First Exam Answers

1.
Objects moves from 0 to 3 meters in first 2 seconds so v = 3/2 m/sec.

Then from 8 to 10 the object moves from 4 to 8 meters so v = 4/2 =2 m/sec.

2.
The slope of the line is continuously changing so the object must be under constant acceleration as the velocity is continuously changing.

3. Object A moved a total of 8 meters in 10 seconds so its velocity is v = 0.8 m/sec.

Object B moved a total of -3 meters in 10 seconds so its velocity is v = -0.3 m/sec.

4. If the velocity of object A is remains constant, a 2 m/sec then its position will use be given by

x(t) = x(o) + vt where x(o) = 8 meters at t = 10 seconds.

so, 5 second later it will have moved (2 m/sec)*5 sec = 10 meters farther and be at position 18 meters.

20 seconds later it will be at position 48 meters.

5. Objects C and D return to their starting position.

6. Object A (see above) comes to rest twice.

7. Hardest question on exam. The instanaeous velocity is the tangent to the curve in the position-time diagram. For objects of constant velocity, the instanaeous velocity and average velocities are equal. For object D, as the object approaches position 8 meters the slope of the line is steeper there than in any of the other cases. Inspection of the graph shows the object moved 2 meter in about 1/3 second.

8. Object D when a total of 16 meters.

9. In both cases the object is in motion under constant acceleration and so the velocity is constantly changing so the position -time diagram is never defined by a straight line.

10. Acceleration is the slope of the line in a velocity vs time graph. Object I has a steeper slope than Object II and therefore has greater acceleration.

11. Impulse if force * time. Object II experienced larger force as a larger time was involved in producing the acceleration. You know that it experienced a larger force because its velocity is higher. This is equivalent to the fan cart and the difference betweeen 0.5 second and 1 seconds of fan action.

12. 6/15 = 1:2.5 d goes as t² for free fall. So if t is twice as long, object falls 4 times more so it would fall t0 60 meters in 2.2 seconds.

13. As demonstrated in class, friction does not depend on contact area.

14. An acceleration produces a change in velocity and is a manifestation of a force.

15. Mass is a measure of resistance to force.

16. v = v_o + at ; initial velocity is zero; object falls for 10 seconds under constant acceleration of 10 m/sec^-2 so it would have a velocity of

10 m/sec^-2 * 10 sec = 100 m/sec

17. More force per unit area keep thinking about what force per unit area is.

18. v = v_o + at; a = 2.0 m/sec^-2 so in 3 seconds the cart would be going 6 m/sec but we observe it to be going 10 m/sec so its initial velocity would have to be 4 m/sec.

19. Transfer of energy is more easily done for baseball; clay absorbs energy and heats up and goes thud.