Would you rather land with your legs bending or stiff?

Question

Q.5 a) Would you rather land with your legs bending or stiff?
b) Why do cricket fielders move their hands backwards when catching a fast ball?
c) Why do railway carriages have dampers at the front and back?
Q.6 In the lecture demonstration you saw that there are 3 balls of equal mass placed in a straight line. The first strikes the second and comes to rest. The second moves and hits the third and comes to rest. The third one then moves off alone.
a) What would happen if the second one was twice the mass of the first or the third, and if the collision was elastic?
b) What would happen if the second was infinitely heavy, and the collision was elastic?
c) What would happen if the collision of the first and second resulted in these two getting stuck to each other? With what speed would they strike the third one?

Answer 1

sorry I'm not doin your work for you.

Answer 2

Q5a When you hit the ground with stiff legs, the force of compression in your thighs can actually reach the breaking tension the bones can take. Meaning you break your legs.

That is why when you are skiing, the instructor yells, "Bend your knees" so that you will land in such a way that the body bends a little more due to your knees being bent.

You have to remember this when you parachute from a plane too. Land with your knees bent. You are coming down with velocity and this makes the momentum of landing have a greater effect on compressing the thigh-bones. And land on two legs so they average the force of impact.

b) and c) has to do with shock absorbers. Moving the hands backwards when catching a fast ball is so the ball doesn't deliver a karate chop onto the hand to break it! Let the force of the hand brake the ball over a longer distance.

Q6 deals with the law of conservation of momentum. This is a set of easy and fun questions.
Play around with three masses each m, and then call the initial velocity of the first mass V.
Remember that the law of conservation of momentum refers to the whole system and not just one ball.
Kinetic energy is lost when the collision is not perfectly elastic.

This question is usually solved by the two equations for a perfectly elastic collision. One equation for momentum conservation, and the other equation for kinetic energy conservation.

It becomes awful only if one moving body is catching up with another moving body to collide. Mathematically messy. Here the target masses are thankfully at rest.

6(b) Ask yourself if any force can move an infinite mass. This kind of question is where momentum conservation breaks down. Why? The law of conservation of momentum refers to an enclosed system of colliding bodies. It is impossible to consider an infinite mass within the boundaries of an enclosed system. So the law doesn't hold.

Initial momentum of the colliding body is changed into a momentum equal in magnitude but in the opposite direction. This is to say the mass bounces off with the same speed but in the opposite direction.

If you can enclose a very large object, then it must have finite mass. Then, in taking part in the collision it has to move a very, very small but still finite distance and acquire a finite but very, very small velocity.

Two things that get stuck during a collision will always result in an inelastic collision. Kinetic energy will always be lost in this collision. Remember that momentum is always conserved -- even for inelastic collisions.

Answer 3

5. a. bending to absorb the impact of the landing with your legs by creating a downward force with them slowing down your body before you hit the ground completely
b. by moving their hands with the ball they reduce the difference in velocity that the ball must undergo (or at least the distance over which it undergoes the change). This reduces the elastic force of the collision and makes it easier for them to catch the ball.
6. a. the second ball would move at half the speed of the first (momentum: p=mv), but the third ball would move at the same speed as the first assuming the second ball comes to rest after striking the third.
b. the second one would move infinitely slowly (not at all)
c. assuming that they are all the same weight again then (mv=2mv') the pair would move at half the speed and the third ball would move at the original speed.

Answer 4

Don't land rigidly, bending the knees will absorb impact and disipate force/shock. That is all, get a textbook you cheeky mare.