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M1 Help

7 A particle P of mass 0.5 kg moves upwards along a line of greatest slope of a rough plane inclined at an angle of 40? to the horizontal. P reaches its highest point and then moves back down the plane. The coefficient of friction between P and the plane is 0.6.

Acceleration up the plane = -10.8 m/s^2
Acceleration down the plane. = -1.8 m/s^2

When P is moving up the plane, it passes through a point A with speed 4m/s.

(a) Find the length of time before P reaches its highest point. = 0.370s

(b) Find the total length of time for P to travel from the point A to its highest point and back to A.

Can anyone explain part b of this? The answers say you have to calculate the displacement first. Why can you not just find v = u + at for the time to travel back to A, and add it to 0.37?

Reply 1

Daniel Freedman

Does it pass A at time 0?

Reply 2

steve10

well, going back you have v = u + at, but all you know is a = -1.8 m/s^2 and u = 0. You don't know either v or t. So find the distance travelled 1st of all, then use that.