Quiz 5 Review: Part 1 Momentum and Collision

Momentum is a vector quantity

TRUE - Momentum is a vector quantity.

h. Total momentum is always conserved between any two objects involved in a collision.

FALSE - Total momentum is conserved only if the collision can be considered isolated from the influence of net external forces.

d. The velocity change of two respective objects involved in a collision will always be equal.

FALSE - Two colliding objects will only experience the same velocity change if they have the same mass and the collision occurs in an isolated system. However, their momentum changes will be equal if the system is isolated from external forces.

j. Two identical objects are moving in opposite directions at the same speed. The forward moving object will have the greatest momentum.

FALSE - When comparing the size of two momentum vectors, the direction is insignificant. The direction of any vector would never enter into a size comparison.

l. A moving object collides with a stationary object; the stationary object has significantly less mass. The stationary object encounters the greater momentum change.

.FALSE - In any collision, the colliding objects will experience equal (and opposite) momentum changes, provided that the collision occurs in an isolated system.

e. While individual objects may change their velocity during a collision, the overall or total velocity of the colliding objects is conserved.

FALSE - This statement is mistaking the term velocity for momentum. It is momentum which is conserved by an isolated system of two or more objects.

f. Momentum is a conserved quantity; the momentum of an object is never changed.

FALSE - To say that momentum is a conserved quantity says that if some objects are isolated from impact from net external forces, then the total momentum of that system is conserved.

i. A less massive object can never have more momentum than a more massive object.

FALSE - A less massive object would have a greater momentum owing to a velocity which is greater than that of the more massive object. Momentum depends upon two quantities * mass and velocity. Both are equally important.

An object with mass will have momentum.

FALSE - An object has momentum if it is moving. Having mass gives an object inertia. When that inertia is in motion, the object has momentum.

c. An object which is traveling east would experience a westward directed impulse in a collision.

FALSE - An object which is traveling east could encounter a collision from the side, from behind (by a faster-moving object) or from the front. The direction of the impulse is dependent upon the direction of the force exerted upon the object. In each of these scenarios, the direction of the force would be different.

d. Object A has more momentum than object B. Therefore, object A will also have more kinetic energy.

FALSE - Consider Object A with a mass of 10 kg and a velocity of 3 m/s. And consider Object B with a mass of 2 kg and a velocity of 10 m/s. Object A clearly has more momentum. However, Object B has the greatest kinetic energy. The kinetic energy of A is 45 J and the kinetic energy of B is 100 J.

b. Most collisions tend to be partially to completely elastic.

FALSE - Few collisions are completely elastic. A completely elastic collision occurs only when the collision force is a non-contact force. Most collisions are either perfectly inelastic or partially inelastic.

c. If an object does not have momentum, then it definitely does not have mechanical energy either.

FALSE - If an object does NOT have momentum, then it definitely does NOT have kinetic energy.

a. Impulse is a force.

FALSE - Impulse is NOT a force. Impulse is a quantity which depends upon both force and time to change the momentum of an object. Impulse is a force acting over time.

d. The kinetic energy of an object remains constant during an elastic collision.

FALSE - In a perfectly elastic collision, in an individual object may gain or lose kinetic energy. It is the system of colliding objects which conserves kinetic energy.

i. When a moving object collides with a stationary object of identical mass, the stationary object encounters the greater collision force.

FALSE - In any collision, the colliding objects exert equal and opposite forces upon each other as the result of the collision interaction. There are no exceptions to this rule.

k. A moving object collides with a stationary object; the stationary object has significantly less mass. The stationary object encounters the greater collision force.

FALSE - In any collision, the colliding objects exert equal and opposite forces upon each other as the result of the collision interaction. There are no exceptions to this rule.

k. a. A moving object collides with a stationary object; the stationary object has significantly less mass. The stationary object encounters the greater collision force.

FALSE - In any collision, the colliding objects exert equal and opposite forces upon each other as the result of the collision interaction. There are no exceptions to this rule.

j. When a moving object collides with a stationary object of identical mass, the stationary object encounters the greater momentum change.

FALSE - In any collision, the colliding objects will experience equal (and opposite) momentum changes, provided that the collision occurs in an isolated system.

g. In a collision between two objects of identical mass, the acceleration values could be different.

FALSE - It the colliding objects have different masses, the equal force which they exert upon each other will lead to different acceleration values for the two objects.

c. Momentum is conserved in an elastic collision but not in an inelastic collision.

FALSE - Momentum can be conserved in both elastic and inelastic collisions provided that the system of colliding objects is isolated from the influence of net external forces. It is kinetic energy that is conserved in a perfectly elastic collision.

a. Momentum is a form of energy.

FALSE - No. Momentum is momentum and energy is energy.

The standard unit on momentum is the Joule.

FALSE - The Joule is the unit of work and energy. The kg m/s is the standard unit of momentum.

e. The Newton is the unit for impulse.

FALSE - The Newton is the unit of force. The standard metric unit of impulse is the N•s.

e. An object can be traveling eastward and slowing down; its momentum is westward.

FALSE - The direction of an object's momentum vector is in the direction that the object is moving.

c. During a collision, the impulse which an object experiences is equal to its velocity change.

FALSE - The impulse encountered by an object is equal to mass multiplied by velocity change - that is, momentum change.

h. A moving air track glider collides with a second stationary glider of identical mass. The first glider loses all of its kinetic energy during the collision as the second glider is set in motion with the same original speed as the first glider. Since the first glider lost all of its kinetic energy, this is a perfectly inelastic collision.

