# Simple pendulum question help needed

1. Nov 19, 2008

### lmlgrey

Simple pendulum question.. help needed!!!!

http://capa.mcgill.ca/res/mcgill/dcmcgill/oldproblems/msuphysislib/Graphics/Gtype26/prob34a_pendSHM1.gif1.[/URL] The diagram shows a simple pendulum consisting of a mass M suspended by a thin string. The magnitude of the tension is T. The mass swings back and forth between ±q0 Choose True or False

True False T is greater than Mg for q = q0.
True False T equals Mg when q = q0.
True False T=Mg at some angle between zero and q0.
True False The vertical component of tension is constant.

2. is it true that T = mg*cos(theta) always?

The vertical component of tension is constant .... true, since the vertical component=force of gravity
T is greater than mg when q=q0. .... false, since q0 is the apex, and T is the smallest when the ampliude is the greatest

I dont know about the other two...especially: T=Mg at some angle between zero and q0
my guess at that question is that since T= Mgcos(theta), therefore, T will only equal to mg when theta=0 or 180? so it would be false?

Last edited by a moderator: Apr 23, 2017
2. Nov 20, 2008

### tiny-tim

Hi lmlgrey!

Don't guess!

Physics is equations, so always find the right equation, and follow it through.

In this case, the right equation is Newton's second law, F = ma.

Most of your answer is wrong, because you've ignored the ma part.

Try again!