physics chapter 6 conceptual questions

consider a field of insects, all of which have essentially the same mass. A. if the total momentum of the insects is zero, what does this imply about their motion? B. if the total kinetic energy of the insects is zero, what does this imply about their motion?

A. If the total momentum of the insects is zero, this could mean 1 of 2 things. Either all of the insects are at rest or an equal amount of insects are moving opposite of each other canceling out the total momentum. B. If the total kinetic energy of the insects is zero, either all of the insects are at rest or an equal amount of insects are moving opposite of one another canceling out the energy.

an astronaut carrying a camera in space finds herself drifting away from a space shuttle after her tether becomes unfastened. if she has no propulsion device, what should she do to move back to the shuttle?

Throwing the camera as hard as she can in the direction opposite of the shuttle would result in an opposing force that would propel her back to the shuttle.

two skaters initially at rest push against each other so that they move in opposite directions. what is the total momentum of the two skaters when they begin moving?

When the skaters push away from each other, their momentum is equal but opposite, so the total momentum is zero.

given the masses of two objects and their velocities before and after a head-on collision, how could you determine whether the collision was elastic, inelastic, or perfectly inelastic?

elastic collision: KE is conserved just as momentum is. KE=0. inelastic collision: delta KE does not = 0 perfectly inelastic collision: when two bodies stick together and move as a single object with the masses added together. delta KE does not = 0. also the final velocities are equivalent

if two particles have equal kinetic energies, must they have the same momentum?

no, The squared velocity in the kinetic energy equation results in different results than momentum.

a truck loaded with sand is moving down the highway in a straight path. A. what happens to the momentum of the truck if the trucks velocity is increasing? B. what happens to the momentum of the truck if sand leaks at a constant rate through a hole in the truck bed while the truck maintains a constant velocity?

A. momentum increases. If the velocity os increasing then the momentum is increasing because P=M x V B. momentum decreases. If the mass is decreasing than the momentum is decreasing

how do car bumpers that collapse on impact help protect a driver?

Car bumpers allow the rest of the car to sink into it during a collision. The bumper helps protect a driver by slowing the car down over a longer period of time. This reduces the abrupt change in momentum.

if you jump from a table onto the floor, are you more likely to be hurt if your knees are bent or if you legs are stiff and your knees are locked?

If you jump from a table onto the floor with knees straight and locked the change in momentum is abrupt and you are more likely to get hurt than if your knees were bent. When knees are bent, they bend more during a landing as your body continues to move down a little but. This prolongs the impulse, spreading the force out over a longer amount of time.

two students hold an open bed sheet loosely by its corners to from a "catching net." the instructor asks a third student to throw an egg into the middle of the sheet as hard as possible. why doesn't the eggs shell break?

Since the bed sheets are loosely held by its corners, it has a lot of slack and the egg has a lot of room to "sink" from the moment it first touches the sheets to the moment it stops. The sheets are slowing the egg down but in a longer duration of time, eliminating abrupt changes in momentum that could potentially crack the egg.

explain how momentum is conserved when a ball bounces against a floor.

Since the velocity or the mass of the floor does not change when a ball bounces off of it, the momentum is conserved.

as a ball falls toward earth, the momentum of the ball increases. how would you reconcile this observation with the law of conservation of momentum?

The Law of Conservation of Momentum states that when the net external force acting on a system is zero, the total momentum remains constant. Since gravity, a non-zero force, is causing the ball to increase in velocity, the momentum is not conserved, so the momentum increases.

in the early 1900s, robert Goddard proposed sending a rocket to the moon. critics took the position that in a vacuum such as exists between earth and the moon, the gases emitted by the rocket would having nothing to push against to propel the rocket. to settle the debate, Goddard placed a gun in a vacuum and fired a blank cartridge from it. (a blank cartridge fired only the hot gases of the burning gunpowder.) what happened when the gun was fired?

The gun moved backwards. The system began with zero momentum because no external force was applied. Due to the Conservation of Momentum, the expelled gasses in one direction pushed the gun in the opposite direction with an equal but opposite momentum.

when a bullet is fired from a gun, what happens to the gun?

The gun recoils backwards due to the Conservation of Momentum. The gun exactly balances the forward momentum of the fired bullet.

in a collision between two soccer balls, momentum is conserved. is momentum conserved for each soccer ball?

The momentum of each ball changes due to the collision, but the total momentum of the two balls together remains conserved

in an elastic collision between two objects, do both objects have the same kinetic energy after the collision as before?

both objects after an elastic collision have the same kinetic energy only when colliding bodies are : 1) of equal masses 2) moving with equal speeds 3) approaching each other while moving in opposite directions if any of the conditions are not satisfied, the kinetic energies of the two bodies after collision are not equal

when a car collision occurs, an air bag is inflated, protecting the passenger from serious injury. how does the air bag soften the blow? discuss the physics involved in terms of momentum and impulse

the impulse momentum theorem is f delta t = delta p. air bags delay the time of impulse as a passenger "sinks" into the inflated bag. the forces is spread out thorough a longer period of time. the longer it takes to change the momentum the less likely the passenger will be seriously injured

if two objects collide and one is initially at rest, is it possible for both to be at rest after the collision? is it possible for one to be at rest after the collision?

if two objects collidie and one is initially art rest, it is not possible for both to be at rest after the collision. one is initially at rest and the other is moving, the total momentum of the two objects before collision is not zero. following the law od conservation of momentum, the total momentum after collision should also be not zero. this condition cannot be satisfied if both of the objects come to rest after collision. yes, it is possible for one object to be at rest after the collision. if one object was initially moving while the other was at rest, it is possible that during collision all of the momentum from the moving object transfers to the other object

gymnasts always preform on padded mats. use the impulse-momentum theorem to discuss how these mats protect the athletes.

the impulse momentum theorem is F delta T = delta p. gymnasts use mats because it allows forces collisions to happen over a longer period of time. when they land on the mat, the mat sinks down a little. The time it takes the gymnasts to first land on the mat to when their full body weight sinks in it reduces the likelihood of a dangerous change in momentum

consider a perfectly inelastic head on collision between a small car and a large truck traveling at the same speed. which vehicle has a greater change in kinetic energy as a result of the collission?

the truck because it has a larger mass (more KE to start)

if an object is not moving what is its momentum?

zero