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SavannahYear 8
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SciencePhysics foundations for GCSE (AQA 8463)30 min★★★ difficulty

Forces, resultants and motion graphs

Objects do not need a force to keep moving. Getting free of that intuition is the single biggest step towards GCSE physics.

Lesson overview

What you'll learn in this lesson

Understand forces as pushes or pulls arising from the interaction between two objects, and balanced and unbalanced forces

Key learning points

  • • Forces come in pairs, between objects
  • • Resultants decide what happens
  • • Falling and terminal velocity
  • • Graphs say it faster than words

This lesson at a glance

  • 30 minutes
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

forcespushesarisinginteractionobjects

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%
1

Watch & discover

Part 1 of 19

Watch first

Forces, resultants and motion graphs

An English teaching video on this topic from GCSE Physics Explained. Watch it, then scroll down for the lesson.

In a nutshell

Understand forces as pushes or pulls arising from the interaction between two objects, and balanced and unbalanced forces

2

What you already know

Part 2 of 19

Before we start

What you already know

You should already know that forces are measured in newtons and how to calculate speed from distance and time.

3

Key words

Part 3 of 19

Key words

Words you'll need today

Resultant force
The single force equivalent to all forces acting.
Balanced forces
Forces whose resultant is zero.
Terminal velocity
The steady speed reached when drag balances weight.
Interaction pair
Two equal, opposite forces acting on two different objects.
Gradient
The steepness of a graph line, which carries physical meaning.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

Balanced and unbalanced forces

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

Learn

Forces come in pairs, between objects

A force is always an interaction: the ground pushes up on you exactly as hard as you push down on it. The two forces of an interaction pair act on different objects, which is why they never cancel each other out on one object.

6

Explore the idea

Part 6 of 19

Learn

Resultants decide what happens

Add forces along a line, taking direction into account. A zero resultant means staying still or continuing at constant velocity. A non-zero resultant means accelerating in that direction — speeding up, slowing down or changing direction.

7

Quick check

Part 7 of 19

Quick check

Zero resultant force means the object...

8

Quick check

Part 8 of 19

Quick check

On a velocity-time graph the gradient shows...

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
10

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Reading motion graphs

A short piece of teaching, then a check to make sure it has landed.

11

Explore the idea

Part 11 of 19

Learn

Falling and terminal velocity

A skydiver accelerates while weight exceeds drag. Drag grows with speed until the two balance, and the resultant becomes zero, so the diver falls at a constant terminal velocity. Opening the parachute increases drag, so they slow to a new, lower terminal velocity.

12

Explore the idea

Part 12 of 19

Learn

Graphs say it faster than words

On a distance-time graph the gradient is speed, and a horizontal line means stationary. On a velocity-time graph the gradient is acceleration, a horizontal line means constant velocity, and the area under the line is distance travelled. Always check which graph you are reading.

13

Quick check

Part 13 of 19

Quick check

At terminal velocity, weight is...

14

Quick check

Part 14 of 19

Quick check

Area under a velocity-time graph gives...

15

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

  • Constant speed means balanced forces, not zero force.
  • A flat line means 'not moving' on a distance-time graph but 'steady speed' on a velocity-time graph.
  • At terminal velocity the diver is still falling fast — the acceleration is what has become zero.
16

Challenge round

Part 16 of 19

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

17

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

A force is always an interaction: the ground ____ up on you exactly as hard as you push down on it.

The two ____ of an interaction pair act on different objects, which is why they never cancel each other out on one object.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Forces, resultants and motion graphs

6 questions, marked with the reasoning shown. No timer.

  1. 1. Forces are measured in...

  2. 2. Interaction pair forces act on...

  3. 3. A horizontal line on a distance-time graph means...

  4. 4. Opening a parachute causes...

  5. 5. Unbalanced forces cause...

  6. 6. Steeper distance-time gradient means...

19

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

  1. Name the variable the club changed the second time, and one variable they kept the same.
  2. Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
  3. Write a conclusion for the club's report, saying how far the evidence supports it.

  • Method and results

    Writing

    Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.

    1 page + results table

  • Conclusion

    Writing

    Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.

    6–8 sentences

  • Labelled diagram

    Diagram

    Draw the apparatus or the process in pencil, label it fully, and photograph it.

    1 labelled diagram

  • Reading and notes

    Reading

    Read the lesson text again and write the key ideas in your own words.

    Half a page of notes

Saved as you type.

Part 1 of 19 · Watch & discover

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