--- title: "What does an unbalanced force do?" subtitle: "Year 7 Physics · Forces and motion · Lesson 3" --- # What does an unbalanced force do? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A curling stone glides 40 metres across the ice. Sweep in front of it and it slides further; let it hit another stone and it stops dead. Every time its motion changes, something has pushed or pulled it. :::teacher-note **In one sentence:** Students explain that unbalanced forces change motion (starting, speeding up, slowing down, stopping, reversing and changing direction) and represent each change with a force diagram (AC9S7U04, node 3). **Students should be able to say:** I can explain that an unbalanced force changes an object's motion, in the direction of the net force. I can name speed and direction as the two things that can change. I can draw force diagrams for two different changes in motion. **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 puck slides to the right across smooth ice. A steady push to the left starts acting on it and keeps acting. What do you predict happens?** ☐ It slows down and stops, then stays still ☐ It keeps sliding right at the same speed: it is already moving ☐ It slows down, stops, then moves off to the left, faster and faster ☐ It stops straight away My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It slows down, stops, then moves off to the left, faster and faster. The net force points left the whole time, so the motion keeps changing towards the left: first slower, then reversed. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: any change in motion :::keep-together ### Why it turned round **On the board, why did the puck turn round?** ☐ The puck ran out of the force that was carrying it right ☐ The push only started working once the puck had stopped ☐ The push kept acting: the net force pointed left, so the puck slowed, stopped and then sped up to the left ::: :::teacher-note **Answer:** The push kept acting: the net force pointed left, so the puck slowed, stopped and then sped up to the left An unbalanced force changes the motion in the direction of the net force for as long as it acts. Slowing down, stopping and reversing were all the same change: towards the left. ::: :::keep-together ### Changing or not **Is the motion changing? (If it is, there must be an unbalanced force.)** Groups: Changing: unbalanced force, Not changing: balanced forces - A cyclist braking at the lights: {{field-15}} - A car going round a roundabout at a steady 30 km/h: {{field-15}} - A lift rising at a steady speed: {{field-15}} - A ball thrown upwards, slowing as it rises: {{field-15}} - A book resting on a desk: {{field-15}} - A netball changing direction when it bounces off the backboard: {{field-15}} - A train sitting at a platform: {{field-15}} ::: :::teacher-note **Answer:** A cyclist braking at the lights → Changing: unbalanced force; A car going round a roundabout at a steady 30 km/h → Changing: unbalanced force; A lift rising at a steady speed → Not changing: balanced forces; A ball thrown upwards, slowing as it rises → Changing: unbalanced force; A book resting on a desk → Not changing: balanced forces; A netball changing direction when it bounces off the backboard → Changing: unbalanced force; A train sitting at a platform → Not changing: balanced forces ::: :::keep-together ### Round the bend **A car drives round a bend at a steady 40 km/h. Are the forces on it balanced?** ☐ No: its direction is changing, so there is a net force towards the inside of the bend ☐ Yes: its speed isn't changing ☐ There are no forces on it ::: :::teacher-note **Answer:** No: its direction is changing, so there is a net force towards the inside of the bend A change of direction is a change in motion, so it needs an unbalanced force. Here the tyres' grip on the road pulls the car towards the inside of the bend. ::: :::keep-together ### Which diagram? **A cyclist rides to the right and is slowing down. Which force diagram fits?** ☐ Forward push 70 N to the right, air resistance and friction 40 N to the left ☐ Forward push 40 N to the right, air resistance and friction 70 N to the left ☐ No forward push, and no other forces ☐ Forward push 50 N to the right, air resistance and friction 50 N to the left ::: :::teacher-note **Answer:** Forward push 40 N to the right, air resistance and friction 70 N to the left Slowing down to the right means a net force to the left: the backward arrow is longer (70 N − 40 N = 30 N to the left). ::: ## Mix it up :::keep-together **Looking back at force arrows: a box rests on a table. Which arrows go in its force diagram?** ☐ Weight down and support up, the same length ☐ Weight and support, both pointing down ☐ Only weight, pointing down ☐ Weight down, longer than the support up ::: :::teacher-note **Answer:** Weight down and support up, the same length Equal forces get equal arrows. Weight points down and support points up, so the arrows balance. ::: :::typical ## Elaborate: from standing still :::keep-together ### The net force **The push is 7 N to the right and friction is 4 N to the left. What is the net force, in newtons?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 3 7 N − 4 N = 3 N to the right: unbalanced, so the box speeds up to the right. With a push of 4 N or less, friction matches it and the box stays still. ::: ::: :::wider ## Elaborate: longer pushes :::keep-together ### A fair test **You want to test whether a bigger push changes a trolley's speed more quickly. Which plan is a fair test?** ☐ Use a heavier trolley for the bigger push ☐ Use the same trolley on the same surface, and change only the size of the push ☐ Push one trolley on carpet and the other on tiles ::: :::teacher-note **Answer:** Use the same trolley on the same surface, and change only the size of the push Change one variable (the push), measure one (how quickly the speed changes) and keep the rest the same (trolley, surface). ::: :::keep-together ### The spearthrower **A spearthrower lets the thrower keep pushing on the spear over a longer arc. Why does the spear leave faster?** ☐ The spearthrower makes the spear lighter ☐ The unbalanced force acts on the spear for longer, so its speed keeps increasing for longer ☐ The spearthrower stores the spear's speed and releases it ::: :::teacher-note **Answer:** The unbalanced force acts on the spear for longer, so its speed keeps increasing for longer The longer an unbalanced force acts, the more the motion changes. Spearthrowers, used by First Peoples across much of mainland Australia, extend the throwing arm so the push lasts longer. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - An **unbalanced** force (a net force that isn't zero) **changes the motion**, in the direction of the net force. - A change in motion can be **starting, speeding up, slowing down, stopping, reversing or changing direction**. Every one needs an unbalanced force. - A force against the motion doesn't just stop an object: if it keeps acting, the object turns round and speeds up the other way. - Steady speed round a bend is still a change in motion (direction), so the forces are unbalanced. ## Exit ticket :::keep-together **A netball flies towards the ring and bounces back off the backboard. Give the change in motion and the force that caused it.** {{lines-3}} ::: :::teacher-note **Answer:** It reversed direction (and slowed, then sped up the other way). The backboard pushed it back: an unbalanced force away from the board. :::