Chapter 6 Flashcards
Momentum
The product of the mass of an object and its velocity. Momentum=mv
Impulse
The product of the force acting on an object and the time during which it acts. Impulse=Ft
Impulse-momentum relationship
Impulse is equal to the change in the momentum of the object that the impulse acts upon. In symbolic notation, Ft=△mv
Law of conservation of momentum
In the absence of an external force, the momentum of a system remains unchanged. Hence, the momentum before an event involving only internal forces is equal to the momentum after the event: mv(before event)=mv(after event)
Elastic collision
A collision in which objects rebound without lasting deformation or the generation of heat.
Inelastic collision
A collision in which objects become distorted, generate heat, and possibly stick together.
Which has a greater momentum: a heavy truck at rest or a moving skateboard?
The moving skateboard.
Distinguish between force and impulse.
Impulse is force times a time interval.
What are the two ways to increase impulse?
Increasing the force or increasing the time interval.
For the same force, why does a long cannon impart more speed to a cannonball than a short cannon?
The force is applied for a longer time in the long cannon.
How is the impulse–momentum relationship related to Newton’s second law?
F=ma=mΔv/Δt,
so Ft=Δ(mv).
To impart the greatest momentum to an object, should you exert the largest force possible, extend that force for as long a time as possible, or both? Explain.
Exert the greatest force over the longest time.
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? Explain.
Long contact time decreases the magnitude of the average force and is favorable.
Why is a force that is applied for a short time more effective in karate?
The average force is increased.
Why is it advantageous to roll with the punch in boxing?
Rolling with the punch increases contact time, which decreases the force.
Which undergoes the greatest change in momentum: (a) a baseball that is caught, (b) a baseball that is thrown, or (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?
For both questions:
A baseball that is caught and then thrown back
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, never.
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.
What does it mean to say that momentum (or any quantity) is conserved?
It means 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.
The momentum of the cannon is equal to the magnitude of the momentum of the cannon ball and points in the opposite direction.
In which is momentum conserved: an elastic collision or an inelastic collision?
Momentum is conserved in elastic and inelastic collisions.
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.
Suppose a ball of putty moving horizontally with 1kg m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1kg m/s of momentum. What is the magnitude of their combined momentum?
1.41 kg·m/s
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.