FALSE - This is a perfectly elastic collision. Before the collision, all the kinetic energy is in the first glider. After the collision, the first glider has no kinetic energy; yet the second glider has the same mass and velocity as the first glider. As such, the second glider has the kinetic energy which the first glider once had.

i. A force of 100 N acting for 0.1 seconds would provide an equivalent impulse as a force of 5 N acting for 2.0 seconds.

TRUE - A force of 100 N for 0.10 s results in an impulse of 10 N•s. This 10 N•s impulse is equivalent to the impulse created by a force of 5 N for 2.0 seconds.

a. Perfectly elastic and perfectly inelastic collisions are the two opposite extremes along a continuum; where a particular collision lies along the continuum is dependent upon the amount kinetic energy which is conserved by the two objects.

TRUE - A perfectly elastic collision is a collision in which the total kinetic energy of the system of colliding objects is conserved. Such collisions are typically characterized by bouncing or repelling from a distance. In a perfectly inelastic collision (as it is sometimes called), the two colliding objects stick together and move as a single unit after the collision. Such collisions are characterized by large losses in the kinetic energy of the system.

h. Two objects of different mass are moving at the same speed; the more massive object will have the greatest momentum.

TRUE - For the same speed (and thus velocity), a more massive object has a greater product of mass and velocity; it therefore has more momentum.

b. If an object has momentum, then it must also have mechanical energy.

TRUE - If an object has momentum, then it is moving. If it is moving, then it has kinetic energy. And if an object has kinetic energy, then it definitely has mechanical energy.

g. A ball is dropped from rest and collides with the ground. The higher that the ball rises upon collision with the ground, the more elastic that the collision is.

TRUE - If large amounts of kinetic energy are conserved when a ball collides with the ground, then the post-collision velocity is high compared to the pre-collision velocity. The ball will thus rise to a height which is nearer to its initial height.

b. Impulse is a vector quantity.

TRUE - Impulse is a vector quantity Like momentum, impulse is not fully described unless a direction is associated with it.

b. During a collision, an object always encounters an impulse and a change in momentum.

TRUE - In a collision, there is a collision force which endures for some amount of time to cause an impulse. This impulse acts upon the object to change its momentum.

g. An object which experiences a net impulse will definitely experience a momentum change.

TRUE - In a collision, there is a collision force which endures for some amount of time to cause an impulse. This impulse acts upon the object to change its velocity and thus its momentum.

d. Objects involved in collisions encounter impulses.

TRUE - In a collision, there is a collision force which endures for some amount of time. The combination of force and time is what is referred to as an impulse.

a. Two colliding objects will exert equal forces upon each other even if their mass is significantly different.

TRUE - In any collision between two objects, the colliding objects exert equal and opposite force upon each other. This is simply Newton's law of action-reaction.

e. Elastic collisions occur when the collision force is a non-contact force.

TRUE - Kinetic energy is lost from a system of colliding objects because the collision transforms kinetic energy into other forms of energy - sound, heat and light energy. When the colliding objects don't really collide in the usual sense (that is when the collision force is a non-contact force), the system of colliding objects does not lose its kinetic energy. Sound is only produced when atoms of one object make contact with atoms of another object. And objects only warm up (converting mechanical energy into thermal energy) when their surfaces meet and atoms at those surfaces are set into vibrational motion or some kind of motion.

g. The momentum of an object varies directly with the speed of the object.

TRUE - Momentum is calculated as the product of mass and velocity. As the speed of an object increases, so does its velocity. As a result, an increasing speed leads to an increasing momentum - a direct relationship.

k. An object with a changing speed will have a changing momentum.

TRUE - Objects with a changing speed also have a changing velocity. As such, an object with a changing speed also has a changing momentum.

f. Most collisions are not inelastic (meaning they ARE ELASTIC) because the collision forces cause energy of motion to be transformed into sound, light and thermal energy (to name a few).

TRUE - See above statement. (Elastic collisions occur when the collision force is a non-contact force. True)

f. The kg•m/s is equivalent to the units on impulse.

TRUE - The N•s is the unit of momentum. The Newton can be written as a kg•m/s^2. When substituted into the N•s expression, the result is the kg m/s.

i. The collision between a tennis ball and a tennis racket tends to be more elastic in nature than a collision between a halfback and linebacker in football.

TRUE - There is significant bounce in the collision between a tennis racket and tennis ball. There is typically little bounce in the collision between a halfback and a linebacker (though there are certainly exceptions to this one). Thus, the ball-racket collision tends to be more elastic.

d) An object which is moving at a constant speed has momentum.

TRUE - This is true. However, one should be quick to note that the object does not have to have a constant speed in order to have momentum.

f. In a collision, the two colliding objects could have different acceleration values.

TRUE - Two colliding objects will exert equal forces upon each other. If the objects have different masses, then these equal forces will produce different accelerations.

a. Two objects of varying mass have the same momentum. The least massive of the two objects will have the greatest kinetic energy.

TRUE - When comparing the momentum of two objects to each other, one must consider both mass and velocity; both are of equal importance when determining the momentum value of an object.

h. In a collision, the net impulse experienced by an object is equal to its momentum change.

TRUE - Yes!!! This is the impulse-momentum change theorem. The impulse encountered by an object in a collision causes and is equal to the momentum change experienced by that object.