Year 7 Science · Unit 3 · Lesson 4
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Learning Goals
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 |
|---|---|---|
| Net force | A. The state of an object whose forces are balanced: still, or moving at a steady speed in a straight line | |
| Balanced forces | B. A drawing of one object with every force on it shown as a labelled arrow | |
| Unbalanced forces | C. Forces that cancel exactly, leaving a net force of zero | |
| Free-body diagram | D. Forces that leave a net force that is not zero, so the motion changes | |
| Equilibrium | E. The force left over once all the forces on an object are combined, taking direction into account |
1. When a force is drawn as an arrow, what does the arrow’s length tell you, and what does the way it points tell you?
Order the steps: reading a free-body diagram
A crane holds a steel beam still in the air. Number the steps for working out what its free-body diagram tells you, from 1 (first) to 5 (last).
| Order (1–5) | Step |
|---|---|
| Compare the sizes of the forces along each line, up against down, to find the net force. | |
| Check your reading by imagining one arrow getting longer, and predict the new motion. | |
| Name each force acting on the beam and the direction it points. | |
| Turn the net force into a statement about whether the beam’s motion changes. | |
| Decide which object the diagram is about. |
Cause and effect: braking
Fill the two empty boxes. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
2. Ava pushes a heavy box forward across the floor with 80 N. Ben pushes it backward with 50 N. While the box slides forward, friction from the floor acts backward on it with 20 N. (a) Find the size and direction of the net force on the box. (b) Predict what happens to the box’s motion.
(a) Box: N
Read the data
Scenario: a delivery drone
A drone carries a parcel from a shop to a house. Table 1 shows the total upward force and the total downward force on the drone, air resistance included, during four parts of the trip.
| Stage | What the drone is doing | Total up (N) | Total down (N) | Net force |
|---|---|---|---|---|
| 1 | Lifting off from the shop roof | 75 | 60 | |
| 2 | Rising at a steady speed | 60 | 60 | |
| 3 | Hovering still above the house | 60 | 60 | |
| 4 | Moving down, and slowing down to land gently | 68 | 60 |
Source: made-up values for this worksheet.
3. For each stage, take the downward total from the upward total and write the result, with up or down, in the empty cells.
4. Stages 2 and 3 have exactly the same forces, but the drone is doing different things. Explain how this is possible.
5. In stage 4 the drone is descending while the leftover force points upward. What effect does this have on how it moves?
6. Kiara rides her bike to school. She pedals hard to get going, then rides along a flat road at a steady speed, then brakes at the school gate. For each part of the trip, compare the forward and backward forces on Kiara and her bike, state the net force, and explain how her motion changes.
Match each term to its meaning
Net force → E; Balanced forces → C; Unbalanced forces → D; Free-body diagram → B; Equilibrium → A.
Question 1
Length: size of the force (1). Way it points: its direction (1). [2 marks]
Order the steps: reading a free-body diagram
Reading down the table as printed, the correct step numbers are 3, 5, 2, 4, 1. In order: choose the object; name each force and its direction; compare sizes to find the net force; say whether the motion changes; check by lengthening one arrow.
Cause and effect: braking
Box 1: braking friction acts backward, bigger than any forward force (1). Box 2: unbalanced; net force backward, against the travel (1). [2 marks]
Question 2
80 − (50 + 20) (1) = 10 N forward (1). Unbalanced, so it speeds up (1). [3 marks]
Read the data: Question 3
1: 15 N up. 2: 0 N. 3: 0 N. 4: 8 N up. All four = 2; two or three = 1. [2 marks]
Question 4
Balanced (net force 0) means the motion does not change (1): stage 2 keeps rising steadily; stage 3 stays still (1). [2 marks]
Question 5
It points against the motion (1), so it slows the drone as it moves down (1). [2 marks]
Question 6
Marking guide, 2 per part (1 forces, 1 motion): starting: pedalling > air resistance and friction, net forward, speeds up; steady: forces equal, net zero, steady speed; braking: backward forces bigger, net backward, slows. [6 marks]
Model answer: At first pedalling beats air resistance and friction: net forward, she speeds up. On the flat the forces are equal: net zero, steady speed. Braking: brake friction and air resistance are now bigger than the pedalling force, so the net force is backward; she slows while still moving forward, because the net force changes her motion, not her direction of travel.
Common error: in Question 2, adding all three forces or leaving out friction.