Physics 高手請進

2010-05-09 10:20 am

1.We have to pedal to keep a bicycle moving at a constant velocity.Does this violate Newton's first law? Explain your ans.

2.Simon is trying to carry too many thing with his car. When the car starts to move forwards, the things fall off at the back.Why?

3.Tony and his fat brother Anthony are having a toboggan ride on an ice surface.
a)If the friction between the toboggans and the ice is negligible, what does Newton's first law predict about their motion? What will happen in reality?
b)The brothers have the same velocity. If they try to slow down using their ski poles,who will find is harder to do so? Why?

4.a)Steering a supertanker is rather different from steering a Star ferry. A supertanker needs a few kilometers of distance to stop or to change direction,while a Star Ferry seldom needs more than50m. Explain this in terms of inertia.
b)(1)A supertanker and a Star Ferry travel steadily at the same velocity. Which of them is acted on by a lager net force?
b)(2)The engine of the supertanker in fact provides much larger propelling force.Does this contradict your answer to (1)?

5.a)Why do ships have a V-shaped from?
b)What is the function of the parachute when a space shuttle lands?
c)Swimmers usually wear swimming caps when they compete.Why?
d)Why do the grips of a bicycle handle have to be rough?

6.Peter takes a lift from the ground floor to the 10th floor. The first accelerates from rest at 1.5m s^-2 for 2 s.It then rises at a constant velocity for 10 s and finally decelerates to a stop in 2.5 s.
The mass of Peter is 75kg .Sketch a graph of his apparent weight against time and describe how his felling of his weight changes during the ride.

回答 (1)

天同

2010-05-09 6:14 pm

✔ 最佳答案

1. No. Force is needed to against friction. The net force on the bicycle is zero.

2. It is because of inertia. All his stuff tend to stay at rest.

3. (a) Newton's first law predicts that they would move at constant speed.
They would still slow down in reality because of air resistance.
(b) Anthony would find it harder to stop, because of his large momentum.

4. (a) The supertanker, having a alrge mass, possesses larger inertia than a Star Ferry. Hence, the supertanker tends to stay in its state of motion more than a Star Ferry does. This makes it difficult to stop the supertanker or change its direction.
(b) (i)The net force acting on them are the same, and are both equal to zero.
(ii) Although the force provided by the engine of the supertanker is large, the water resistance force acted on the supertanker is also large. To achieve a constant speed, the net force acting on the supertanker needs to be zero.

5. (a) To reduce water resistance.
(b) To increase air drag, which helps to slow down the space shuttle.
(c) To redice water resistance.
(d) To provide larger friction so that slipping between the hands (when wetted with sweat) and the handles would not occur.

6. Let R be the normal reaction acting on Peter (which is the same as his apparent weight).
During acceleration, R - 70g = 70 x 1.5
hence, R = 805 N (take g, the acceleration due to gravity, to be 10 m/s2)
Speed attained after 2 s = 1.5 x 2 m/s = 3 m/s
hence, during rising up at constant speed, R - 70g = 70 x 0
i.e. R = 70g N = 700 N
During decceleration,
decceleration = 3/2.5 m/s^2 = 1.2 m/s^2
hence, 70g - R = 70 x 1.2
R = (70g - 70x1.2) N = 616 N
From the above calculated results, you could plot the graph easily.

During the lift ride, Peter first felt that his weight increased during the acceleration, then return to his normal weight when the lift is going up at constant speed, and he felt his weight decreased at the decceleration of the lift until it came to a stop.

👍 0 👎 0