Solving for Minimum Velocity: Ball & Rock

[frostchaos123]

Homework Statement

As in the attachement, find the minimum v such that the ball does not touch the rock.

The Attempt at a Solution

The solution given is mg*cos(theta) - mv^2/R = n < 0, where theta is the angle between the ball and the person.

My question is why is there a normal force acting on the ball? Since the ball is falling downwards without hitting the rock, wouldn't there be no forces acting on it other than centripetal acceleration?

 

[AC130Nav]

What is a "normal force" according to your definition?

This is simply a case of the rock moving along the x-axis (the kick) farther than it would by falling in the y axis.

I don't see centripetal anything entering into the problem.

 

[Doc Al]

My question is why is there a normal force acting on the ball?

Note that to find the minimum speed they set the normal force to zero.

Since the ball is falling downwards without hitting the rock, wouldn't there be no forces acting on it other than centripetal acceleration?

As long as the ball does not make contact with the rock, the only force on the ball is gravity. (Centripetal acceleration is not a force.)

The idea is this. First have the ball making contact with the rock as it slides down. There's a non-zero normal force, given by the equation you quoted. Find the speed which makes that normal force zero.

 

[AC130Nav]

Note that to find the minimum speed they set the normal force to zero.

As long as the ball does not make contact with the rock, the only force on the ball is gravity. (Centripetal acceleration is not a force.)

The idea is this. First have the ball making contact with the rock as it slides down. There's a non-zero normal force, given by the equation you quoted. Find the speed which makes that normal force zero.

Is the ball moving?

 

[Doc Al]

Is the ball moving?

Sure.