Lesson focus: A zero resultant preserves velocitySpeed in a particular direction, such as 5 m/s north.; a non-zero resultant changes speed or direction.
Calculate resultants and explain straight and circular motionMoving in a circle, where the direction keeps changing..
Build on earlier learning
Forces at a distanceCombine multiple forces, including fields, into a resultant.
You will be successful when you can…
- Calculate resultants.
- Interpret zero resultant.
- Explain acceleration as velocity change.
- Explain inward resultant in turning.
Words that carry the science
- balanced forces
- Forces with zero vector resultant.
- unbalanced forces
- Forces with non-zero resultant.
- velocity
- Speed in a stated direction.
- acceleration
- Change in velocity per unit time.
- circular motion
- Motion with continuously changing direction around a centre.
Use velocity, not speed alone
01Zero resultant
An object at rest stays at rest; a moving object continues in a straight line at constant speed when resultant is zero.
02Non-zero resultant
A resultant can speed up, slow down or change direction. Direction matters because velocitySpeed in a particular direction, such as 5 m/s north. is a vector.
velocitySpeed in a particular direction, such as 5 m/s north. is speed in a direction. So if a car goes round a bend at the same speed, its direction changes, so its velocity changes. Changing velocity needs an unbalanced forceWhen the forces on an object are not equal, so it speeds up, slows down or changes direction.. For a car on a bend, that force pushes in towards the middle of the bend.
03Turning at constant speed
An inward resultant changes velocity direction continuously. No outward force balances it in an inertial force diagram.
Balanced and unbalanced forces
A car's speed only changes when the forces on it are unbalanced. When the driving force equals the resistive forces, the car keeps a steady speed — or stays still.
Controls
Forces are illustrative (car mass 1000 kg). In space there is no air or road, so with the engine off the car keeps going at a steady speed: Newton’s first law.
Diagnose a bend
A car moves around a level bend at constant 12 m/s.
- Speed is constant.
- Direction changes, so velocity changes.
- Changing velocity means acceleration.
- Resultant force points inward toward curve centre.
How to make the reasoning visible: Use “constant velocity” only for constant speed and direction.
Resultant cases
All horizontal forces are on the same object.
| Forces | Resultant | Motion statement |
|---|---|---|
| 40 N right, 40 N left | 0 N | Constant velocity possible |
| 60 N right, 20 N left | 40 N right | Accelerates right |
| 10 N up, 25 N down | 15 N down | Accelerates downward |
| Inward force while turning | Inward | Direction changes |
Can forces be large but balanced?
Why does a satellite accelerate at constant speed?
Use the table. Work out the resultant force for 10 N up and 25 N down.
Check your understanding
Answer from memory, then check against the suggested answer.
What is balanced forces?
What is unbalanced forces?
What is velocity?
What happens to a still object when the forces on it are balanced?
What can an unbalanced force do to a moving object?
A lift rises at constant speed. Compare tension and weight.
Why is “centrifugal force balances gravity” misleading for an orbit diagram?
A car travels at a steady speed in a straight line. Are the forces on it balanced or unbalanced? Explain.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A car goes round a bend at a steady speed. Answer the parts to explain why there is an unbalanced force on it.
What is velocity?
(1)What changes as the car goes round the bend?
(1)So what happens to its velocity?
(1)Which way does the unbalanced force point?
(1)Total for this question: 4 marks
Command word: Speed and velocity are not the same. Velocity includes direction.