AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
So, the velocity (speed and/or direction) of an object will only change if…
Full specification wording
Newton's First Law: So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
Velocity is a vector: it has both magnitude (speed) and direction.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 6 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Velocity is a vector: it has both magnitude (speed) and direction. A resultant force is needed to change either the speed, the direction, or both. If the resultant force is zero, the object continues at constant velocity in a straight line. For example, a car turning a corner at constant speed still changes velocity because its direction changes, so a resultant force acts towards the centre of the turn. Similarly, a cyclist accelerating from rest has a forward resultant force. This statement is the core of Newton's First Law: without a resultant force, velocity cannot change.
What earns the marks
- Define velocity as speed in a stated direction, so a change in either speed or direction is a change in velocity.
- State that a resultant force is required to change an object's velocity.
- Explain that if the resultant force is zero, the object moves at constant velocity in a straight line.
- Give an example where speed is constant but direction changes, such as circular motion, and identify the resultant force.
- Give an example where direction is constant but speed changes, such as a car accelerating along a straight road.
- Link the size and direction of the resultant force to the change in velocity produced.
Common mistakes and how to put them right
Believing that an object moving at constant speed cannot be accelerating.
Put it right: Changing direction at constant speed is still a change in velocity and requires a resultant force.
Thinking that a resultant force is needed to keep an object moving.
Put it right: A resultant force is needed only to change velocity, not to maintain it.
Treating speed and velocity as identical.
Put it right: Velocity includes direction, so a change in direction alone changes velocity.
Exam tips
- Use the word 'velocity' rather than 'speed' when direction may change, and state the direction where possible.
- When explaining circular motion, mention the resultant force acting towards the centre of the circle.
- Check whether the question asks about speed or velocity; a change in direction alone is enough to change velocity.
Try the lesson
The start of the Newton's First Law: So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object. lesson, made from checked teaching notes for this specification point.
Have a go first
A cyclist is travelling along a straight, level road at a steady 6 m/s. Which statement describes the resultant force acting on the cyclist?
Listen
Velocity is a vector: it has both magnitude (speed) and direction. A resultant force is needed to change either the speed, the direction, or both. If the resultant force is zero, the object continues at constant velocity in a straight line. For example, a car turning a corner at constant speed still changes velocity because its direction changes, so a resultant force acts towards the centre of the turn. Similarly, a cyclist accelerating from rest has a forward resultant force. This statement is the core of Newton's First Law: without a resultant force, velocity cannot change. A strong answer earns marks for these points. Define velocity as speed in a stated direction, so a change in either speed or direction is a change in velocity. State that a resultant force is required to change an object's velocity. Explain that if the resultant force is zero, the object moves at constant velocity in a straight line. Give an example where speed is constant but direction changes, such as circular motion, and identify the resultant force. A common slip: Believing that an object moving at constant speed cannot be accelerating. Instead, changing direction at constant speed is still a change in velocity and requires a resultant force. Exam tip: Use the word 'velocity' rather than 'speed' when direction may change, and state the direction where possible.
See it
Key words
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Newton's First Law
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Velocity as a vector
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Resultant force
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Acceleration and change of direction
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See every definition
- Newton's First Law
- Without a resultant force, velocity cannot change, so the object continues at constant velocity in a straight line.
- Velocity as a vector
- Speed in a stated direction, so a change in either speed or direction is a change in velocity.
- Resultant force
- Required to change an object's velocity; if it is zero, the object moves at constant velocity in a straight line.
- Acceleration and change of direction
- Changing direction at constant speed is still a change in velocity and requires a resultant force.
Label the diagram
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1Box 1
2Box 2
3Box 3
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