--- title: "Does balanced mean no forces?" subtitle: "Year 7 Physics · Forces and motion · Lesson 2" --- # Does balanced mean no forces? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A car cruises down the highway at a steady 100 km/h, engine running. Is its engine pushing harder than the air and the road are holding it back? :::teacher-note **In one sentence:** Students identify balanced and unbalanced forces and predict the motion, recognising that balanced forces mean no change in motion, not no forces (AC9S7U04, node 2, a hinge node). **Students should be able to say:** I can tell balanced from unbalanced forces in a diagram. I can explain that balanced forces mean no change in motion, not no forces. I can predict the motion from a force diagram. **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 car drives along a straight road at a steady 100 km/h. Compare the forward force from the engine with the backward forces (air and road friction).** ☐ Forward force is bigger, or the car would stop ☐ They are equal: the forces are balanced ☐ There are no forces on it while it cruises ☐ Backward forces are bigger My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** They are equal. At a steady speed the forces are balanced: the net force is zero, so the motion doesn't change. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: balanced and unbalanced :::keep-together ### Steady speed **When the box moved at a steady speed, what was true?** ☐ There were no forces on it ☐ The push was bigger than friction ☐ The push equalled friction: balanced, net force 0 ::: :::teacher-note **Answer:** The push equalled friction: balanced, net force 0 Balanced forces: equal and opposite. They are both still acting; they cancel out. ::: :::keep-together ### What the net force does **What happens with each net force? (The box is already moving to the right.)** Groups: Speeds up, Steady speed, Slows down - Push 9 N, friction 6 N: {{field-15}} - Push 6 N, friction 6 N: {{field-15}} - Push 2 N, friction 6 N: {{field-15}} - Push 12 N, friction 6 N: {{field-15}} ::: :::teacher-note **Answer:** Push 9 N, friction 6 N → Speeds up; Push 6 N, friction 6 N → Steady speed; Push 2 N, friction 6 N → Slows down; Push 12 N, friction 6 N → Speeds up ::: :::keep-together ### Balanced or not **Balanced or unbalanced?** Groups: Balanced, Unbalanced - A book resting on a desk: {{field-15}} - A car cruising at a steady speed: {{field-15}} - A ball speeding up as it rolls down a slope: {{field-15}} - A skydiver falling at a steady speed: {{field-15}} - A cyclist braking: {{field-15}} - A plane taking off down the runway: {{field-15}} ::: :::teacher-note **Answer:** A book resting on a desk → Balanced; A car cruising at a steady speed → Balanced; A ball speeding up as it rolls down a slope → Unbalanced; A skydiver falling at a steady speed → Balanced; A cyclist braking → Unbalanced; A plane taking off down the runway → Unbalanced ::: ## Mix it up :::keep-together **Looking back at force arrows: a diagram shows a 30 N push to the right and 10 N of friction to the left. How do the arrows look?** ☐ The two arrows are the same length ☐ The push arrow is three times as long as the friction arrow, and they point opposite ways ☐ Both arrows point to the right ☐ The friction arrow is longer, because friction is stronger ::: :::teacher-note **Answer:** The push arrow is three times as long as the friction arrow, and they point opposite ways The length of an arrow shows the size of the force, and it points the way the force acts. 30 N is three times 10 N. ::: :::typical ## Elaborate: the falling skydiver :::keep-together ### Still falling? **Her forces are balanced. Is she still falling?** ☐ No, she has stopped in mid-air ☐ Yes, at a steady speed ☐ She is speeding up ::: :::teacher-note **Answer:** Yes, at a steady speed This steady speed is her terminal velocity: weight and air resistance cancel, so her speed stays the same. ::: ::: :::wider ## Elaborate: slowing down and stopping :::keep-together ### Friction at rest **Once the box has stopped, with a push of 2 N, the friction is shown as 2 N, not 6 N. Why?** ☐ Friction only resists as much as it needs to, up to its limit ☐ The board is wrong ☐ Friction disappears when things stop ::: :::teacher-note **Answer:** Friction only resists as much as it needs to, up to its limit A stationary object's friction matches the push, up to a limit (6 N here). Beyond that, the object starts sliding. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Balanced** forces are equal and opposite: the **net force is zero**. They are still acting; they cancel. - Balanced forces mean **no change in motion**: still objects stay still; moving objects keep a steady speed in a straight line. - **Unbalanced** forces change the motion: the object speeds up, slows down or changes direction, the way the net force points. - A car at a steady speed has balanced forces: the engine's push equals air resistance plus friction. ## Exit ticket :::keep-together **A cyclist rides at a steady 20 km/h on a flat road. Are the forces on her balanced? Explain.** {{lines-3}} ::: :::teacher-note **Answer:** Yes: she isn't speeding up or slowing down, so her forward force equals the backward forces (air resistance and friction). :::