Conceptual Physics: Momentum, Impulse, and Collisions

How can we relate the law of conservation of momentum to collisions?

- In a collision between 2 bodies, if the forces between the bodies are much larger than any external forces, we can neglect the external forces and treat the bodies as an isolated system. - Therefore, momentum is conserved and the total momentum of the system has the same value before and after the collision.

What are the principles stated in the law of conservation of momentum?

- In the absence of an external force, the momentum of a system remains unchanged. - When no external force is present, no external impulse is present, and no change in momentum is possible. - If the vector sum of the external forces on a system is zero, the total momentum of the system is constant (If no external forces act on the isolated system the total momentum of this system is conserved).

What is a completely inelastic collision? How can completely inelastic collisions be mathematically expressed?

A collision when colliding bodies stick together, move as one body, and move at a shared velocity after the collision.

Two vehicles with equal magnitudes of momentum traveling at right angles to each other undergo an inelastic collision. The magnitude of momentum for the combined wreck is A) greater than the magnitude of momentum of either car before collision. B) less than the magnitude of momentum of either car before collision. C) the same as the magnitude of momentum of either car before collision.

A) greater than the magnitude of momentum of either car before collision. We can use the formula to find the resultant vector's magnitude since the momentum vectors are traveling at right angles. Looking at the formula, it is clear that no matter what magnitude we have for the cars, the resultant vector will always be greater than the individual cars' magnitudes.

Compared with falling on a carpet floor, a wine glass may break when it falls on a stone floor because the: A) stopping time is shorter on the stone. B) stopping time is longer on the Stone C) stopping time is longer on the carpet. D) stopping time is shorter on the carpet.

A) stopping time is shorter on the stone.

A karate chop is more effective if one's hand A) follows through upon impact. B) bounces upon impact. C) extends the time upon impact

B) bounces upon impact.

In jumping, bending your knees when your feet make contact with the ground reduces the force on you. Why is this the case?

By extending the airtime during your jump, the momentum decreases.

A 5000-kg freight car collides with a 10,000-kg freight car at rest. The cars couple upon collision and move at 2 m/s. What was the initial speed of the 5000-kg car? A) 5 m/s B) 8 m/s C) 6 m/s D) 4 m/s

C) 6 m/s Given the situation here, the collision can be considered a completely inelastic collision due to coupling upon hitting each other. We can therefore use the equation for completely inelastic collisions to solve for the initial speed of the 5000 kg car, getting a final answer of 6 m/s.

Which has the greater momentum when moving? A) A bullet B) A container ship C) Depends on each object's speed

C) Depends on each object's speed. Since we do not know if the speeds of each object share a constant speed, we can determine which one has the greater momentum when moving. A bullet has a lot of speed, but its mass is relatively small. A container sip has a relatively high mass, but its speed is relatively small.

Which one of the following statements concerning momentum is true? A) Momentum is a scalar quantity. B) The SI unit of momentum is kg*m^2/s. C) Momentum and impulse are measured in the same units. D) Momentum is a force. E) The momentum of an object is always positive.

C) Momentum and impulse are measured in the same units.

The total momentum of a flock of identical birds could be zero only if the birds are: A) taking off from the ground. B) very tired and coming down to rest. C) flying in different directions. D) flying in the same direction.

C) flying in different directions. Knowing that momentum is a vector quantity that always has the same direction as a particle's velocity vector, in order for the total momentum to be zero, then the sum of the velocity vectors need to equal zero. So the direction of the velocity vectors needs to the opposite and the magnitudes need to be equal.

Compared with falling on a stone floor, a wine glass may not break when it falls on a carpeted floor because of the: A) stopping time is shorter on the carpet. B) carpeted floor provides a smaller impulse. C) stopping time is longer on the carpet. D) carpet provides a smaller impulse and a longer time.

C) stopping time is longer on the carpet. Throughout the motion, the impulse of the force is the same throughout. However, since the carpet extends the time before fully landing, we can say that reduces the force.

Complete the following statement: Momentum will be conserved in a two-body collision only if: A) the kinetic energy of the system is conserved. B) both bodies come to rest. C) the net external force acting on the two-body system is zero. D) the internal forces of the two body system cancel in action-reaction pairs. E) the collision is perfectly elastic.

