We claim that momentum is conserved, yet most moving objects slow down and stop. Explain.
For momentum to be conserved, the system under analysis must be closed and defined. No forces can act from outside the system. A coasting car has air friction and road friction on it, which are outside forces. But if they were considered part of the system, and their velocities analyzed, the momentum of the entire system would be conserved but not necessarily of any single component.
When a person jumps from a tree to the ground, what happens to the momentum of the person upon striking the ground?
Consider this problem as a very light object hitting and sticking to a very heavy one. The large object-small object combo would have some momentum after the collision but due to the very large mass of Earth, the velocity of the combo is not measurable. Jumper lands on Earth, and nothing more.
When you release an inflated but untied balloon, why does it fly across the room?
When you release an inflated but untied balloon at rest, the gas inside the balloon rushes out the open end. That escaping has and the balloon form a closed system and so the momentum of the system is conserved. The balloon and remaining has acquires a momentum equal and opposite to the momentum of the escaping was and so move in the opposite direction of the escaping gas.
It is said that in ancient times, a rich man with a bag of gold coins froze to death while stranded on a frozen lake. Because the ice was frictionless, he could not push himself to shore. What could he have done to save himself had he not been so miserly?
If the rich man would have faced away from the shore and thrown the bag of coins directly away from the shore, he would have acquired a velocity towards the shore by conservation of momentum. Since ice is frictionless, he would slide all the way to the shore.
How can a rocket change direction when it is far out in space and is essentially a vacuum?
When a rocket expels gas in a given direction, it puts a force on that gas. The momentum of the gas rocket system stays constant, so if the gas is pushed to the left, the rocket will be pushed to the right due to Newton's third law. So the rocket must carry some kind of material to be ejected to change direction.
The longer the impact time of an impulse, the smaller the force can be for the same momentum change and hence the smaller the deformation of the object on which the force acts. On this basis, explain the value of air bags which are intended to inflate during an automobile collision.
The airbag greatly increases the amount o time over which the stopping force acts on the driver. If a hard object like a steering wheel stops the impact, the impact is short and strong. If an airbag stops the driver, then a much smaller force is exerted over a much longer time. It can increase the time of stopping by a factor of 10.