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
- 25 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 25
Watch first
Forces, resultants and motion graphs
Balanced and unbalanced forces, read straight off the motion graphs.
In a nutshell
Understand forces as pushes or pulls arising from the interaction between two objects, and balanced and unbalanced forces
What you already know
Part 2 of 25
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.
Key words
Part 3 of 25
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.
Reset break
Part 4 of 25
Pause
That's sitting 1 of 6 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.
Learning cycle
Part 5 of 25
Learning cycle 1 of 2
Balanced and unbalanced forces
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 6 of 25
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.
Explore the idea
Part 7 of 25
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.
Reset break
Part 8 of 25
Pause
That's sitting 2 of 6 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.
Quick check
Part 9 of 25
Quick check
Part 10 of 25
Learning cycle
Part 11 of 25
Learning cycle 2 of 2
Reading motion graphs
A short piece of teaching, then a check to make sure it has landed.
Reset break
Part 12 of 25
Pause
That's sitting 3 of 6 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.
Explore the idea
Part 13 of 25
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.
Explore the idea
Part 14 of 25
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.
Quick check
Part 15 of 25
Reset break
Part 16 of 25
Pause
That's sitting 4 of 6 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.
Quick check
Part 17 of 25
Common mix-ups
Part 18 of 25
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.
Challenge round
Part 19 of 25
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
Reset break
Part 20 of 25
Pause
That's sitting 5 of 6 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.
Challenge round
Part 21 of 25
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.
Challenge round
Part 22 of 25
Game · Recall cards
Zero resultant force means the object...
Card 1 of 4
Exit quiz
Part 23 of 25
Exit quiz
Show what you've learned
6 questions, marked together at the end. Nothing is timed.
1. Forces are measured in...
2. Interaction pair forces act on...
3. A horizontal line on a distance-time graph means...
4. Opening a parachute causes...
5. Unbalanced forces cause...
6. Steeper distance-time gradient means...
Mastery quiz
Part 24 of 25
Marked quiz
End of lesson quiz: Forces, resultants and motion graphs
4 questions, marked with the reasoning shown. No timer.
1. Zero resultant force means the object...
2. On a velocity-time graph the gradient shows...
3. At terminal velocity, weight is...
4. Area under a velocity-time graph gives...
Lesson round-up
Part 25 of 25
Lesson round-up
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Ask LunaPart 1 of 25 · Watch & discover