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# Simple harmonic motion - positive or negative acceleration? watch

1. In questions like the following, how do you know when acceleration is positive or negative when you're only told that displacement is a certain value, since it could be in either direction of the equilibrium?

A simple pendulum is given a small displacement from its equilibrium position and performs simple harmonic motion.
Calculate the frequency of the oscillations of a simple pendulum of length 984 mm.
Calculate the acceleration of the bob of the simple pendulum when the displacement from the equilibrium position is 42 mm.

So I worked out the frequency and got 0.504 Hz which is correct, but in the second part I got -0.42, but the mark scheme says it's positive 0.42.

I used the equation a = -(2pif)^2(x)

If x is positive, it's impossible for a to be positive isn't it? Since the square of anything is positive?
2. The mark scheme is just giving the examiner the magnitude of the acceleration to check your numerical answer.
#You will not lose a mark over the minus sign. In fact, there is nothing wrong with your answer. Be careful of mark schemes, they are not model answers for students, they are notes for the examiner.
3. (Original post by Stonebridge)
The mark scheme is just giving the examiner the magnitude of the acceleration to check your numerical answer.
#You will not lose a mark over the minus sign. In fact, there is nothing wrong with your answer. Be careful of mark schemes, they are not model answers for students, they are notes for the examiner.
Ah ok, thanks!
4. (Original post by jf1994)
Ah ok, thanks!
One thing worth thinking about here, though, is if you understand the significance of that minus sign in this context.
The acceleration at that point is certainly "0.42 ms-2" but, for example, can it be determined from the information given whether that means the pendulum is speeding up or slowing down?

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