1) In a projectile motion the velocity
a) is always perpendicular to the acceleration
b) is never perpendicular to the acceleration
c) is perpendicular to the acceleration for one instant only
d) is perpendicular to the acceleration for two instants

2) Figure shows the displacement -- time graph of a particle moving on the X-axis.
a) The particle is continuously going in positive x-direction.
b) The particle is at rest.
c) the velocity increases up to a time t_{0} & then becomes constant.
d) the particle moves at a constant velocity up to a time t_{0} and then stops.

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2014-09-01T17:43:36+05:30

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1) c) is perpendicular to the acceleration for one instant only. In general the velocity has two components, horizontal and vertical. Acceleration is vertical. So there is always some angle between them. But at the top most point of flight, the velocity is horizontal and acceleration is vertical.  just at one instant

2) d) when displacement is increasing wrt time, its slope is velocity. Slope of a straight line is constant. after t0, the displacement is not changing. So position is not changing. so it is at rest.
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