Physics ch 6
Newton's third law states that the force a cannon exerted on a cannonball is equal and opposite to the force the cannonball exerts on the cannon. Does it follow that the impulse the cannon exerts on the cannonball is equal and opposite to the impulse the cannonball exerts on the cannon?
yes because they both occur within the same time interval
is momentum conserved for all collisions?
yes, whenever external forces do not interfere
to impart the greatest momentum to an object, should you exert the largest force possible, extend that force for as long a time possible, or both? explain.
you should exert the largest force possible because it extends the time of contact which increases momentum
the momentum before firing a cannonball is
zero
after firing a cannonball, the net momentum is ____. why?
zero because the momentum of the cannon is equal and opposite to the momentum of the cannonball
To catch a fast-moving ball, you extend your hand forward before contact with the ball and let it ride backward in the direction of the ball's motion. Doing this reduces the force of contact on your hand principally because the A) force of contact is reduced. B) relative velocity is less. C) time of contact is increased. D) time of contact is decreased. E) none of the above
c
consider an air track. suppose the gliding cart of mass 0.5kg bumps into, and sticks to, a stationary cart of mass 1.5kg. If the speed of the gliding cart before the collision is v-before, how fast will the coupled carts glide after the collision?
(o.5kg)v-before = (0.5kg + 1.5 kg)v-after v-after = (0.5kg)v-before/(0.5kg+1.5kg) 0.5 v-before/2 v-before/4 1/4 as fast
elastic collision
A collision in which colliding objects rebound without lasting deformation or the generation of heat
inelastic collision
A collision in which the colliding objects become distorted, generate heat, and possibly stick together
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) the same as the magnitude of momentum of either car before collision. B) less than the magnitude of momentum of either car before collision. C) greater than the magnitude of momentum of either car before collision. D) none of the above
c
When a vertically falling firecracker bursts, the vector sum of momentum fragments A) in the horizontal direction cancels to zero. B) in the vertical direction equals the momentum of the firecracker before bursting. C) both of these D) neither of these
c
Whether a truck comes to a stop by crashing into a haystack or a brick wall, the impulse is A) greater with the haystack. B) greater with the brick wall. C) both the same
c
Which produces more force: driving into a very massive concrete wall with no "give," or having a head-on collision with an identical car moving toward you at the same speed? A) car B) wall C) both the same D) need more information
c
You're driving down the highway and a bug spatters into your windshield. Which undergoes the greater change in momentum during the time of contact? A) the bug B) your car C) both the same
c
if no net force or net impulse acts on a system, the momentum of the system can or cannot change?
cannot
Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?
car b will have the same initial speed as car a
impulse is equal to
change in momentum
no external force acts on the cue-ball and 8-ball systems and momentum is
conserved
As Bronco Brown steps off a hovering high-flying helicopter and falls through the air, he experiences A) an impulse. B) an increasing speed. C) a decreasing acceleration. D) all of the above
d
The force that accelerates a rocket in outer space is exerted on the rocket by the A) rocket's nose cone. B) rocket's wings. C) atmospheric pressure. D) exhaust gases. E) none of the above
d
When Peter tosses an egg against a sagging sheet, the egg doesn't break due to A) reduced impulse. B) reduced momentum. C) both of these D) neither of these
d
an external force acts on the cue-ball system and its momentum
decreases
momentum has both
direction and magnitude
A falling 1-N apple hits the ground with a force of about A) 1 N. B) 2 N. C) 4 N. D) 10 N. E) need more information
e
A sandbag motionless in outer space is hit by a three-times-as massive sandbag moving at 12 m/s. They stick together and move at a speed of A) 3 m/s. B) 4 m/s. C) 6 m/s. D) 8 m/s. E) none of the above
e
when the boxer moves away (rides with the punch), he _____ the time and _____ the force
extends; diminishes
impulse
force x time
force is ____ when bouncing occurs
greater
to increase the momentum of an object, you apply the _____ force possible for as ____ as possible
greatest; long
brief time of contact makes the force of impact
huge
When does impulse equal momentum?
impulse equals a change in momentum; if the initial momentum of an object is zero when the impulse is applied, final momentum = impulse applied. if an object is brought to rest, initial momentum = impulse delivered
how does the force of the karate expert exert on the bricks compared with the force exerted on her hands?
in accord with Newton's third law, the forces are equal
law of conservation of momentum
in the absence of an external force, the momentum of a system remains unchanged
an external force acts on the 8-ball system and its momentum
increases
In the preceding question, what is the total momentum of the balls of putty before and after the collision? preceding question: Suppose a ball of putty moving horizontally with 1 kg·m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg·m/s of momentum. What is the magnitude of their combined momentum?
it is the same: square root of 2 or 1.41 kgm/s
If the change in momentum occurs over a short time, the hitting force is _____
large
a ____ change in momentum over a long time requires a safely small average force
large
When you are struck by a moving object, is it favorable that the object makes contact with you over a short time or over a long time?
