## Friday, August 22, 2008

### AP Physics & Degree Entrance Kinematics- Questions on One Dimensional Motion

The following questions numbered 1, 2 and 3 are based on the velocity-time graph of a particle in one dimensional motion shown in the adjoining figure.
(1) The displacement of the particle during the first second of its motion is nearly
(a) 0.25 m
(b) 0.5 m
(c) 1 m
(d) 1.5 m
(e) 2 m
The area under the velocity-time curve gives the displacement. The portion of the graph for the interval from zero to 1 second is straight and the area under the curve is triangular and is equal to (½)×1×4 = 2 m.
(2) The average accelerations during the 3rd second and 7th second respectively are nearly
(a) 0.5 ms–2 and 2 ms–2
(b) 1 ms–2 and –2 ms–2
(c) 1 ms–2 and 2 ms–2
(d) 1 ms–2 and –2 ms–2
(e) zero and –2 ms–2
The 3rd second is the interval from 2 seconds to 3 seconds and during this time the velocity increases from 7 ms–1 to 8 ms–1. The acceleration is therefore (8 – 7)/1 = 1 ms–2.
The 7th second is the interval from 6 seconds to 7 seconds and during this time the velocity decreases from 8 ms–1 to 6 ms–1.
The acceleration is therefore (6 – 8)/1 = – 2 ms–2. [Option (b)]
(3) Which one among the following acceleration–time graphs most closely represents the motion of the particle?
The acceleration is positive and uniform initially. Afterwards the acceleration becomes zero (since the velocity remains constant) for some time and then becomes negative (since the velocity goes on decreasing) and uniform. The curve shown in (b) therefore represents the motion of the particle.
Let us leave the velocity time graph here and consider a couple of different questions:
(4) Two boys running at uniform speeds v1 and v2 respectively along a straight line path in opposite directions get 9 m closer each second. While running along the same direction with their speeds reduced by 50%, they get 0.5m closer each second. The speeds v1 and v2 are respectively
(a) 6 ms–1 and 3 ms–1
(b) 5 ms–1 and 4 ms–1
(c) 5 ms–1 and 4.5 ms–1
(d) 5 ms–1 and 3.5 ms–1
(e) 5.5 ms–1 and 3.5 ms–1
This is a simple question involving relative velocity. Wile running along opposite directions, we have
v1+ v2 = 9
While running along the same direction, we have
v1/2 v2/2 = 0.5, from which v1v2 = 1
Solving the above equations, we obtain v1 = 5 ms–1 and v2 = 4 ms–1
(5) The rear end of a train running on a straight track with uniform acceleration has velocities 6 ms–1 and 10 ms–1 respectively when passing points A and B in its path. The velocity of the rear end midway between these points is approximately
(a) 7 ms–1
(b) 7.5 ms–1
(c) 8ms–1
(d) 8.2 ms–1
(e) 8.4 ms–1
We have v2 = u2 + 2as where u and v are the initial ans final velocities respectively, a is the acceleration and s is the displacement.
If the distance between A and B is s, we have
102 = 62 + 2as from which 2as = 64
If v1 is the velocity midway between A and B we have
v12 = 62 + 2a(s/2) = 62 + 32 = 68
Therefore, v1 = 8.2 ms–1 nearly.
You will find more questions (with solution) on one dimensional motion and other sections at AP Physics Resources

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