Phy Qs. !!Urgent..pls help!

2008-10-12 12:25 am

1.If an object falling freely were somehow equipped with an odometer to measure the distance it travels, then the amount of distance it travels each succeeding second would be
a. greater than the second before.
b. doubled.
c. less and less each second.
d. constant.

2.At one instant a heavy object in air is moving upward at 50 meters per second. One second later its speed in meters per second is approximately
a. 40 m/s.
b. 50 m/s.
c. 60 m/s.
d. 55 m/s.

3.An apple at rest weighs 1 N. The net force on the apple when it is in free fall is
a. 0 N.
b. 1 N.
c. 0.1 N.
d. 9.8 N.
e. none of these

4. car traveling at 100 km/hr strikes an unfortunate bug and splatters it. The force of impact is
a. greater on the car.
b. greater on the bug.
c. the same for both.

5.When a rock thrown straight upwards gets to the exact top of its path, its
a. velocity is zero and its acceleration is zero.
b. velocity is about 10 m/s and its acceleration is zero.
c. velocity is about 10 m/s and its acceleration is about 10 meters per second per second.
d. velocity is zero and its acceleration is about 10 meters per second per second.
e. none of these

回答 (1)

Audrey Hepburn

2008-10-12 12:32 am

✔ 最佳答案

1. The answer is A.

By the equation of motion, v = u + at = u + 10t

As for successive seconds, the value of u increases, hence v increases with time. By definition, v speed is the rate of change of distance. Hence, the distance travelled must be greater than the second before.

2. Take upward direction be positive.

By v = u + at

v = 50 - 10(1)

v = 40 ms-1

The answer is A.

3. The answer is B.

The net force on the apple when it is in free fall is its weight, which is the same as it is at rest.

4. The answer is C.

By Newton's 3rd law, the force exerted on the car by the bug and the force exerted on the bug by the car are action and reaction pair. They have the same magnitude but opposite in direction. Hence, the force of impact is the same for both.

5. The answer is D.

When the rock is at it's maximum height, the instantaneous velocity is zero, but the acceleration is 10 ms-2 (Acceleration due to gravity).

參考: Myself~~~

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