Physics - Circular Motion Concept?

Question

This is the situation:

A ball is attached to a rope which is spinning (vertically) at a constant rate. Now when you need to calculate the acceleration being exerted on the ball AT THE TOP, you subtract gravity from from the NORMAL force, because the gravity is pulling down and the Normal force us pushing up.

My question is: why is the Normal force pushing up. Shouldn't it be pushing in the same direction as gravity.

Answer 1

Good question - it tells us exactly where the problem is. And you are right, there are no forces pushing up at the top.

So let's forget about "normal" force and recognize three forces:

T = tension on the rope
W = weight of the ball (assume rope is massless)
N = net force on the ball.

Because the ball is spinning at a constant angular velocity, the magnitude of the acceleration is constant, but not its direction - it is always in the direction of the center of the circle.

net force = mass x net acceleration

so the net force N also has a constant magnitude but different direction.

When the ball is at the top, the tension on the rope T, and the force of gravity W are both in the same direction - down. So:

N = T + W or T = N - W

When the ball is at the bottom, the tension T is in the same direction as the net force - up toward the center of the circle - but the force of gravity is still down.

So if we take up as being positive, we have:

N = T - W or T = N + W

If we take down as positive, we get the same result:

-N = -T + W or -T = -N - W or T = N + W

The term "normal force" is usually used in the context of friction, where it refers to the force on an object by a surface. If the surface is horizontal and the only other force acting on the object is gravity, then:

1. The box isn't moving so its acceleration must be 0

2. acceleration = 0 means the net force = 0

3. net force = W (down) - N (up)

4. so W = N

(Note, if the surface is a ramp or someone is pushing on the object, this result is no longer valid)

Answer 2

a million. The directional rigidity is tangential from each area of reference. What I advise is that the process the rigidity applies in all guidelines alongside the lateral floor of the dryer casing, in any respect factors in time (for the duration of the operation). it fairly is the action of the dryer's casing, which exerts this centripetal rigidity. 2. by way of rotational velocity and our inertia, we tend to be in a action, a action that's to set us tangentially different than for the earth's floor. besides the undeniable fact that it fairly is the earth's gravity, which balances this rigidity, and makes our weight the way it fairly is. No. The ball won't land in our hand. bear in mind that speed has the two - magnitude and course. as long by way of fact the ball is on your hand, its speed in terms of magnitude and course is taken care of. once you throw it up the air off a automobile moving alongside a curve, the speed is made to be consistent, it is the magnitude is conserved, besides by way of fact the course. So, the ball won't land on your hand, yet could be hurled far flung from you. HTH