AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
So, when a vehicle travels at a steady speed the resistive forces balance…
Full specification wording
Newton's First Law: So, when a vehicle travels at a steady speed the resistive forces balance the driving force.
A vehicle moving at a steady speed has zero resultant force, so the driving force exactly balances the total resistive forces.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A vehicle moving at a steady speed has zero resultant force, so the driving force exactly balances the total resistive forces. The driving force is actually provided by the static friction between the driven tyres and the road pushing the vehicle forward. Resistive forces include air resistance (drag) and rolling resistance. If the driving force were larger, the vehicle would accelerate; if smaller, it would decelerate.
For example: A car cruising at a constant 20 m/s on a level road has a forward driving force of 600 N and a total resistive force of 600 N, giving a resultant of 0 N. This is a direct application of Newton's First Law: a body continues at constant velocity unless acted on by a resultant force.
What earns the marks
- State that steady speed means the vehicle's velocity is constant, so acceleration is zero.
- Identify the driving force acting forwards (due to friction between driven tyres and road) and the resistive forces acting backwards.
- Explain that the resultant force is zero because the two sets of forces are equal in size and opposite in direction.
- Use the equation resultant force = driving force − resistive force and show that it equals 0 N for steady speed.
- Apply the idea to a numerical example, such as 600 N forward and 600 N backward giving a resultant of 0 N.
- Recognise that resistive forces include air resistance and friction, and that air resistance increases with speed, whereas solid friction generally does not.
Common mistakes and how to put them right
Thinking that a moving vehicle must have a resultant force in the direction of motion.
Put it right: At steady speed the resultant force is zero, and motion continues because of inertia.
Confusing balanced forces with no forces acting.
Put it right: Forces are still present and balanced, not absent.
Assuming the driving force is always larger than resistive forces whenever the vehicle moves.
Put it right: The driving force is larger only when the vehicle is accelerating.
Exam tips
- Always state the direction of each force when describing a balanced-force situation.
- Use the phrase 'resultant force is zero' rather than 'no forces' to show precise understanding.
- If a numerical value is given, calculate the resultant force explicitly before concluding that the speed is steady.
Try the lesson
The start of the Newton's First Law: So, when a vehicle travels at a steady speed the resistive forces balance the driving force. lesson, made from checked teaching notes for this specification point.
Have a go first
A car travels along a straight, level road at a steady speed. Which statement describes the forces acting on the car?
Listen
A vehicle moving at a steady speed has zero resultant force, so the driving force exactly balances the total resistive forces. The driving force is actually provided by the static friction between the driven tyres and the road pushing the vehicle forward. Resistive forces include air resistance (drag) and rolling resistance. If the driving force were larger, the vehicle would accelerate; if smaller, it would decelerate. For example, a car cruising at a constant 20 m/s on a level road has a forward driving force of 600 N and a total resistive force of 600 N, giving a resultant of 0 N. This is a direct application of Newton's First Law: a body continues at constant velocity unless acted on by a resultant force. A strong answer earns marks for these points. State that steady speed means the vehicle's velocity is constant, so acceleration is zero. Identify the driving force acting forwards (due to friction between driven tyres and road) and the resistive forces acting backwards. Explain that the resultant force is zero because the two sets of forces are equal in size and opposite in direction. Use the equation resultant force = driving force − resistive force and show that it equals 0 N for steady speed. A common slip: Thinking that a moving vehicle must have a resultant force in the direction of motion. Instead, at steady speed the resultant force is zero, and motion continues because of inertia. Exam tip: Always state the direction of each force when describing a balanced-force situation.
Key words
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Balanced forces
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Resultant force
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Steady speed and constant velocity
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Newton's First Law
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See every definition
- Balanced forces
- When a vehicle travels at a steady speed the resistive forces balance the driving force.
- Resultant force
- At steady speed it is zero, because the driving force and the total resistive forces are equal in size and opposite in direction.
- Steady speed and constant velocity
- A vehicle travelling at steady speed has constant velocity, so its acceleration is zero.
- Newton's First Law
- A body continues at constant velocity unless acted on by a resultant force.
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