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# Mechanics Watch

1. A ball, attached to the end of an elastic string, is moving in a vertical line. The motion of the ball is modelled as a particle B moving a long a vertical axis so that its displacement, x m, from a fixed point O on the line at time t seconds is given by x=0.6cos(pit/3). Find
a) the distance of B from O when t=1/2
b) the smallest positive value of t for which B is instantaneously at rest

I'm finding b) problematic.

I found dx/dt and let that equal zero to get t=3, whereas the book has t=3/2 (which is a solution to x=0) so I imagine the book may have made an error here but could somebody please check, cheers.
2. (Original post by fayled)
A ball, attached to the end of an elastic string, is moving in a vertical line. The motion of the ball is modelled as a particle B moving a long a vertical axis so that its displacement, x m, from a fixed point O on the line at time t seconds is given by x=0.6cos(pit/3). Find
a) the distance of B from O when t=1/2
b) the smallest positive value of t for which B is instantaneously at rest

I'm finding b) problematic.

I found dv/dt and let that equal zero to get t=3, whereas the book has t=3/2 (which is a solution to x=0) so I imagine the book may have made an error here but could somebody please check, cheers.
Your solution to dx/dt is correct.
3. Thanks edexcel, that's another half hour of revision time I won't get back thanks to your crap solution bank. Glad I haven't completely forgotten C3 trig though.
4. (Original post by jpepsred)
Thanks edexcel, that's another half hour of revision time I won't get back thanks to your crap solution bank. Glad I haven't completely forgotten C3 trig though.

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