Block moving on a frictionless loop

[phosgene]

Homework Statement

A block moves on the frictionless slide shown in the diagram. What is the minimum height from which the block can be released if it is to complete the loop without leaving the surface of the slide? (Assume that the dimensions of the block are small compared to R, and that there is no energy associated with rotational motion of the block.) Hint: Think about the minimum speed of the block at the top of the loop if it is to remain in contact with the surface.

Homework Equations

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The Attempt at a Solution

I have no idea at all! I mean, for the block to stay in contact with the top of the loop, it would need some kind of force to counteract gravity. Even if it was moving really fast, wouldn't the slightest influence of gravity pull it away from the loop? But I can't think of any force acting against gravity. Well, I'm pretty sure that this reasoning is wrong, because I've seen something like this done in real life. So I'm totally stuck..

 

[SHISHKABOB]

haven't you ever seen a rollercoaster go around a loopdeloop? Or swinging water over your head in a bucket and it not coming out? What force is applied to something that is moving in a circle?

 

[PhanthomJay]

But I can't think of any force acting against gravity.

There does not necessarily have to be a force acting against gravity at the top of the loop if the object is accelerating vertically downward at that point. Is it?