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# 4. Movement of a Particle in Circular Motion w/ Polar Coordinates

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7 thoughts on “4. Movement of a Particle in Circular Motion w/ Polar Coordinates”

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45:47 Suppose that the particle is initially at rest in a frictionless horizontal pipe. Now suppose the pipe rotates. By Newton 2, the particle "wants" to accelerate tangentially due to the normal force of the wall of the pipe on it, so the particle moves from rest (i.e. accelerates) along the pipe as there is no resistance. An observer fixed to the pipe is accelerating and hence is not in an inertial frame, so Newton's second law those not apply. The radial acceleration of the particle that this observer measures is due to a fictitious radial force.

An observer in an inertial frame will measure the radial acceleration as 0.

If the particle remained at rest relative to the pipe, then to someone in an inertial frame, the particle would have non-zero radial acceleration and hence a radial force due to pipe (friction) would have to act on the particle.

Wait wait wait,,, if a driver is drunk and drives side to side like the figure 49:00 the slope of the tangent is NOT the velocity because the figure is the plot of X and Y in a function of trajectory, if you want to obtain the velocity the function should be time in X and angular displacement in Y.

Based on case 3 where there is no restriction on the ball flying out of the tube, does this mean that the force in the r hat direction is zero in cases 1 and 2 as well? Since the r hat direction represents the centrifugal 'force'?

I have actually learned to derive all of these formulas. xD

I don't like memorising things, I prefer to know how to do the actual derivations so I don't have to carry formulas around everywhere.

This is the most awful teaching method ever!

suppose if it is an inertial reference frame?

Sir it's a very useful for me..