F.4 physics (force and motion)

1. There is no motion between an object and the surface when static frcition is acting, otherwise it becomes kinetic friction.

Static friction acts opposite to the direction that the object tends to move. When the bus moves forward, because of inertia, the man tends to remain behind. Hence, static friction acts in the forward direction.

2. An object moving at constant speed under balanced forces has no increase in kinetic energy. But it doesn't not necessarily mean that there is no work done.

An example is a force pulling an object to move at constant speed on a rough surface. The work done by the pulling force produces heat due to friction.

3. A reference level is just a convenient arbitrary level. Generally, it it the lowest level that can be attained by an object in question.

4. Your diagram is not clear enough. What does the large circle represent? Is the system in equilibrium?

5. You don't need to do any calculation. It is clearly 1:1 (oprion A). Object B is being pushed to move by object A. The two are in contact and hence possess the same acceleration.

6. In using the formula: W = F.s, the force F must be in the same direction as that of the displacement s.
In the track from A to B, F (the frictional force) changes its direction at the bottom of the slope, varies its direction continuously at the circular track, and finally at the level track. Therefore, you cannot simply use the displacement from A to B (i.e. a straight line joining A and B) in calculating the work done by friction.

Since friction is always in direction opposite to the motion (the velocity), the displacement s under such situation follows the actual path covered by the roller coaster.




2013-04-05 07:43:14 補充：
Your suppl questions:
4. If the two circles represent smooth pulleys, the system wouldn't be in equilibrium. The weight would fall.

2013-04-05 07:47:46 補充：
Your suppl question:
5. The 120N force is applied to A, NOT to B. The force that A exerts on B is smaller than 120N. The net force (i.e. 120N minus the force B exerts on A) gives an acceleration to A.

2013-04-05 07:49:30 補充：
see "opinion section" for answer to Q5

2013-04-05 07:53:20 補充：
Two objects linked together (that means they would not be separated physically) can be considered as a single object. Naturally, they have the same velocity and acceleration.

2013-04-05 07:57:03 補充：
A daily example is a MTR train. A train is composed of 10 individual train-cars linked together. Each train-car has the same acceleration and velocity. If not, you could imagine what would happen.

i am busy preparing dse now, so i don't have time to ans all your question(and the reward is 5pt only--no offense), but i would still like to help you a bit.

1) i think you should think about why a person accelerates when the bus move, it's just because of friction! 1. the bus moves relative to the person, a friction f is exerted on the bus floor by the person (and is in a direction which is opposite to the motion of bus) 2. by newton's 3rd law, a force is exerted on people in oppoiste direction of f (which is in the same direction as the motion of bus) 3. the person moves together with the bus

a bit messy, hope u understand...

2)as net force=0>> no work done

3)i think it is not important as usually we find our ans by using increase/decrease in PE which is the same , no matter a reference point is considered or not

2013-04-04 01:03:27 補充：
1. the bus moves relative to the person
(because the person doesn't move at the beginning)

1)由於 物體(人) 與 正在加速的物體在"同一個參考系統(System)"上,因此沒有相對 運動(Relative motion).若沒有相對運動的趨勢,則不會產生摩擦力(Friction).

2)於此時因為沒有淨力(Net force),根據公式 W = F x d ,也沒有作功(Work).

3)......

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