1. A 1300-kg car moving at 20m/s hits the fence and comes to a stop in 2 s. what is the average forced acted to the car?
2. A lady tennis player hits an approaching ball with a force of 750 N. If she hits the ball in 0.002, how much impulse is imparted to the tennis ball?
Answers
Answer:
I'm just confused because according to newton's third law of motion, " any action has an opposite but equal reaction", hence forces in nature are balanced, and balanced forces means bodies are at rest, so how come in reality, we can observe motion?
ANSWER:
You do not understand the third law. If there are two objects, A and B, and A exerts a force on B, Newton's third law says that B exerts a force of equal magnitude and opposite direction on A. So, you see, the two forces are not on the same object and are therefore not "balanced".
QUESTION:
If a man was to be proportionately increased in size, would it be more difficult for him to perform a pull-up? All his body percents are exactly the same including his muscle, fat, ability, ect. The pull-up bar is also high enough that his feet would not touch the ground in either situation.
ANSWER:
Scaling of biological organisms has been a topic of interest for a long time. I think Leonardo de Vinci was the first to think about it. Anyhow, I think the answer to your question is that it would be harder to do a pullup. Here is why: The weight of something is proportional to the cube of its dimensions. So, if you doubled all the dimensions of a man, his weight would increase eightfold. But, the strength of bones or muscles is dependent on their cross sectional areas, a bone or muscle doubled in size would have about four times the strength.
QUESTION:
I am confused if I put a book from ground to a table at height 'h' ,note that the book is at rest before and after putting , total work done on book is 0, as change in k.e. Is 0 ,therefore total transfer of energy on book should also be 0 but after putting the book on the table at height 'h' the book gain a p.e. Of 'mgh'.how?
ANSWER:
Yes, you are confused! The idea of a potential energy function is that it keeps track automatically of the work done by a force always present in a particular problem. The gravitational potential energy represents the work done by the weight mg of an object. So, if you use potential energy, you do not count the work weight does, that is ΔK+ΔU=Wext where Wext is the work done by all forces other than gravity, in your example that is you. So, to lift the book you must exert a force upward of magnitude mg for a distance h and so the work you do is Wext=mgh; since kinetic energy has not changed, ΔU=mgh.