物理问题.最好能用英文回答.中文也没关系.

物理问题.最好能用英文回答.中文也没关系.
以下链接里的问题.http://zhidao.baidu.com/question/72814482.html

1.If an object has no acceleration,can you conclude that no forces are exerted on it?Explain
Ans:According to the second Newton's law,if an object has no acceleration,then we can conclude that the sum of external forces exerted on it is 0.
2.What is the acceleration of a rock at the top of its trajectory when thrown straight upward?Explain whether or not the answer is zero by using the equation a=f/m as a guide to your thinking.
Ans:the acceleration is equal to -g
3.A rocket fired from its launching pad not only picks up speed,but its acceleration also increases significantly as firing continues.Why is this so?( hint:about 90% of the mass of a newly launched rocket is fuel)
Ans:the velocity of the rocket increases is because of the acceleration.Assume that the driving force remains unchanged,then because the mass of rocket decreases,the acceleration is also increasing.
4.When blocking in football,why does a defending lineman often attempt to get his body under that of his opponent and push upward?What effect does this have no the friction force between the opposing lineman’s feet and the ground?
Ans:according to the formular of friction force f=μN ,so in that way our player can reduce N and finally reduce the friction force f.
5.A horizontal force of 100N is required to push a crate across a factory floor at a constant speed.What is the net force acting on the crate?What is the force of friction acting on the crate.
Ans:since the crate moves at a constant speed,the net force acting on the crate is 0.the friction force is equal to the horizontal force of 100N.
6.Why is easier to walk on a carpeted floor than on a smooth,polished floor?
Ans:the fraction coefficient of a carpeted floor is much more than the fraction coefficient of a smooth polished floor.So the friction force provided by the carpeted floor is much larger than the smooth floor.
7.Suppose you’re weighing yourself while standing next to the bathroom sink.Using the idea of action and reaction,explain why the scale reading will be less when you push down on the top of the sink.Why will the scale reading be more if you pull up on the bottom of the sink.
Ans:(doing the force analysis,it is very easy to explain.)When you push down on the top of the sink,the reaction force on you is upwards,then the force on the scale is less.And vice versa.
8.What is the reaction force to an action force of 1000N exerted by the earth on an orbiting communications satellite?
Ans:the amount of reaction force is equal to the action force,and it is also 1000N.
9.A speeding bus makes contact with a bug that splatters onto the windshield.Because of the sudden force,the unfortunate bug undergoes a sudden deceleration.Is the corresponding force that the bug exerts against the windshield greater,less or the same?Is the resulting deceleration of the bus greater than ,less than,or the same as that of bug.
Ans:the corresponding force on the bus is the same.but the deceleration of the bus is much less than that of bug.(because the mass of bus is much larger than the mass of bug.)
10.A pair of 50-N weights are attached to a spring scale as shown.Does the spring scale read 0,50 or 100N?(hint:would it read any differently if one of the strings were held by your hand instead of being attached to the 50-N weight?.)
Ans:Sorry I cannot see the figure.but I think that the spring sacle reading is also 50N.
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