C) the net external force acting on the two-body system is zero.

A cannon recoils while firing a cannonball. The speed of the cannon's recoil is relatively small because the: A) momentum is mainly concentrated in the cannonball B) force against the cannon is smaller than against the ball. C) momentum of the cannon is smaller. D) cannon has much more mass than the cannonball.

D) cannon has much more mass than the cannonball.

When Peter tosses an egg against a sagging sheet, the egg doesn't break due to: A) reduced momentum B) reduced impulse C) all of the above. D) none of the above

D) none of the above.

Two objects of equal mass traveling toward each other with equal speeds undergo a head on collision. Which one of the following statements concerning their velocities after the collision is necessarily true? A) Their velocities will be reduced. B) Their velocities will be zero. C) They will exchange velocities. D) Their velocities will be unchanged. E) Their velocities may be zero.

E) Their velocities may be zero. If two objects with the same mass and equal velocities will collide straight head-on, then the objects after the collision theoretically could be zero. However, the direction of the objects is unclear, so it may be a possibility.

Complete the following statement: Different types of collisions between interacting bodies are categorized on the basis of: A) linear momentum conservation. B) mechanical energy conservation. C) the temporal duration of the collision. D) the magnitude of the forces involved. E) kinetic energy conservation.

E) kinetic energy conservation.

When can a collision be considered elastic?

Elastic collisions occur when the forces between the colliding bodies are conservative forces. There is no disfigurement or heat generation between the objects, and the kinetic energy as well the momentum is conserved.

When a boxer is moving toward a punch, the force experienced is increased because the time of contact is increased. True or False?

False.

When a moving object hits a stationary object and causes it to move, some of the moving object's kinetic energy is transformed into momentum in the object that was at rest. True or False?

False.

When a baseball player hits a home run, the baseball receives a greater impulse from the bat than the bat does from the ball. True or False?

False. According to Newton's third law, there is always an equal and opposite reaction force to an action force. Since the bat is exerting a force onto the ball, then the ball is exerting a equal and opposite force back at the bat, in this case being impulse.

A rubber ball moving horizontally at 8.50 m/s hits a brick wall and bounces back with exactly the same speed but in the opposite direction. During this collision, its change in momentum is zero. True or False?

False. Knowing that

If a force acts perpendicular to an object's direction of motion, that force cannot change the object's kinetic energy or momentum. True or False?

False. Only if we consider the force's work acting perpendicular to the object's direction of motion would this be true.

A ball of clay is thrown against a wall and sticks there. In this process, momentum is not conserved because the clay stops moving. True or False?

False. According to the law of conservation of momentum, ALL collisions' net momentum before and after the collision are conserved. This includes if one of the such object's motion stops.

In what direction does the impulse vector always point to?

In the same direction as the particle's net force vector.

In what direction does the momentum vector always point to?

In the same direction as the particle's velocity vector.

When can a collision be considered inelastic?

Inelastic collisions occur in which the total kinetic energy after the collision is less than before the collision. Some kinetic energy is lost, as it transforms into heat. The momentum is conserved, but not the kinetic energy.

What is the SI unit for impulse?

N*s

Why is it preferable for an out of control car to hit through a haystack rather than into a concrete wall?

The greater the impulse exerted on something, the greater the change in momentum. In this case, the impulse is the same. Knowing this, we can assume that by extending the hitting time, the overall force is reduced.

How does the momentum of an object change if the object's velocity doubles?

The momentum also doubles.

What is momentum?

The product of an object's mass and velocity.

A brief force applied over a short time interval produces a smaller change in momentum than the same force applied over a longer time interval. True or False?

True.

Kinetic energy is conserved only in perfectly elastic collisions, but momentum is conserved in all collisions. True or False?

True.

The change in momentum of a particle during a time interval equals the impulse of the net force that acts on the particle during that interval. True or False?

True.

IF an object is at rest, what is its momentum?

Zero.

What are the two types of collisions?

elastic and inelastic collisions

What is the SI unit for momentum?

kg*m/s

A primary difference between momentum and kinetic energy is:

momenta can cancel; kinetic energy cannot.

What is impulse?

the change in momentum of an object.

How can you find the net force in terms of momentum and Newton's second law?

𐤃p = change in momentum 𐤃t = time period