--- title: "Does a moving thing need a push?" subtitle: "Year 10 Physics · Newton's laws · Lesson 2" --- # Does a moving thing need a push? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Kick a ball on grass and it rolls to a stop. Slide a puck on ice and it goes much further. In space, a probe launched decades ago is still coasting out of the solar system with its engines off. :::teacher-note **In one sentence:** Students state Newton's first law and explain why a moving object with no net force continues at constant velocity, confronting the Aristotelian idea that motion needs a continuous force (AC9S10U05, node 2, a hinge node). **Students should be able to say:** I can state Newton's first law. I can explain why a moving object with no net force keeps a constant velocity. I can identify friction as the hidden force that slows everyday objects. **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 **A hockey stick hits a puck, then stops touching it. The puck slides across perfectly smooth ice (no friction). What do you predict happens?** ☐ It stops straight away ☐ It speeds up ☐ It slows down as the force of the hit runs out ☐ It keeps sliding at the same speed My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It keeps sliding at the same speed. With no net force, nothing changes its motion. The 'force of the hit' isn't stored inside it: the force only acted while the stick touched it. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the first law :::keep-together ### Coasting **With no push and no friction, what did the puck do?** ☐ Slowed down gradually ☐ Stopped ☐ Kept moving at a constant speed ::: :::teacher-note **Answer:** Kept moving at a constant speed No net force, no change in motion: Newton's first law. ::: :::keep-together ### The law **Which statement is Newton's first law?** ☐ Heavier objects are harder to keep moving ☐ Every object eventually comes to rest ☐ A moving object needs a force to keep it moving ☐ An object stays at rest or keeps moving at a constant velocity unless a net force acts on it ::: :::teacher-note **Answer:** An object stays at rest or keeps moving at a constant velocity unless a net force acts on it ::: :::keep-together ### Why things slow down **So why does a ball rolling on grass slow down and stop?** ☐ Balls naturally stop ☐ Friction and air resistance act against it: a net force backwards ☐ Gravity pulls it backwards ☐ Its force runs out ::: :::teacher-note **Answer:** Friction and air resistance act against it: a net force backwards The 'hidden' forces, friction and air resistance, are the net force that slows it. ::: :::keep-together ### Inertia **What is inertia?** ☐ The friction on a moving object ☐ An object's tendency to resist changes in its motion (more mass, more inertia) ☐ A force that keeps objects moving ::: :::teacher-note **Answer:** An object's tendency to resist changes in its motion (more mass, more inertia) ::: :::typical ## Elaborate: friction is the hidden force :::keep-together ### What slows it **Friction is 2 N and the puck's mass is 1 kg. While it slides, what is happening?** ☐ A net force of 2 N backwards makes it slow down steadily ☐ Its push slowly runs out ☐ Friction is balanced by its motion ::: :::teacher-note **Answer:** A net force of 2 N backwards makes it slow down steadily The only horizontal force is friction, so it is the net force. The graph slopes down in a straight line: steady slowing. ::: ::: :::wider ## Elaborate: seatbelts :::keep-together **A car hits a barrier and stops suddenly. A passenger isn't wearing a seatbelt. Using the first law, what happens to them?** ☐ They stop with the car ☐ They keep moving forward at the car's old speed until something (the dashboard, windscreen) exerts a force on them ☐ They are thrown forward by a force from the crash ::: :::teacher-note **Answer:** They keep moving forward at the car's old speed until something (the dashboard, windscreen) exerts a force on them A seatbelt applies the force that stops the passenger along with the car, spread over a little more time and a wide area. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Newton's first law:** an object stays at rest, or keeps moving at a constant velocity, unless a **net force** acts on it. - A moving object does **not** need a force to keep moving. A force is needed to **change** motion. - Everyday objects slow down because of **friction and air resistance**: hidden forces that are the net force. - **Inertia** is an object's resistance to changes in its motion; more mass, more inertia. ## Exit ticket :::keep-together **A spacecraft coasts through empty space with its engines off. Describe its motion and explain.** {{lines-3}} ::: :::teacher-note **Answer:** It keeps moving at a constant velocity: there is no net force (no friction or air resistance), so by the first law its motion doesn't change. :::