long period of time because the hitting force would therefore be small
momentum
mass x velocity
what does it mean to say that momentum (or any quantity) is conserved?
momentum does not change
when a cannonball is fired, momentum is conserved for the system cannon + cannonball. would momentum be conserved for the system if momentum were not a vector quantity? explain.
momentum would not be conserved if force, which is therefore impulse, was not a vector quantity
Which has a greater momentum: a heavy truck at rest or a moving skateboard?
moving skateboard because it has velocity
Net momentum before collision =
net momentum after collision
Can you produce a net impulse on an automobile if you sit inside and push on the dashboard? can the internal forces within a soccer ball produce an impulse on the soccer ball that will change its momentum?
no
a boxer being hit with a punch contrives to extend time for best results, whereas a karate expert delivers a force in a short time for best results. Isn't there a contradiction here?
no because the best results for each are different. the best result for the boxer is reduced force which is accomplished by maximizing time whereas the best result for the karate expert is increased force delivered in minimal time
if the boxer is able to increase the duration of impact to three times as long by riding with the punch, by how much will the force of impact be reduced?
reduced only a third
if the boxer moves into the gloved fist, the time is ______ and he must withstand a ______ force
reduced; greater
If the change in momentum occurs over a long time, the hitting force is ______
small
does a runner down a hill have more momentum than the boulder chasing him or does the boulder have more momentum?
the boulder has more momentum
For the same force, why does a long cannon impart more speed to a cannonball than a short cannon?
the force acts over a longer period of time which imparts more speed
why is it advantageous to roll with the punch in boxing?
the impact would occur over a long period of time which means less forceful
How will the impulse resulting from the impact differ if her hand bounces back upon striking the bricks?
the impulse will be greater if her hand bounces back from the bricks on the time of impact
for the same force, which cannon imparts a greater impulse to a cannonball: a long cannon or a short one?
the long cannon because the force acts over a longer period of time
If the equally massive cars of the preceding question stick together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A? preceding q: Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?
their combined speed, after the inelastic collision, is half the initial speed of car A
which has more momentum: a 1-ton car moving at 100km/h or a 2-ton truck moving at 50km/h?
they have the same momentum
why are the engines of a supertanker normally cut off 25km from port?
timing is especially important when changing momentum
when you push with the same force for twice the time, you impart ____ the impulse and produce _____ the change in momentum
twice and twice
if the boxer moves into the punch so as to decrease the duration of impact by half, by how much will the force of impact be increased?
two times greater
Does a moving object have momentum?
yes
Is it correct to say that, if no net impulse is exerted on a system, then no change in the momentum of the system will occur?
yes
does following through extend the time of contact?
yes
does the force of impact on a golfball vary throughout the duration of impact?
yes
is impulse greater when bouncing occurs?
yes
is momentum a vector quantity
yes
Newton's second law states that, if no net force is exerted on a system, no acceleration occurs. Does it follow that no change in momentum occurs?
yes because no acceleration means no change in occurs in velocity or momentum; no net force means no net impulse and thus no change in momentum
distinguish between force and impulse
F = ma I = Ft
Does a moving object have impulse?
No, because impulse isn't something an object has
A fan attached to an ice sailcraft stalled on a windless day blows air into the sail that bounces backward upon impact. The boat A) moves in the direction of the wind impact force. B) does not move by this wind impact. C) moves, but in the opposite direction of the wind impact force.
a
A tiny gun made of a strong but very light material fires a bullet more massive than the gun itself. For such a weapon A) the target would be safer than the shooter. B) recoil problems would be lessened. C) conservation of energy would not hold. D) conservation of momentum would not hold. E) both conservation of energy and momentum would not hold.
a
If a monkey floating in outer space throws his hat away, the hat and the monkey will both A) move away from each other, but at different speeds. B) move away from each other at the same speed. C) move a short distance and then slow down. D) move a short distance and then go faster. E) come to a stop after a few minutes.
a
If you can't avoid being hit by a fast-moving object, you'll suffer a smaller contact force if you can extend that force over a A) longer time. B) shorter time. C) both the same
a
Padded dashboards in cars are safer in an accident than non-padded ones because passengers hitting the dashboard encounter A) lengthened time of contact. B) shorter time of contact. C) decreased impulse. D) increased momentum.
a
Recoil is noticeable if we throw a heavy ball while standing on roller skates. If instead we go through the motions of throwing the ball but hold onto it, our net recoil will be A) zero. B) the same as before. C) small, but noticeable.
a
The conservation of momentum is most closely related to A) Newton's 1st law. B) Newton's 2nd law. C) Newton's 3rd law. D) Newton's law of gravity
a
Two identical objects in outer space, one moving at 2 m/s, the other at 1 m/s, have a head-on collision and stick together. Their combined speed after the collision is A) 0.5 m/s. B) 0.33 m/s. C) 0.67 m/s. D) 1.0 m/s. E) none of the above
a
When Freddy Frog drops vertically from a tree onto a horizontally-moving skateboard, the speed of the skateboard A) decreases. B) increases. C) neither decreases nor increases.
