Physics: force and motion

code PLUS

2013-11-03 12:15 am

圖片參考：http://imgcld.yimg.com/8/n/HA00647229/o/20131102154728.jpg

'If a block is moving with constant velocity on a rough horizontal surface, there must be a net force acting on the block.'
Is this correct?

圖片參考：http://imgcld.yimg.com/8/n/HA00647229/o/20131102155745.jpg

What is difference of A and C?

圖片參考：http://imgcld.yimg.com/8/n/HA00647229/o/20131102154822.jpg

A stone is thrown vertically upward. Neglect air resistance.
'The acceleration of the stone decreases throughout the upward motion.'
Why is this statement wrong?

圖片參考：http://imgcld.yimg.com/8/n/HA00647229/o/20131102160616.jpg

An astronaut lands on the moon and finds that his weight is about one-sixth of that on earth.
Which of the following deductions is/are correct? (1) If he throws an object upwards on the moon, it will reach a higher level than throwing the object with the same speed on earth.
(2) If he release an object on the moon, it will take a shorter time to reach the ground than releasing the object from the same height on earth.
(3) The maximum weight he can lift on the moon is greater than on earth. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1),(2) and (3)

回答 (1)

天同

2013-11-03 7:41 am

✔ 最佳答案

1. Option D.

2. Not correct. Constant velocity imples no net force. (Newton's Second Law of Motion).

3. Option C.
A seat-belt helps to spread the impulse (product of force and time) over a longer time, thus lowering the force acted on the driver.
The curve in option A is physically impossible, as it shows the force provided by the seat-belt comes into effect after the driver has crashed onto the steering wheel of his car.

4. F = 1g.sin(30) + Ff where Ff is the downward acting frictional force
i.e. 11 = 10.sin(30) + Ff (take g = 10 m/s^2)
Ff = (11 - 5) N = 6 N

When F is withdrawn, the downward weight component 10.sin(30) is only 5 N, thus frictional force (which has a maximum of 6 N) is strong enough to balance the weight component.
The answer is thus option A.

5. The stone is only under the action of its own weight. Hence, the acceleration is g, the acceleration due to gravity, which is constant and pointing vertically downward.

6. Frictional force = (10 - 2) N = 8 N
When only the 2 N force remains, frictional force is strong enough to balance such 2 N force. Hence, the resultant force is zero.
The answer is option A.

7. Statement (1) is correct.
Use equation: v^2 = u^2 + 2gs (where g is the acceleration due to gravity)
hence, s = u^2/2g
A lower value of g on the moon indicates a larger value for s (height reached) than on earth.

Statement (2) is wrong.
Use equation: s = ut + (1/2)gt^2
hence, t^2 = 2s/g
Thus, for a given s (height), a lower g on moon indicates a longer time of falling than that on earth.

Statement (3) is clearly wrong.
The weight that a person can lift doesn't depend on location.

Therefore, the answer is option A.

👍 0 👎 0