Momentum Short Answer Questions
Explain the differences between elastic and inelastic collisions.
-Elastic-KE and momentum are both conserved; no energy is lost -Inelastic-the objects stick together. Momentum is conserved but KE is not
Explain the differences between impact and impulse as they relate to momentum.
-Impact-just the force (N) acting (do not have to change the motion of a body) -Impulse-the force in a finite amount of time changed the motion of a body (F(delta)t=(delta)p) & (mvf-mvi)
Two train cars that look the same collide. One was originally at rest. They stick together and move off at more than half the original velocity of the moving train car. What can you say about the mass (contents) of the two trains and everything you know about the collision? Repeat this question but change to "move off at half the original velocity" and "move off at less than the original velocity".
-Inelastic collision, momentum is conserved, total energy is conserved but some KE (motion) is converted to heat -the train at rest has less mass than the train that hits from behind -they have the same mass -the train at rest has more mass than the train that hits from behind.
Why can a batter hit a pitched baseball further than a ball tossed in the air by the batter?
-The pitched ball is moving toward the bat almost horizontally so it has horizontal momentum. The bag is swung at the ball adding impulse. The bat changes the direction of the original horizontal momentum plus the added momentum by swinging at the ball allows it to travel farther. -The ball tossed into the air has zero initial horizontal momentum. The momentum you can give it is by (F(delta)t=(delta)p). This is less momentum so the ball will not go as far.
When a person jumps from a tree to the ground, what happens to the momentum of the person upon striking the ground? What happens to the kinetic energy?
Consider the system to be a light person hitting a very heavy ground. The momentum of the person (p=mv) is transferred to the ground. The earth doesn't move. The kinetic energy is converted to other forms of energy such as sound and mostly heat. The total is always conserved. This is an inelastic collision.
Cars used to be built as rigid as possible to withstand collisions. Today, though, cars are designed to have 'crumple zones' that collapse upon impact. What is the advantage of this new design?
During a collision an outside impulse acts on the car and changes the car's momentum. The change in the momentum of the car remains the same whether there are crumple zones or not. The crumple zone increase the time during which the force exerted by the outside object acts. The increase in time results in a smaller average force acting on the car.
You are sitting at a baseball game when a foul ball comes in your direction. You catch it bare-handed. In order to catch it safely, should you move your hands toward the ball, hold them still, or move them in the same direction as the moving ball?
F(delta)t=(delta)p. Same direction as ball as moving to increase time to decrease the force of the ball. Increase time to decrease force.
We claim that momentum is conserved, yet all moving objects eventually slow down and stop. Explain.
For momentum to be conserved, the system under analysis must be closed (no forces outside system acting on it). Friction is an outside force acting on a system which we cannot remove so all objects eventually come to rest.
Describe a collision in which all kinetic energy is lost, yet the total energy is conserved.
If you have two objects that are moving in opposite directions with the same momentum (p=mv) and have a totally inelastic collision they will stick together. The momentum is conserved and will be zero. Since they are not moving after the collision the kinetic energy (of motion) is transferred to mostly heat. The total energy is always conserved. There can be no motion after the collision.
According to the impulse formula (F(delta)t=(delta)p), the longer the impact time of an impulse, the smaller the force can be for the same momenutm change, and hence the smaller 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 auto collision and reduce the possibility of fracture or death.
The air bag greatly increases the amount of time over which the force acts on the driver to the same change in momentum. Air bags can increased the time approximately 10x so this can decrease the force approximately 10x. (the force does the damage to the person).
Is it possible for an object to receive a larger impulse from a smaller force than from a larger force? Explain.
Yes, impulse is the product of the force of impact times the time the force is impressed to change the momentum. The impulse from a small force applied over a long time can be larger than the impulse applied by a large force over a small time.
a. The white cue ball travels across a pool table and collides with the stationary 8-ball. The two balls have equal mass. After the collision the cue ball is at rest. What is everything you know about this collision? b. What if, after the collision, each ball has some of the original momentum?
a. This is an elastic collision, the KE and momentum are conserved, it is a head on collision since the cue ball is at rest, no heat generated, and there is no deformity in either object. (Assume that there is enough friction to 'hold' the balls in place, same mass) b. This is an elastic collision, it is a glancing blow, they hit at some angle other than head on they each have momentum after the collision and move away from each other at a right angle. there is no deformity, KE and momentum are both conserved. (assume same 2 things)