• Quick retrieval from earlier learning (Newton's laws of motion; Mass and weight (W = mg)). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. The same force is applied to an empty trolley and a full one. Which accelerates more? (neither moves / the full trolley / both the same / the empty trolley) → the empty trolleyPrompt: a = F ÷ m
2. What is the weight of a 35 kg object on Earth? Use g = 9.8 N/kg. → 343 NPrompt: weight = mass × gravitational field strength (W = m g)
3. Newton's laws: which statement is true? (Heavier objects need less force to accelerate. / For every force there is an equal and opposite force on the other object. / A moving object needs a constant force to keep moving at steady speed in space. / The two forces in a Newton's third law pair act on the same object.) → For every force there is an equal and opposite force on the other object.Prompt: Think about inertia, F = ma and action–reaction pairs.

• Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
• Share the objective: Force, mass and acceleration (F = ma). Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: What resultant force is needed to accelerate a 80 kg mass at 2.5 m/s²?
• Step 1: F = 80 × 2.5 = 200
• Step 2: So the answer is: 200 N
• Answer: 200 N
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. What resultant force is needed to accelerate a 50 kg mass at 0.5 m/s²? → 25 NPrompt: F = m × a
2. A resultant force of 20 N acts on a 10 kg object. What is its acceleration, in m/s²? → 2 m/s²Prompt: Rearrange F = m a to find a.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. A 1500 kg cart is pushed forward with 770 N. Friction acts backwards with 20 N. What is its acceleration, in m/s²? → 0.5 m/s²Prompt: First find the resultant force.
2. A resultant force of 4 N acts on a 2 kg object. What is its acceleration, in m/s²? → 2 m/s²Prompt: Rearrange F = m a to find a.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.