Phy Qs. Need Help!!Urgent..pls help!

[匿名]

2008-10-11 11:24 pm

1.Twelve seconds after starting from rest, an object falling freely will have a speed of
A. 10 m/s.
B. 100 m/s.
C. 50 m/s.
D. more than 100 m/s.

2.If a projectile is fired straight up at a speed of 10 m/s, the total time to return to its starting position is about
A. 10 seconds.
B. 20 seconds.
C. 1 second.
D. 2 seconds.
E. not enough information to estimate

3.A ball is thrown upwards and caught when it comes back down. In the presence of air resistance, the speed with which it is caught is always
A. the same as the speed it had when thrown upwards.
B. more than the speed it had when thrown upwards.
C. less than the speed it had when thrown upwards.
D. impossible to determine.

4.A rock is thrown vertically into the air. At the top of its path, its acceleration in meters per second per second is
A. zero.
B. greater than 9.8.
C. between 0 and 9.8.
D. 9.8.

5.A vehicle that weights 400 N on the surface of the Earth is traveling in outer space at a speed of 400 m/s . It can be stopped by applying a constant force of 20 N for
A. 80 seconds.
B. 2 seconds.
C. 800 seconds.
D. 400 seconds.
E. 4 seconds.

回答 (1)

Audrey Hepburn

2008-10-12 12:03 am

✔ 最佳答案

1. By v = u + at

v = 0 + 10(12)

Speed, v = 120 ms-1

The answer is D. more than 100 ms-1.

2. By s = ut + 1/2 at2

0 = 10t + 1/2 (-10)t2

t2 - 2t = 0

t = 2s or 0 (rejected)

So, the answer is D. 2 seconds.

3. The answer is C. less than the speed it had when thrown upwards.

In the presence of air resistance, when the ball is moving downward, the air resistance acts upward, which is in the opposite to the direction of the weight of the ball. Hence, the net force acting on the ball is less than the weight of the ball (In the case that air resistance is absent)

Hence, the ball has a smaller acceleration when it is moving downward.

4. The answer is D.

At the top of the path, the instantaneous speed of the rock is zero, but not acceleration. The acceleration is still the acceleration due to gravity.

5. Mass of the vehicle = 400 / g = 400 / 10 = 40 kg

By Newton's 2nd law of motion

F = ma

-20 = 40a

Acceleration, a = -0.5 ms-2

By v = u + at

0 = 400 - 0.5t

Time, t = 800 s

The answer is C.

參考: Myself~~~

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