--- title: "What forces act on it?" subtitle: "Year 10 Physics · Newton's laws · Lesson 6" --- # What forces act on it? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A plane cruising at 900 km/h has four big forces on it, each hundreds of thousands of newtons. Yet the passengers can pour a drink without spilling it. How? :::teacher-note **In one sentence:** Students draw free body diagrams for objects with several forces, find the net force in each direction, and use Newton's first and second laws to say what the motion does (AC9S10U05, node 6). **Students should be able to say:** I can draw a free body diagram with every force on an object. I can find the net force in each direction. I can say what the object's motion will do from its free body 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 plane cruises at a steady speed and height. What can you say about the forces on it?** ☐ Thrust is bigger than drag, or it would slow down ☐ There are no forces on it ☐ They balance both ways: lift equals weight, thrust equals drag ☐ Lift is bigger than weight, or it would fall My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** They balance in both directions. Steady speed in a straight line means no change in motion, so no net force (Newton's first law): lift balances weight, and thrust balances drag. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: reading a free body diagram :::keep-together ### What goes on it **What goes on a free body diagram?** ☐ Every object in the scene and all the forces between them ☐ One object, and every force acting on that object ☐ Only the forces that make the object move ::: :::teacher-note **Answer:** One object, and every force acting on that object One object, every force on it, each as an arrow starting at the object, labelled with what causes it and its size. ::: :::keep-together ### The skydiver **A 600 N skydiver has 450 N of air resistance acting up on her. What is the net force on her, in newtons, downwards?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 150 600 − 450 = 150 N downwards: she is still speeding up. ::: :::keep-together ### The box **A 2 kg box is pushed right with 14 N against 6 N of friction. What is its acceleration, in m/s²?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 4 Net force 14 − 6 = 8 N right. a = F ÷ m = 8 ÷ 2 = 4 m/s². ::: :::keep-together ### The lift **A lift's cable pulls up with exactly its weight. The lift is moving up. What is happening?** ☐ It must be stopped ☐ It is speeding up, because it is moving up ☐ It moves up at a steady speed: balanced forces, no change in motion ::: :::teacher-note **Answer:** It moves up at a steady speed: balanced forces, no change in motion First law: balanced forces keep the motion the same, here steady upward motion. ::: :::typical ## Elaborate: four forces on a plane :::keep-together ### More thrust **The pilot now increases thrust a little and leaves everything else the same. What happens?** ☐ The plane climbs straight up ☐ The plane speeds up: a net force forward ☐ Nothing changes ::: :::teacher-note **Answer:** The plane speeds up: a net force forward Thrust bigger than drag gives a net force forward, so the plane accelerates (until the extra speed raises the drag to match). ::: ::: :::wider ## Elaborate: the skydiver's fall :::keep-together **As a skydiver falls faster, air resistance grows. What happens to her free body diagram over time?** ☐ The arrows stay the same throughout ☐ The upward arrow grows until it equals her weight; then she falls at a steady speed (terminal velocity) ☐ Her weight arrow shrinks ::: :::teacher-note **Answer:** The upward arrow grows until it equals her weight; then she falls at a steady speed (terminal velocity) Net force downward shrinks to zero, so the acceleration does too. Opening the parachute makes air resistance jump, and she slows to a new, lower steady speed. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **free body diagram** shows one object and every force acting on it, as labelled arrows from the object. - Work out the net force in each direction separately (up–down, left–right). - Net force zero in every direction: no change in motion (still, or steady speed in a straight line). - A net force: the object accelerates in that direction, with a = F ÷ m. ## Exit ticket :::keep-together **A 1000 kg car has 3000 N of driving force and 1000 N of drag. Find the net force and the acceleration.** {{lines-3}} ::: :::teacher-note **Answer:** 2000 N forward; a = 2000 ÷ 1000 = 2 m/s². :::