CE Phy mechanics

SAM

2009-07-02 5:35 am

Bruce fires a bullet of mass 0.05 kg at a block of mass 1 kg on a smooth table as shown below. The bullet takes 0.1 s to be totally embedded and to stop inside the block. After the collision, the block, together with the bullet, moves at a speed of 6 m s−1.

6.What is the average force acting on the block by the bullet when the bullet embeds into the block?

A. 60 N
B. 63 N
C. 67 N
D. 70 N

14. Pulling forces of 10 N and 13 N are exerted on an object. The angle between them is 45. What is the magnitude of the resultant force?

A. 16.3 N
B. 19.2 N
C. 21.2 N
D. 23 N

20. Danny falls to the lowest point and bounces up. Consider this period of time only. Which of the following statements is/are correct?
(1) At the lowest point, the gravitational potential energy of Danny is the lowest.
(2) At the lowest point, the kinetic energy of Danny is the highest.
(3) The mechanical energy of Danny is conserved.

A. (1) only
B. (2) only
C. (1) and (2) only
D. (1) and (3) only

26. 1st statement
An object having an increasing acceleration must have an increasing velocity.
2nd statement
An accelerating object has a net force acting on it.
A.
B.
C.
D.

30. 1st statement
In an elastic collision, both kinetic energy and momentum are conserved.
2nd statement
In an elastic collision, an object must have the same speed before and after the collision.
A.
B.
C.
D.

回答 (1)

Prof. Physics

2009-07-02 5:55 am

✔ 最佳答案

6. Consider the block, By impulse = Change of momentum

Ft = mv - mu

F = (1)(6 - 0) / 0.1 = 60 N

By Newton's 3rd law of motion, the force acting on the bullet by the block and that acting on the block by the bullet are action and reaction pair.

So, the required answer is 60 N.

It is answer A.

14. By parallelogram method,

The resultant force, F

= sqrt[(10)2 + (13)2 - 2(10)(13)cos135*]

= 21.2 N

Answer: C.

20. Option (1) is correct. Taking the reference point at any level, the G.P.E. of the lowest point is still the lowest.

Option (2) is correct. It is because the loss of G.P.E. has been converted to gain in K.E. So, the K.E. should be the greatest there.

Option (3) is not necessary correct. Energy may lost during collision

So, the answer is C.

26. Answer is D.

The 1st statement is wrong.

The 2nd statement is right. It is governed by Newton's 2nd law of motion, F = ma

Whenever there is an acceleration on a mass, there must be a net force acting on it.

30. Answer is C.

The 1st statement is right. It is given by conservation of energy and momentum

The 2nd statement is wrong. The speed of the object before and after collision is not necessary the same. Just the energy and the momentum of the whole system conserve.

參考: Physics king

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