匿名
匿名 發問於 科學及數學其他 - 科學 · 10 年前

Some simple physics question

1.Under air resistance,why a heavier object have a higher acceleration than a lighter object?

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2.g is acceleration due to gravity.Is g a vector?Does g only indicate a value of 10ms^-2 but not direction?

3.Why a tumbler will never fall down?Explain with concept of centre of gravity.

4.Is water rocket involve action and reaction pair?How does a water rocket operate?

5.How does a brake work?Why does a car decelerate when the brake is applied?

更新:

Extended Q.1 :Then why a book falls quickly on the floor than a paper with the same size?

更新 2:

And I don't understand the explanation of Q.3

更新 3:

Before braking and during braking,is there any friction act oon the car by the road?

更新 4:

is the friction the same?

1 個解答

評分
  • 天同
    Lv 7
    10 年前
    最愛解答

    1. The statement is not always true. In fact, air resistance bepends very much on the size, shape and geometry of the object, not on its mass. For two objects of the same material but of different masses, the heavier one have larger volume, and hence larger air resistance. It would then fall slower than the lighter object because air resistance acting on it (the heavier object) is larger.

    Whether an object falls faster or slower in air is strongly affected by its size and shape.

    2. The acceleration due to gravity, g, is defintely a vector. It has both magnitude and direction. The acceleration due to gracity has a value (magnitude) of about 9.81 m/s^2 at sea level over the equator, and has a direction always pointing to the centre of the earth (i.e. vertically downward).

    3. It is because the centre of gravity of a tumbler is lower down close to the bottom. As such, when it is tilted to even a large angle, the centre of gravity will not fall outside its base. Hence, the torque produced will turn the tumbler back to its upright position.

    4. Water rocket propels because of the reaction force produced by the ejecting water. There is a pair of action-reaction forces acting.

    By ejecting water from the tail of the rocket (this is the action force), the rocket itself experiences an equal and opposite reaction force (Newton's Third Law), thus lifting up the rocket from the ground.

    5. Simply speaking, In a disc brake of a vehicle, there is a brake pad mounted on a caliper adjacent on a rotating disc fixed to the wheel. During braking, the brake pads squeeze the rotating disc. Friction between the pads and the disc slows the wheels down. The slowing down of the wheels thus bring the vehicle to a stop.

    In fact, car braking is just a transformation of energy. The kinetic energy of the car is transformed to heat by friction acting between the brake pad and the rotating disc.

    The following web-page may help:http://auto.howstuffworks.com/auto-parts/brakes/br...

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