Chapter 6: Momentum (From Lecture Slide) Flashcards
Momentum (Definition)
1) Property of moving things
2) More specifically, mass of an object multiplied by its velocity
Momentum (Formula)
Mass x Velocity (Vector quantity: p = mv)
Why does p stand for momentum?
It really stands for impetus, which is from the Latin impellere from im + pellere
Pellere meant to
“Push forcefully, im- was a prefix meaning “Inner”.
Impellere meant
Pushing with an inner source of energy
A moving boulder has more momentum than
A stone rolling at the same speed
A faster boulder has more momentum than
A slow boulder with the same mass
A boulder at rest, has
No momentum
A moving object can have a large momentum if:
1) Mass is large
2) Velocity is large
3) Or both!
Momentum is directly proportional to
Mass and Velocity
An object at rest has
Zero momentum
A moving object has all
1) Momentum
2) Energy
3) Speed
When the speed of an object is doubled, its momentum
Doubles
Which vehicle has a greater momentum? A truck at rest or a man walking 3 mph.
The man
Which vehicle has a smaller momentum? A 2000 kg car moving at 30 m/s or a 4000 kg light truck moving at 15 m/s.
They have the same momentum
Momentum can change by changing
1) Mass (usually does not change)
2) Velocity (most often changes)
3) Or both!
Impulse
Changing velocity means acceleration occurs
What produces acceleration?
Net force
Net force
Acceleration, change in velocity, and change in momentum
Increase net force equals to
Increase change in momentum
Impulse is time also important for
Changing momentum
How long does the force act?
Case A: Unpadded wall-immediate smack
Case B: Paddled wall-cushion absorbs impact
Case A: Unpadded wall-immediate smack
Greater force over shorter time
Case B: Paddled wall-cushion absorbs impact
Lesser force over a longer time
Both force and time are important in
Changing momentum
The product of force and time is
The impulse
Impulse (Formula)
Force x time interval
A moving object experiences impulse only if
A force acts on it and changes its momentum
An object traveling at constant speed has
No net forces acting on it (dynamic equilibrium)
Does it have momentum? Yes
It is p = mv
Is there an impulse imparted on the object? Why?
No, net force to change momentum
The greater the impulse exerted on an object,
The greater its change in momentum
Impulse and change in momentum are always linked (3 cases)
Case 1: Increasing momentum
Case 2: Decreasing momentum over a long time
Case 3: Decreasing momentum over a short time
Case 1: Increasing momentum
1) Apply the greatest force for as long as possible
2) Increasing the time of contact
Forces involved in impulse vary from
Instant to instant
Only concerned with the average force
1) Not instantaneous
2) Defined over some velocity and time interval
Average force
Defined over same velocity and time interval
A cannonball shot from a cannon with a long barrel will emerge with greater speed because
The cannonball receives a greater (Impulse)
Case 2: Decreasing momentum over a long time
When a car is out of control,it is better to hit a haystack than a concreate wall = both have the initial speed and come to rest
When landing after a jump, gradually bending your knees increases the time your momentum decreases,
Reducing the force exerted on you by the ground
Increasing time equal to
Small force
A fast moving car hitting a haystack or hitting a cement wall produces vastly different results
They are both experiences the same change in momentum and the same impulse
Case 3: Decreasing momentum over a short time
A short time interval produces a large force (Rise into the punch: Decrease time = large force)
Time of contact is
Brief and force of impact is huge
Impulses are generally greater when
The objects is bounce
Catching a falling item with your hands
You provide the impulse to reduce its momentum is zero
If you quickly throw the item up again, you provide an additional
Impulse
Double impulse occurs when something
Bounces
Pelton wheel designed with curve-shaped paddles to cause incoming water to make a U-turn upon impact or bounce
Produces a large impulse that turns the wheel
Conservation of momentum (Newton’s 2nd Law)
To accelerate an object, we apply to a net force
To change the momentum of an object,
Exert an impulse on it (Impulse-momentum theorem)
Only an external impulse to a system can
Change the momentum of the system
If no external force is present, then
No change in momentum is possible…Momentum is conserved!
External net force applied by apple on orange
Acceleration exists
No external net force, force are internal and cancel
Acceleration does not exist
If no net force or net impulse acts on a system,
The momentum of that system cannot change
When momentum, or any quantity in physics,
Does not change we says it conserved
Law of Conservation of Momentum
In the absences of an external force, the momentum of a systme remains unchanged
Collision is
The absence of external forces
Net momentum before collision equals
Net momentum after collision and momentum is conserved
Two types of collisions
Elastic and Inelastic
1) Elastic Collision
Occurs when colliding objects rebound without lasting deformation or any generation of heat
2) Inelastic Collision
Occurs when colliding object result in deformation and/or the generation of heat
