposite ones cancel). [3 marks]
Question 3
Weight down, normal force up (1), equal lengths (1). Net force zero, no change in motion (1). [3 marks]
Question 4
(a) 120 − 80 = 40 N toward shore (1). (b) No: net force is not zero in every direction (1). (c) Speeding up (1). [3 marks]
Question 5
Net force zero (1): steady speed, not stopping (1). [2 marks]
Read the data: Question 6
2: 1100 N up (1). 4: 800 N down (1). [2 marks]
Question 7
Wrong: at 3 it moves up (1). Balanced means no change in motion (1), so forces alone cannot tell still from steady (1). [3 marks]
Question 8
Net force changes motion, not the travel direction (1); it slows the rising lift (1). [2 marks]
Question 9
Marking guide: 1 judgement (wrong); 1 still example, forces named; 1 they cancel; 1 moving example, forces named; 1 they cancel though it moves; 1 zero net force = no change in motion. [6 marks]
Model answer: Ella is wrong: the forces cancel. A still book: weight down, equal normal force up. A skydiver at terminal velocity: weight down, equal air resistance up, yet she falls fast. Zero net force only means no change in motion.
Common error: reading a downward net force as “moving down”.