Year 7 Science · Unit 3 · Lesson 2
Challenge Worksheet
Learning Goals
1. Name the instrument that measures mass and the instrument that measures weight. Give the unit each one reads.
Find the error
Priya’s working
Task: find the weight of a 0.60 kg rock sample on Earth (g = 9.8 N/kg) and on Venus (g = 8.9 N/kg).
Line 1: Weight on Earth = 0.60 × 9.8 = 5.9 kg.
Line 2: On Venus the rock’s mass falls to 0.54 kg, because Venus pulls less strongly than Earth.
Line 3: Weight on Venus = 0.60 ÷ 8.9 = 0.067 N.
Line 4: Check: the Venus weight is smaller than the Earth weight, as it should be.
2. Three of Priya’s four lines contain one error each. Name each wrong line and rewrite it correctly, with the right numbers and units.
3. A small crane is built for a base on Mars. Its cable is safe for a load weighing up to 1480 N. The engineers want to use it to lift a 350 kg water tank. (a) Is the lift safe on Mars (g = 3.7 N/kg)? (b) Before launch, they test the crane with the same tank on Earth (g = 9.8 N/kg). Is that lift safe? Show your working for both.
Mars: safe or not safe?
Earth: safe or not safe?
Read the data: how far does gravity reach?
Scenario
Table 1 shows how strong Earth’s gravity is at different heights above the ground, and what an 80 kg astronaut would weigh at each height. The International Space Station (ISS) orbits at about 400 km.
| Height above Earth’s surface (km) | g (N/kg) | Weight of the 80 kg astronaut (N) |
|---|---|---|
| 0 (the ground) | 9.8 | 784 |
| 400 (the ISS) | 8.7 | |
| 1,000 | 7.3 | 584 |
| 10,000 | 1.5 | 120 |
| 36,000 | 0.22 |
Source: g values calculated for this worksheet and rounded.
4. Calculate the two missing weights and write them in Table 1.
5. Describe how g changes as the height increases. Predict whether g would ever become exactly zero if the astronaut kept travelling away, and explain your prediction.
6. A website says: “Astronauts on the ISS float because there is no gravity in space.” Evaluate this claim. Use the data in Table 1 and explain why the astronauts really float.
Question 1
Mass: a balance scale, reading kilograms (kg) (1). Weight: a spring scale (newton meter), reading newtons (N) (1). [2 marks]
Find the error
2. Line 1: 5.9 N, not kg; weight is a force (1). Line 2: the mass stays 0.60 kg on Venus; only g changes (1). Line 3: multiply, not divide: 0.60 × 8.9 = 5.3 N (1). Line 4 is correct. [3 marks]
Question 3
(a) W = 350 × 3.7 = 1295 N (1), which is below 1480 N, so the lift is safe on Mars (1). (b) W = 350 × 9.8 = 3430 N, far above 1480 N, so the Earth test is not safe (1). [3 marks]
Read the data: how far does gravity reach?
4. 400 km: 80 × 8.7 = 696 N (1). 36,000 km: 80 × 0.22 = 17.6 N (1). [2 marks]
5. g gets smaller as height increases, quickly at first and then more slowly (1). It would not become exactly zero: gravity keeps getting weaker with distance but never switches off (1). [2 marks]
Question 6
Marking guide: 1 the claim is wrong; 1 at the ISS height g is 8.7 N/kg, about 90% of the 9.8 N/kg at the ground; 1 so the 80 kg astronaut still weighs about 696 N; 1 gravity weakens with distance but does not switch off (still 0.22 N/kg at 36,000 km); 1 the ISS and the astronauts are falling toward Earth together at the same rate (free fall), so the floor does not push up on them and they float; 1 they also move sideways so fast that they keep missing Earth, which is an orbit. [6 marks]
Model answer: The claim is wrong. Table 1 shows g at the ISS is 8.7 N/kg, about 90% of its value on the ground, so an 80 kg astronaut there still weighs about 696 N. Gravity gets weaker with distance but never switches off; even at 36,000 km it is 0.22 N/kg. The astronauts float because they and the ISS are all falling toward Earth at the same rate, so nothing pushes up on them. They also move sideways so fast that they keep missing Earth: that endless falling-and-missing is an orbit.
Common error: saying there is no gravity in orbit, or that astronauts float because they have no mass.