Solving for Velocity: Metal Ball Swinging in a Circle

[Nusc]

Homework Statement

A metal ball is attached to a rope with length 2.40 m and swung in constant velocity in a circle with velocity 3.0 m/s

A light at the same height casts a shadow from the center to 0.8 m, what is the velocity at this point?

Homework Equations

The Attempt at a Solution

 

[Liquidxlax]

again, you must show some work. Look at your equations relate them and draw a picture to further your understanding

 

[Nusc]

We don't know the mass of the metal ball. We can't find the acceleration without first finding the velocity.

 

[Nusc]

Actually it's asking for the velocity at 1.0 not 0.8 m.

Where do I start?

 

[gneill]

Actually it's asking for the velocity at 1.0 not 0.8 m.

Where do I start?

I think you could start by clarifying the problem.

It's not clear where this light is supposed to be situated, where the shadow is being cast, and by what (the ball? The rope?) Was there a diagram accompanying the problem?

 

[Nusc]

You're not give anymore detail.

I would think that the shadow is cast long the plane of the circle so when the image is projected on the screen, it looks like a line. The shadow of the ball.

No diagram.

 

[gneill]

You're not give anymore detail.

I would think that the shadow is cast long the plane of the circle so when the image is projected on the screen, it looks like a line. The shadow of the ball.

No diagram.

What screen? There was no screen mentioned in the problem statement.

Perhaps you should list all of the unstated assumptions you are making. What (precise) setup do you have in mind? Is gravity working? There's a screen (where?). What is casting a shadow and exactly where and upon what? The light source is at the same height as:________?

A quick calculation shows that, if gravity is involved, the radius of the circle that the ball revolves in would be more than a meter (you can calculate it). So where is this light casting a shadow at 0.8m?

 

[Nusc]

You're already given that the metal ball swings in a circle. The radius of this circle is 2.4 m.


The question doesn't specify anything about the shadow but asks you to calculate the speed of the shadow at 0.8 from the center.

 

[gneill]

You're already given that the metal ball swings in a circle. The radius of this circle is 2.4 m.


The question doesn't specify anything about the shadow but asks you to calculate the speed of the shadow at 0.8 from the center.

No, so far the problem specifies the length of the rope. It says nothing about whether the rope is horizontal (no gravity) or describes a cone shape as it turns (conical pendulum). In a conical pendulum the ball still follows a circular path.

http://en.wikipedia.org/wiki/Conical_pendulum"

 

[Nusc]

Suppose it is horiztonal, no gravity.

 

[gneill]

Suppose it is horiztonal, no gravity.

Fine. Then there's no shadow from the ball at 0.8 m, since it's revolving at a radius of 2.4 m. How does that help?

 

[Nusc]

There's a shadow. Suppose the radius is half that size. Find the velocity 0.8 m from the center.

If the light was in the plane then we would see a straight line on the screen. Assuming the radius is 1.2, then we get a speed of 2.4 m/s which is not correct.