velocity sign in SHM?? (1 Viewer)

[school4nerds]

Hey, with the 2000 q4 d) the question is 'a particle is moving in SHM about a fixed point O. Its amplitude is 3cm and its period is 4pi seconds.

find its speed at point O'.

i got v= +/- 3/2 cm/s

however the answer is only +3/2 cm/s??

i seem to always get this little bit wrong, how do you know whether the sign is supposed to be positive or negative?

 

[arrogant]

if it asked for speed, then assume positive. Speed is not a vector quantity, while velocity is

 

[school4nerds]

if it asked for speed, then assume positive. Speed is not a vector quantity, while velocity is

thanks!

 

[kman16]

As mentioned above if it asks for speed then you take the positive value as speed is not a vector quantity. However sometimes it can be confusing when they ask for velocity and you have two solutions: one positive and one negative.
In this case you choose the one that matches the initial conditions.
E.G:
Q: If v^2 = (x-1)^2 Find v in terms of x. Initially the particle is at the origin moving at v=1.

A: v = +- (x-1) But now we need to choose whether we take the positive or negative solution.
Taking positive and subbing in initial conditions: (1) = (0-1). Which is false.
Taking negative and subbing in initial conditions: (1) = -(0-1).
= 1. Which is true

Therefore v = -(x-1)

 

[school4nerds]

As mentioned above if it asks for speed then you take the positive value as speed is not a vector quantity. However sometimes it can be confusing when they ask for velocity and you have two solutions: one positive and one negative.
In this case you choose the one that matches the initial conditions.
E.G:
Q: If v^2 = (x-1)^2 Find v in terms of x. Initially the particle is at the origin moving at v=1.

A: v = +- (x-1) But now we need to choose whether we take the positive or negative solution.
Taking positive and subbing in initial conditions: (1) = (0-1). Which is false.
Taking negative and subbing in initial conditions: (1) = -(0-1).
= 1. Which is true

Therefore v = -(x-1)

thanks that was helpful

 

[kayven]

If we put + and - in the exam, do we lose marks?