Science lesson plan · 8th grade (Year 9 in England) · 45 minutes
Find the resultant of several forces
Forces and motion
CCSS NGSS.MS-PS2-2CCSS NGSS.HS-PS2-1England: KS3 Physics: Motion and forcesEngland: GCSE Combined Science: Forces (resultant forces)
Objective and success criteria
Students will: Find the resultant of several forces.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Balanced and unbalanced forces
Leads to: the next skill in this area.
Materials: Force meters, Toy cars and ramps, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Balanced and unbalanced forces). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Balanced forces: which statement is NOT true? (When forces are balanced, a still object stays still. / When forces are balanced, a moving object keeps moving at the same speed in the same direction. / A book resting on a table has balanced forces acting on it. / A still object has no forces acting on it.) → A still object has no forces acting on it.Prompt: Can something move steadily with balanced forces?
Hook · 5 min
- Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
- Share the objective: Find the resultant of several forces. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: A car's engine provides a driving force of 2000 N forwards. Air resistance is 300 N and friction is 150 N, both backwards. What is the resultant force, in N?
- Step 1: 300 + 150 = 450
- Step 2: 2000 − 450 = 1550
- Step 3: The resultant acts forwards, so the car speeds up.
- Step 4: So the answer is: 1550 N
- Answer: 1550 N
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- A car's engine provides a driving force of 1000 N forwards. Air resistance is 400 N and friction is 100 N, both backwards. What is the resultant force, in N? → 500 NPrompt: Add the forces acting backwards.
- Two forces act on a box at right angles: 60 N east and 80 N north. What is the size of the resultant force, in N? → 100 NPrompt: Draw the forces nose to tail to make a right-angled triangle.
You do: independent practice · 10 min
- 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.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Two forces act on a box at right angles: 100 N east and 240 N north. What is the size of the resultant force, in N? → 260 NPrompt: Draw the forces nose to tail to make a right-angled triangle.
- A car's engine provides a driving force of 800 N forwards. Air resistance is 200 N and friction is 200 N, both backwards. What is the resultant force, in N? → 400 NPrompt: Add the forces acting backwards.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Balanced and unbalanced forces.
- Use concrete materials: force meters.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-8.
Free lesson plan from eurekami.com/lesson-plan-builder