Year 7 Science · Unit 3 · Lesson 2
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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 |
|---|---|---|
| Mass | A. The unit of force, symbol N | |
| Weight | B. The amount of matter in an object, measured in kilograms | |
| Gravity | C. How strongly gravity pulls on each kilogram, measured in N/kg | |
| Gravitational field strength | D. The force of gravity pulling on an object, measured in newtons | |
| Newton | E. The non-contact force that attracts all objects with mass toward each other |
1. Write the equation for weight in words or symbols. Give the unit of each of its three quantities.
Order the steps: a netball on Earth and on Mars
The lesson’s method for finding a 0.42 kg netball’s weight on Earth and on Mars. Number the steps from 1 (first) to 5 (last).
| Step | What you do |
|---|---|
| Work out the Earth weight: W = 0.42 × 9.8 = 4.1 N. | |
| Check that the Mars weight is smaller than the Earth weight, in the same ratio as the g values. | |
| List the values with units: m = 0.42 kg; g = 9.8 N/kg (Earth), 3.7 N/kg (Mars). | |
| Keep m = 0.42 kg and change only g: W = 0.42 × 3.7 = 1.6 N on Mars. | |
| Write the equation linking the listed values, W = m × g; check kg × N/kg gives N. |
2. Ravi works out that the same netball weighs 4.1 N on Earth and 11 N on Mars. Without redoing his working, explain how you can tell that his Mars answer is wrong.
Read the data: a model rover on four worlds
Scenario
Engineers work out the weight of a 25 kg model rover in four places, to help design its wheels.
| Place | Moon | Mars | Venus | Earth |
|---|---|---|---|---|
| g (N/kg) | 1.6 | 3.7 | 8.9 | 9.8 |
| Weight (N) | 40 | 92.5 | 245 |
Source: values calculated for this worksheet with W = m × g.
3. Calculate the rover’s weight on Venus and write it in Table 1.
Weight on Venus N
4. Plot weight (y-axis) against g (x-axis) for all four places. Start both axes at zero, label them with units, and draw a line of best fit.
5. Describe the pattern in your graph. Use it to predict the rover’s weight on a world where g = 6.0 N/kg.
6. You wake up in a windowless spacecraft. It has landed on the Moon, on Mars or back on Earth, but you do not know which. You have a balance scale with a set of known masses, a spring scale, and a 1.0 kg block. Explain which instrument can tell you where you are, how you would use it, and what reading you would expect in each place. Explain why the other instrument cannot help.
Match each term to its meaning
Mass → B; Weight → D; Gravity → E; Gravitational field strength → C; Newton → A.
Question 1
W = m × g, weight = mass × gravitational field strength (1). W in newtons (N), m in kilograms (kg), g in newtons per kilogram (N/kg) (1). [2 marks]
Order the steps: a netball on Earth and on Mars
Reading down the table as printed, the correct step numbers are 3, 5, 1, 4, 2. In order: list the values with units; write W = m × g and check the units; work out the Earth weight (4.1 N); keep the mass and change only g (1.6 N on Mars); check the answers make sense.
Question 2
The mass is the same and Mars’s g (3.7 N/kg) is smaller than Earth’s (9.8 N/kg), so the Mars weight must be smaller than 4.1 N (1). 11 N is bigger than the Earth weight, so it must be wrong (the correct value is 1.6 N) (1). [2 marks]
Read the data: a model rover on four worlds
3. W = 25 × 8.9 (1) = 222.5 N (accept 223 N) (1). [2 marks]
4. Axes from zero, labelled g (N/kg) and weight (N), with even scales (1); four points plotted correctly (1); straight line of best fit through the points and the origin, and a title (1). [3 marks]
5. A straight line through the origin: weight increases in step with g, and doubling g doubles the weight (1). At g = 6.0 N/kg: 150 N (accept 145–155 N read from the line) (1). [2 marks]
Question 6
Marking guide: 1 the spring scale; 1 hang the 1.0 kg block on it and read its weight in newtons; 2 readings 1.6 N (Moon), 3.7 N (Mars), 9.8 N (Earth): two correct = 1, all three = 2; 1 the balance compares the block with known masses and gravity pulls both sides equally, so it reads 1.0 kg everywhere; 1 mass does not change with place but weight = m × g does. [6 marks]
Model answer: Hang the 1.0 kg block on the spring scale. Since W = m × g and m = 1.0 kg, the reading equals g: about 1.6 N on the Moon, 3.7 N on Mars or 9.8 N on Earth. The balance cannot help: gravity pulls both pans equally, so it reads 1.0 kg everywhere. Mass never changes with location; weight does.
Common error: changing the mass as well as g when an object moves to another planet.