--- title: "What is the net force?" subtitle: "Year 10 Physics · Newton's laws · Lesson 1" --- # What is the net force? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A car's engine pushes it forward with 2000 newtons. So why doesn't it keep speeding up for ever? :::teacher-note **In one sentence:** Students represent forces as vectors and add or subtract forces along one line to find the net force, then say what the net force does to motion (AC9S10U05, node 1). **Students should be able to say:** I can explain why force is a vector. I can find the net force from several forces along one line. I can say whether a net force will change an object's 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 car's driving force is 2000 N forward. Air resistance is 500 N and rolling resistance 300 N, both backward. What is the net force on the car?** ☐ 2800 N forward ☐ 1200 N forward ☐ Zero: the forces cancel ☐ 2000 N forward My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 1200 N forward. The two backward forces add to 800 N; opposite to the 2000 N forward, so they subtract: 2000 − 800 = 1200 N, in the direction of the bigger side (forward). ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: forces as vectors :::keep-together ### Size and direction **Why is force called a vector?** ☐ It can only act in a straight line ☐ It has a size and a direction, and both matter ☐ It is measured in newtons ::: :::teacher-note **Answer:** It has a size and a direction, and both matter A vector has size and direction. 10 N left and 10 N right are different forces, and they cancel. ::: :::keep-together ### A push and friction **Two people push a stalled car forward with 300 N and 250 N. Friction acts backward with 400 N. What is the net force, in newtons? (forward is positive)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 150 Forward: 300 + 250 = 550 N. Backward: 400 N. Net: 550 − 400 = 150 N forward. ::: :::keep-together ### Tug of war **In a tug of war, Team A pulls left with 1800 N and Team B pulls right with 2100 N. What is the size of the net force, in newtons?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 300 Opposite directions: subtract. 2100 − 1800 = 300 N, to the right (Team B's way). ::: :::keep-together ### Balanced **A skydiver weighs 700 N. Air resistance on her is 700 N up. What is happening to her motion?** ☐ She must be stopped in the air ☐ Nothing is changing: net force zero, so she falls at a steady speed ☐ She is speeding up, because weight is pulling her down ::: :::teacher-note **Answer:** Nothing is changing: net force zero, so she falls at a steady speed Balanced forces give a net force of zero: the motion doesn't change. She keeps falling at the same speed (terminal velocity). ::: :::keep-together ### What happens next **What will the motion do?** Groups: Change (speed up, slow down or turn), Stay the same - Net force 500 N forward: {{field-15}} - Net force 200 N backward on a moving bike: {{field-15}} - Forward 900 N, backward 900 N: {{field-15}} - A puck sliding on ice with no friction: {{field-15}} ::: :::teacher-note **Answer:** Net force 500 N forward → Change (speed up, slow down or turn); Net force 200 N backward on a moving bike → Change (speed up, slow down or turn); Forward 900 N, backward 900 N → Stay the same; A puck sliding on ice with no friction → Stay the same ::: :::keep-together ### Touching or not **Contact or non-contact force?** Groups: Contact force, Non-contact force - Friction between the tyres and the road: {{field-15}} - Air resistance on a cyclist: {{field-15}} - A table pushing up on a book (the normal force): {{field-15}} - Gravity pulling a skydiver down (her weight): {{field-15}} - A magnet pulling a paperclip from a few centimetres away: {{field-15}} ::: :::teacher-note **Answer:** Friction between the tyres and the road → Contact force; Air resistance on a cyclist → Contact force; A table pushing up on a book (the normal force) → Contact force; Gravity pulling a skydiver down (her weight) → Non-contact force; A magnet pulling a paperclip from a few centimetres away → Non-contact force ::: :::typical ## Elaborate: a four-person tug of war :::keep-together ### Dee's pull **Ana pulls 400 N and Ben 300 N to the left. Cai pulls 450 N to the right. How hard must Dee pull right for the net force to be zero?** ☐ 150 N ☐ 250 N ☐ 700 N ::: :::teacher-note **Answer:** 250 N Left: 400 + 300 = 700 N. Right must also be 700 N: 450 + 250 = 700. ::: ::: :::wider ## Elaborate: driving at a steady speed :::keep-together ### Why no speeding up **The car's engine is still pushing with 1500 N, but it isn't speeding up. Why?** ☐ The engine's force is used up ☐ Moving things always slow down unless pushed harder ☐ Air and rolling resistance add up to 1500 N backward, so the net force is zero ::: :::teacher-note **Answer:** Air and rolling resistance add up to 1500 N backward, so the net force is zero Net force zero: no change in motion. The engine only needs to cancel the resistance to keep a steady speed. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A force is a **vector**: it has a size (in newtons) and a direction. - Along one line, forces in the same direction **add**; forces in opposite directions **subtract**. Choose one direction as positive. - What is left is the **net force** (resultant force), in the direction of the bigger side. - A net force changes motion (speeds up, slows down, turns). A net force of zero leaves the motion unchanged: still, or at a steady speed. ## Exit ticket :::keep-together **A cyclist pedals with a forward force of 120 N. Air resistance is 70 N and friction 20 N. Find the net force and say what happens to her speed.** {{lines-3}} ::: :::teacher-note **Answer:** 120 − (70 + 20) = 30 N forward: she speeds up. :::