Ch. 6
A cannon recoils while firing a cannonball. The speed of the cannon's recoil is relatively small because the
cannon has much more mass than the cannonball.
A cannonball following a parabolic path explodes into fragments. The momentum of the fragments
continue along the path as if the explosion didn't occur.
If all people, animals, trains and trucks all over the world began to walk or run towards the east (opposite the direction of Earth's spin), then
Earth would spin a bit slower.
Distinguish between force and impulse.
Impulse is force times a time interval.
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
Longer time
In which is momentum conserved: an elastic collision or an inelastic collision?
Momentum is conserved in elastic and inelastic collisions.
Which has a greater momentum: a heavy truck at rest or a moving skateboard?
Moving skateboard
Why is a force that is applied for a short time more effective in karate?
The average force is increased.
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?
The speed of car B is equal to the initial speed of A.
Two automobiles, each of mass 1100kg , are moving at the same speed, 23m/s , when they collide and stick together. In what direction and at what speed does the wreckage move if one car was driving north and one south.
The wreckage after collision is motionless.
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?
Their speed is half the initial speed of car A.
To impart the greatest momentum to an object, you must
exert the greatest force over the longest time.
The total momentum of a flock of identical birds could be zero only if the birds are
flying in different directions.
Freddy Frog drops vertically from a tree onto a horizontally-moving skateboard. The reason he doesn't slip off the skateboard is due to
friction between his feet and the board.
A heavy truck and a small car rolling down a hill at the same speed are forced to stop in the same amount of time. Compared with the force that stops the car, the force needed to stop the truck is
greater
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
greater than the magnitude of momentum of either car before collision.
Whether a truck comes to a stop by crashing into a haystack or a brick wall, the stopping force is
greater with the brick wall
What is the magnitude of the impulse on an 7.6-kg ball rolling at 1.4m/s when it bumps into a pillow and stops? [Impulse = change in momentum: Ft=Δmv]
impulse = 11 N⋅s
An impulse can be increased by
increasing the force or increasing the time interval.
A slowly moving ship can have a greater momentum than a fast-moving racing car when _______.
its mass times velocity is greater than that of the car.
Padded dashboards in cars are safer in an accident than non-padded ones because passengers hitting the dashboard encounter
lengthened time of contact.
A large heavy truck and a small baby carriage roll down a hill. Neglecting friction, at the bottom of the hill, the baby carriage will likely have
less momentum
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
momentum
An astronaut floating alone in outer space throws a baseball. If the ball moves away at 20 m/s, the astronaut will
move in the opposite direction at a lower speed.
What impulse occurs when an average force of 12N is exerted on a cart for 2.1s ? [Impulse = Ft]
p = 25 (kg⋅m)/s
What is the momentum of an 8.4-kg bowling ball rolling at 1.0m/s ? {Momentum = mv}
p = 8.4 (kg⋅m)/s
The quantity that is called impulse can be measured by the _______.
product of force and time.
When a big fish swims into an oncoming smaller fish and swallows it, the momentum of the two-fish system _______.
remains the same
Compared with falling on a stone floor, a wine glass may not break when it falls on a carpeted floor because the
stopping time is longer on the carpet.
A tiny gun made of a strong but very light material fires a bullet more massive than the gun itself. For such a weapon
the target would be safer than the shooter.
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
2 m/s.
The speed of a 4-kg ball with a momentum of 12 kg m/s is
3 m/s
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
3,000 kg m/s.
A 5000-kg freight car collides with a 10,000-kg freight car at rest. They couple upon collision and move at 2 m/s. What was the initial speed of the 5000-kg car?
6 m/s
How is the impulse -momentum relationship related to Newton's second law?
F=ma=mΔv/Δt, so Ft=Δ(mv)
What does it mean to say that momentum is conserved?
It means momentum does not change.
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 contact time decreases the magnitude of the average force and is favorable.
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, always
It is correct to say that impulse is equal to
a corresponding change in momentum.
A red car has a head-on collision with an approaching blue car with the same magnitude of momentum. A green car driving with the same momentum as the other cars collides with an enormously massive wall. Which of the three cars will experience the greatest impulse?
all the same
Two vehicles with equal magnitudes of momentum traveling at right angles to each other undergo an inelastic collision. The combined wreck moves in a direction
at 45 degrees to the direction of either car before collision.
When a vertically falling firecracker bursts, the vector sum of momentum fragments
both of these
Whether a truck comes to a stop by crashing into a haystack or a brick wall, the impulse is
both the same
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?
both the same
A 4.2-kg fish swimming 1.4m/s swallows an absentminded 1.0-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Find the speed of the approaching smaller fish before lunch.
v = 5.9 m/s
When a falling firecracker explodes, the momenta of its pieces _______.
vectorally add up to equal the initial momentum of the firecracker
When two vehicles collide, momentum is conserved _______.
whether the collision is elastic or inelastic.
