# Applying Newton's Laws in a Circular Motion

A child ties a rock to a rope and turns it around describing a circle. horizontal. My Classical Physics teacher has told me that the string cannot be perfectly horizontal, but I don't know why. I have tried to make a balance of forces, but I don't draw any conclusions.

$$\mathrm T=\text{tension of the rope}$$

• What software have you used to make the graphics? Sometimes I need to make figures of inclined planes and other physical systems on the computer, and I would like to know. – aprendiendo-a-programar Oct 19 '18 at 12:04
• GeoGebra. You can insert equations in $\LaTeX$. – Frobenius Oct 19 '18 at 12:21

There is gravitational force pull the rock down, therefore to main rock's position in height, the string had an angle with respect to horizon and therefore produced an upwards counter action force.

• So the shape is conical isn't it? (With its end in the child's hand) – santimirandarp Oct 18 '18 at 16:53
• @santimirandarp theoretically yes. – user9976437 Oct 18 '18 at 16:56
• @user9976437@santimirandarp So, is the trajectory a conic or a circumference? – aprendiendo-a-programar Oct 18 '18 at 18:31
• @aprendiendo-a-programar Hi, trajectory is circular – santimirandarp Oct 23 '18 at 6:40

Draw a force diagram at any one instant: There are only 2 forces acting on the rock, gravity which points down vertically, and the tension force from the string. Look at the vertical direction. We want the rock to be making horizontal circles so it can't be moving in the vertical direction. This means the forces in the vertical direction must be balanced. If the string is perfectly horizontal there can't be a force to balance the force of gravity in the vertical direction because there are no other forces at play (neglecting the air). The string tension must have at least a vertical component that cancels out the force of gravity. No matter how strong the string tension is, there's no way for a purely horizontal force to cancel out a purely vertical one.

• This is the same as the previous answer. It'd be better only with some calculations or a graph at least...btw "there is only two forces" or "there are only ..."? – santimirandarp Oct 18 '18 at 17:10
• @santimirandarp I feel like my answer is more explicit than previous answers. If you feel it's a poor answer, you are of course free to downvote. – enumaris Oct 18 '18 at 17:12
• No, of course not. Because I like it I comment it. I just think it can be improved... – santimirandarp Oct 18 '18 at 17:16
• @santimirandarp unfortunately, I do not currently have access to the tools needed to actual draw a figure, I was hoping my description was descriptive enough for the OP to be able to visualize it. – enumaris Oct 18 '18 at 17:17

Your teacher is correct about it, since the ball is in vertical equilibrium and hence forces are balanced in the vertical direction meaning that a component of tension balances the weight of the rock implying that the the string must be inclined at some angle however small the angle may be.