# Simple magnetics questions

1. Feb 20, 2004

### Chris420

Hi, I had a thought in my head and was just wondering something.

If I have 2 exact steel balls with the same inital velocity shooting staight up but will one of the balls there is a magnet just above a point where the velocity of that ball will be 0. The steel ball does not come into contact with the magnet but it gets close enough to it so that the magnetic pull does affect the ball a little.

Will both of these balls hit the ground at the same time when they fall about down? Will they both hit the ground with the same force?

Chris

2. Feb 20, 2004

### NateTG

The ball that is affected by the magnet will hit later. Both balls will hit with the same velocity.

3. Feb 20, 2004

### HallsofIvy

Do you understand that this makes no sense at all?

"will one of the balls there is a magnet just above a point where the velocity of that ball will be 0. The steel ball does not come into contact with the magnet but it gets close enough to it so that the magnetic pull does affect the ball a little."

A magnetic field will "affect the ball a little" at any distance whatsoever. What strength "affect" are you talking about?

"Will both of these balls hit the ground at the same time when they fall about down? Will they both hit the ground with the same force?"

Where is the other ball? Is it so far away that we can ignore the magnetic field? A simple way to handle this problem is to take the force due to the magnet to be a constant- then the total force is "M-mg" and you can calculate the acceleration, time to hit the ground again, etc. from d2x/dt2= M/m- g.

If you want to be very accurate you could use A/r2 where r is the distance from the ball to the magnet- but then the differential equation for x becomes very difficult.

4. Feb 20, 2004

### Chris420

Sorry I screwed up that post, I didn't really look over it before I posted. Thanks for your replies though.

5. Feb 21, 2004

### dodik

this is wrong

[zz)] Well that would depend on how much strength the magnet has and your problem makes no sense at all. [b(]

6. Feb 21, 2004

### ahrkron

Staff Emeritus
Hey guys,

I think we should not be too harsh with how people ask questions. They ask because they are learning, and I would much rather have people asking questions with less than perfect wording than refraining from asking because of not being sure of how to ask them properly.

7. Feb 21, 2004

### cider_drinker

The following assumes that the magnet is not close enough and strong enough to suck in and hold the ball. If this is the case then some interesting things happen.

The magnet will exert an upwards force on the ball as it approaches, countering some of the weight force, which will result in less deceleration. This force will increase as the ball approaches the magnet, and so it will experience a slower deceleration than the other ball, and therefore reach a greater altitude than the ball without a magnet above it.

This means that it will start to fall from a greater height, at a latter time than the first ball, and so will not only reach the ground later, but will have a greater velocity.

Note that the velocity of an object travelling thought a medium like the atmosphere around earth is subject to a phenomenon known as terminal velocity, and this means if the speed is great enough that the air resistance force the object experiences becomes the same as the weight force (the force the mass of the object experiences due to gravity) then acceleration ceases and the velocity remains constant. If the balls are fired hard enough, then both will return to the surface with terminal velocity, but the one with the magnet above will still be later.

In a vacuum, the ball with the magnet above will always have a greater velocity when hitting the surface of whatever body they are launched from, provided they are not launched hard enough to escape the gravity of the body. To escape, they would have to be launched with what is known as the escape velocity, or even faster.

I agree, don't be too hard on questioners, they are 'hear' tee learn! Wee all make mistooks sometimes :)

8. Feb 22, 2004

When the ball is going up, it feels an upward force from the magnet. But it also feels that same force when it's going down, so it will accelerate downward more slowly. This will in fact (actually, I made an assumption, so this isn't fact) be exactly opposite and the balls will hit with the ground at the same speed they were launched.

If you don't believe me, consider the energy of each of the balls at each starting point. If we take the forces to be conservative with respect to the initial position (a simplifying assumption that is not always true), the energy when the ball comes back to this position will be the same as when it started. The potentials are the same, so the kinetic energies must also be the same to satisfy conservation of energy.

The question is poorly worded, but certainly not beyond comprehension. I've seen worse questions on some tests.