--- title: "Would you weigh less on the Moon?" subtitle: "Year 7 Physics · Forces and motion · Lesson 4" --- # Would you weigh less on the Moon? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Astronauts on the Moon bounce around like they're on a trampoline. Did they lose something on the way there? :::teacher-note **In one sentence:** Students distinguish mass (kg) from weight (N), explain gravitational force as depending on mass, and calculate weight = mass × g in different places (AC9S7U04, node 4, a hinge node). **Students should be able to say:** I can explain the difference between mass and weight, with units. I can calculate weight from mass and g. I can explain why weight is different on the Moon and other planets. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **An astronaut has a mass of 60 kg on Earth. She flies to the Moon. What is her mass there?** ☐ More than 60 kg ☐ 0 kg: there's no gravity on the Moon ☐ 60 kg: mass doesn't change ☐ About 10 kg My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Still 60 kg: she is made of the same matter. Her weight (the pull of gravity on her) is what changes: about 96 N on the Moon instead of about 588 N on Earth. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: mass and weight :::keep-together ### Which is which **Mass or weight?** Groups: Mass, Weight - Measured in kilograms (kg): {{field-15}} - Measured in newtons (N): {{field-15}} - The same on Earth and on the Moon: {{field-15}} - A force, pulling towards the planet: {{field-15}} - How much matter an object has: {{field-15}} - Changes if g changes: {{field-15}} ::: :::teacher-note **Answer:** Measured in kilograms (kg) → Mass; Measured in newtons (N) → Weight; The same on Earth and on the Moon → Mass; A force, pulling towards the planet → Weight; How much matter an object has → Mass; Changes if g changes → Weight ::: :::keep-together ### Weight on Earth **A 5 kg bag on Earth (g = 9.8 N/kg). What is its weight, in N?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 49 Weight = mass × g = 5 × 9.8 = 49 N. ::: :::keep-together ### Weight on Mars **The same 5 kg bag on Mars (g = 3.7 N/kg). Its weight, in N?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 18.5 5 × 3.7 = 18.5 N. Same mass, smaller weight. ::: :::keep-together ### Why the Moon pulls less **Why is the astronaut's weight smaller on the Moon?** ☐ She has less mass there ☐ The Moon is further from the Sun ☐ The Moon has much less mass than Earth, so its gravity pulls less (smaller g) ☐ There is no air on the Moon ::: :::teacher-note **Answer:** The Moon has much less mass than Earth, so its gravity pulls less (smaller g) Gravity depends on mass. The Moon has about 1/80 of Earth's mass, so its g is only 1.6 N/kg. ::: ## Mix it up :::keep-together **Looking back at balanced forces: a 2 kg bag hangs still from a spring balance. Its weight is about 20 N. What force does the spring pull up with?** ☐ About 2 N ☐ 0 N, because the bag is not moving ☐ About 20 N: the forces are balanced ☐ About 40 N, so that it can lift the bag ::: :::teacher-note **Answer:** About 20 N: the forces are balanced The bag is still, so the forces are balanced: the spring pulls up with a force equal to the weight, about 20 N. ::: :::typical ## Elaborate: a rover on Jupiter :::keep-together ### Built for Mars **A rover's wheels are designed for its weight on Mars. It is sent to Jupiter instead (if it could land). What is the problem?** ☐ Its weight would be over six times bigger, so its wheels and legs might collapse ☐ Nothing: weight is the same everywhere ☐ Its mass would be bigger, so it would need more fuel ::: :::teacher-note **Answer:** Its weight would be over six times bigger, so its wheels and legs might collapse 24.8 ÷ 3.7 ≈ 6.7, so its weight would be nearly seven times bigger. ::: ::: :::wider ## Elaborate: do heavy things fall faster? :::keep-together **On the Moon (no air), an astronaut dropped a hammer and a feather together (Apollo 15, 1971). What happened?** ☐ They hit the ground together ☐ The feather floated away ☐ The hammer landed first ::: :::teacher-note **Answer:** They hit the ground together The heavier hammer is pulled harder, but it also has more mass to get moving, so both speed up at the same rate. Air resistance is what makes feathers fall slowly on Earth. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Mass** (kg): how much matter. The same everywhere. - **Weight** (N): the force of gravity on an object. **Weight = mass × g.** - **g** is the gravitational field strength: 9.8 N/kg on Earth (about 10), 1.6 on the Moon, 3.7 on Mars, 24.8 on Jupiter. - Gravity depends on mass: the Moon has far less mass than Earth, so it pulls less (smaller g, smaller weight). - Without air, all objects fall together, whatever their mass. ## Exit ticket :::keep-together **A 2 kg rock on Earth (g = 9.8 N/kg). What are its mass and its weight on the Moon (g = 1.6 N/kg)?** {{lines-3}} ::: :::teacher-note **Answer:** Mass 2 kg (unchanged); weight 2 × 1.6 = 3.2 N. :::