The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

ANSWERS TO REVIEW QUESTIONS OF Home Work 2

R5. Average speed = distance/time = 15km/0.5hr = 30km/hr

R6. Distance = speed x time = 25km/hr x 0.5hr = 12.5km

R8. Yes

R9. Yes it maintains a constant speed. No it does not maintain a constant velocity because it`s direction is changing.

R11. Acceleration = (change in velocity)/time = 100km/h/10s = 10km/h/s.

R12. Acceleration = 0km/h/s because it`s velocity is constant.

R20. Velocity (after 5s) = g x time = 10m/s/s x 5s = 50m/s

Velocity (after 6s) = 10m/s/s x 6s = 60m/s

ANSWERS TO EXCERISES OF Home Work 2

E3. Instantaneous speed.

E4. The speed of both airplanes are the same and equal to 300 km/h. However, their velocities are not the same - one is 300 km/h, North while the other is 300 km/h, South.

E6. Yes, the automobile in this case would be slowing down, i.e. decelerating.

E8. (a) Yes, your velocity does change because your direction changes from north to east. (b) Yes, you are accelerating because your velocity is changing. Recall Acceleration = change in velocity/change in time.

E11. No, you don`t know if both cars accelerated over the same or different periods of time.

(Recall that acceleration = change in velocity/time.)

E14. (a) Yes, if an object is moving at constant velocity then its acceleration is zero. (b) Yes, eg. if you throw a ball in the air, at the top of its flight, its speed is zero while its acceleration due to gravity is still 10m/s/s, down.

ANSWERS TO PROBLEMS OF Home Work 2

P1. From v = d/t therefore t = d/v. We convert 3m to 3000mm and therefore the time t = 3000mm/1.5mm/year = 2000 years.

P2. Acceleration = (final velocity - initial velocity)/time = (0 - 100 km/h)/10 s = -10 km/h/s

P3. Since it starts going up at 30m/s and loses 10m/s each second, its time going up is 3 seconds. Its time returning is also 3 seconds, so it`s in the air for a total of 6 seconds. Distance up (or down) is 0.5gt² = 5 x 3² = 45m.

P5. Instantaneous velocity v = gt = 10m/s/s x 10s = 100m/s, down.

Average velocity = (v_i + v_f)/2 = (0 + 100)/2 = 50m/s, down.

Distance, d = 0.5gt² = 0.5 x 10 x 10² = 500m