a
Which case requires the greatest impulse: a. a baseball that is caught b. a baseball that is thrown c. a baseball that is caught and then thrown back (if all of the baseballs have the same speed just before being caught and just after being thrown)
a baseball that is caught and then thrown back
Which undergoes the greatest change in momentum a. a baseball that is caught b. a baseball that is thrown c. a baseball that is caught and then thrown back (if all of the baseballs have the same speed just before being caught and just after being thrown)
a baseball that is caught and then thrown back because it provides extra impulse
A 1-kg chunk of putty moving at 1 m/s collides with and sticks to a 5-kg bowling ball initially at rest. The bowling ball and putty then move with a momentum of A) 0 kg m/s. B) 1 kg m/s. C) 2 kg m/s. D) 5 kg m/s. E) more than 5 kg m/s.
b
A golf ball moving forward with 1 unit of momentum strikes and bounces backward off a heavy bowling ball that is initially at rest and free to move. The bowling ball is set in motion with a momentum of A) less than 1 unit. B) more than 1 unit. C) 1 unit. D) not enough information
b
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
The impulse-momentum relationship is a direct result of A) Newton's 1st law. B) Newton's 2nd law. C) Newton's 3rd law. D) Newton's law of gravity.
b
Suppose a ball of putty moving horizontally with 1 kg·m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg·m/s of momentum. What is the magnitude of their combined momentum?
(1kgm/s)^2 + (1kgm/s)^2 = p^2 p = square root of 2 or 1.41 kgm/s
consider a fish that swims toward and swallows a smaller fish at rest. if the larger fish has a mass of 5kg and swims 1 m/s toward a 1-kg fish, what is the velocity of the larger fish immediately after lunch?
(5kg)(1 m/s) + (1kg)(0 m/s) = (5 kg + 1kg)v 5kg x m/s = (6kg)v v = 5/6 m/s
Two identical freight cars roll without friction (one at 1 m/s, the other at 2 m/s) toward each other on a level track. They collide, couple together, and roll away in the direction of A) the slower car. B) the faster car. C) neither, for they stop.
b
When a boxer is moving away from a punch, the force experienced is reduced because A) momentum transfer is reduced. B) the time of contact is increased. C) the force is less effective. D) all of the above
b
Which of the following has the largest momentum relative to Earth's surface? A) a tightrope walker crossing Niagara Falls B) a pickup truck speeding along a highway C) a Mack truck parked in a parking lot D) the Science building on campus E) a mouse running across your room
b
How is the impulse-momentum relationship related to Newton's second law?
because the law states that if no net force is exerted on a system, no acceleration occurs. therefore, no acceleration means no change in velocity or momentum which means no net impulse
Why is a force that is applied for a short time more effective in karate?
because the time of contact is brief, meaning the force of impact is large
in which is momentum conserved: an elastic collision or an inelastic collision?
both
what are two ways to increase impulse?
by increasing force or time of application
A 5-kg fish swimming at 1 m/s swallows an absent-minded 1-kg fish at rest. The speed of the larger fish after lunch is A) 1/2 m/s. B) 2/5 m/s. C) 5/6 m/s. D) 6/5 m/s. E) 1 m/s.
c
A 5000-kg freight car moving at 2 m/s collides with a 10,000-kg freight car at rest. They couple upon collision and move away at A) 2 m/s. B) 1 m/s. C) 2/3 m/s. D) 1/3 m/s.
c
A motorcycle of mass 100 kilograms slowly rolls off the edge of a cliff and falls for three seconds before reaching the bottom of a gully. Its momentum upon hitting the ground is A) 1,000 kg m/s. B) 2,000 kg m/s. C) 3,000 kg m/s. D) 4,000 kg m/s. E) 9,000 kg m/.
c
A piece of putty moving with 1 unit of momentum strikes and sticks to a heavy bowling ball that is initially at rest. Both move with a combined momentum of A) less than 1 unit. B) more than 1 unit. C) 1 unit. D) need more information
c
A same-size iron ball and wooden ball are dropped simultaneously from a tower and reach the ground at the same time. The iron ball has a greater A) speed. B) acceleration. C) momentum. D) all of the above E) none of the above
c
Consider massive gliders that slide friction-free along a horizontal air track. Glider A has a mass of 1 kg, a speed of 1 m/s, and collides with Glider B that has a mass of 5 kg and is at rest. If they stick upon collision, their speed after collision will be A) 1/4 m/s. B) 1/5 m/s. C) 1/6 m/s. D) 1 m/s. E) none of the above
c
The mass of a ball moving at 3 m/s with a momentum of 48 kg m/s is A) 4 kg. B) 12 kg. C) 16 kg. D) 144 kg. E) none of these
c
The total momentum of a flock of identical birds could be zero only if the birds are A) taking off from the ground. B) flying in the same direction. C) flying in different directions. D) very tired and coming down to rest.
c
