IB Physics SL & HL

Circular Motion
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It is a common misconception that objects have to move in the direction of the force. This is false; the acceleration points in the direction of the force. This means the change in velocity points in the direction of the force. It is not the velocity that points in the direction of the force.

For example, as the man approaches the top of the loop, he has very some upwards velocity from running up through the curve. At the apex, all that vertical velocity is converted into horizontal, and his vertical velocity is very small. At this point, gravity begins to act and increases his downward velocity.

However, gravity can only increase that downward velocity so fast. Before gravity can make him start falling, the much larger horizontal velocity allows the man to reach the downwards sloping part of the curve, which will accelerate him downwards faster.
Because of this, gravity will not make the man lose contact with the loop (assuming the man is going fast enough).

A similar system you can think of that you are probably familiar with is projectile motion. At the top of the trajectory the force points down, the velocity is horizontal, and the projectile continues on its parabolic path with both horizontal and vertical velocity. The difference between the projectile and the man in this example is that the net force is constant for the projectile. The horizontal component of the velocity never changes.

For the man, the net force is always changing so that the motion is circular. The vertical and horizontal components of the velocity are always changing around the circle. The projectile is falling, but the man isn't purely falling.
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Physics with Mo

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