Q3.16 Perspectives (T)

Flora is blind-fold, and a passenger in a car. The car is driven along a road, and rounds a bend, at which point she makes painful contact with the car door. Fred watches from the roadside. Give an account of the physics
  1. from Fred’s perspective
  2. from Flora’s perspective
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Hint

Fred’s frame is inertial; for him, there are no fictitious forces. But Flora’s frame is non-inertial …
 

Solution

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Solution

  1. As it goes round the bend the car displays the appropriate centripetal acceleration, towards the centre of the bend. For Fred (whose reference frame, the earth, is effectively inertial) Newton’s 2nd law holds, with no fudges necessary. The centripetal acceleration which Flora has can be attributed to the push that the door exerts on her.
  2. Flora’s account is different. She finds herself at rest (not-accelerating) with respect to her reference frame. And yet she is aware that the car door presses on her. She makes sense of this by saying that she is being acted on by a force which pushes her against the car door. This is the centrifugal force. From her standpoint, she is at rest because the two forces exerted on her (the door-contact-force and the centrifugal force) are equal and opposite.

    Even those who would deny any knowledge of Newton’s laws instinctively ‘invent’ such forces to ‘make sense of’ their experience in non-inertial frames.