--- title: "How big, which way?" subtitle: "Year 7 Physics · Forces and motion · Lesson 1" --- # How big, which way? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Push a shopping trolley gently, then hard. Same direction, different force. Push it from the other side: same size, different force. Two things to get right every time. :::teacher-note **In one sentence:** Students represent forces as arrows with the right size and direction, and name them (AC9S7U04, node 1, a hinge node). **Students should be able to say:** I can draw a force as an arrow with the right direction and relative size. I can name the forces acting on an object. I can draw a force diagram that fits a situation. **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 **In a force diagram, each force is drawn as an arrow. What do you predict the arrow shows?** ☐ Both how big it is (its length) and which way it acts (its direction) ☐ Only how big the force is ☐ Only which way the force acts ☐ Which way the object will move My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Both. The arrow's length shows the size of the force and it points the way the force acts. (It does not always point the way the object moves.) ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: reading a force diagram :::keep-together ### Length is size **When you made the push bigger, what happened to its arrow?** ☐ It got longer, pointing the same way ☐ It stayed the same; only the number changed ☐ It changed direction ::: :::teacher-note **Answer:** It got longer, pointing the same way Length shows size. A force twice as big gets an arrow twice as long. ::: :::keep-together ### Naming forces **Match each force to what causes it.** A. Air pushing against a moving object B. Surfaces rubbing, acting against sliding C. Earth's gravity pulling the object down D. Something in contact pushing or pulling E. A surface pushing back up on the object {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Weight` | | | `Support (normal force)` | | | `Friction` | | | `Air resistance` | | | `Push or pull` | | ::: :::teacher-note **Answer:** Weight → C. Earth's gravity pulling the object down; Support (normal force) → E. A surface pushing back up on the object; Friction → B. Surfaces rubbing, acting against sliding; Air resistance → A. Air pushing against a moving object; Push or pull → D. Something in contact pushing or pulling ::: :::keep-together ### Two equal forces **The trolley's weight is 49 N and the support is 49 N. How should their arrows look?** ☐ Both pointing down ☐ Weight longer, because it is heavier ☐ The same length, pointing in opposite directions ::: :::teacher-note **Answer:** The same length, pointing in opposite directions Equal sizes, equal lengths. Weight points down; the support from the ground points up. ::: ## draw it right :::typical ## Elaborate: four situations :::keep-together **Which diagram fits each situation? (Forward is the way the object moves.)** Groups: Forward arrow longer than backward arrow, Forward and backward arrows equal, Backward arrow longer than forward arrow - A cyclist speeding up: {{field-15}} - A car driving at a steady 60 km/h: {{field-15}} - A sled slowing down on flat snow: {{field-15}} - A box being pushed but not moving: {{field-15}} - A runner speeding up out of the blocks: {{field-15}} ::: :::teacher-note **Answer:** A cyclist speeding up → Forward arrow longer than backward arrow; A car driving at a steady 60 km/h → Forward and backward arrows equal; A sled slowing down on flat snow → Backward arrow longer than forward arrow; A box being pushed but not moving → Forward and backward arrows equal; A runner speeding up out of the blocks → Forward arrow longer than backward arrow ::: ::: :::wider ## Elaborate: a boomerang :::keep-together **First Peoples of Australia shaped boomerangs with a curved upper surface, like an aircraft wing. In flight, as air moves over it, which force does that shape mainly create?** ☐ Thrust: the shape keeps pushing it forward, like an engine ☐ More weight ☐ Lift: air pushing the boomerang up (and sideways, which curves its path) ::: :::teacher-note **Answer:** Lift: air pushing the boomerang up (and sideways, which curves its path) The aerofoil shape makes the air push on it: lift. Spinning, the lift tilts the boomerang so it curves back. Its force diagram: weight down, lift, and air resistance against its motion. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A force has a **size** (newtons, N) and a **direction**, so it is drawn as an **arrow**. - The arrow's **length** shows the size; it **points** the way the force acts. - A **force diagram** shows every force on one object, each one named. - Common forces: weight (down), support (up from a surface), friction (against sliding), air resistance (against motion through air), pushes and pulls. ## Exit ticket :::keep-together **A box sits on the floor. Name the two forces on it and describe their arrows.** {{lines-3}} ::: :::teacher-note **Answer:** Weight, pointing down, and support from the floor, pointing up; the same length because they are equal. :::