Edexcel A-Level Physics 9PH0
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Further Mechanics
Centripetal force and what provides it
- 2 things to know
- Papers 1 and 3
Overview
Recall questions on why a resultant force is needed, why centripetal force is not an extra force, why it does no work, and what supplies it for a car and a satellite.
What you need to know
Answer each one before you reveal it.
A vehicle rounds a banked track at exactly the design speed with no sideways friction. Which forces act, and what conditions apply?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Why must there be a resultant force on any object moving in a circle?
Model answer
The object is accelerating towards the centre, and by Newton's second law an acceleration always requires a resultant force acting in the same direction.
- 2
Is centripetal force an extra force that acts in addition to the others on a body?
Model answer
No, it is simply the name given to the resultant of the real forces already acting, such as tension, gravity, friction or the normal contact force, when that resultant points towards the centre.
- 3
State the equations for centripetal force:
Model answer
The centripetal force is F = ma = mv²/r = mrω².
- 4
Why does the centripetal force do no work on an object in uniform circular motion?
Model answer
The force is always perpendicular to the velocity, so there is no displacement in the direction of the force and no energy is transferred, which is why the speed remains constant.
- 5
What provides the centripetal force for a car turning a corner on a level road?
Model answer
Friction between the tyres and the road surface, acting horizontally towards the centre of the bend.
- 6
Why does a car skid outwards if it takes a bend too quickly?
Model answer
The maximum friction available is less than the mv²/r needed at that speed, so the resultant inward force is too small and the car follows a path of larger radius than the bend.
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