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Momentum and Impulse
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46
Physics
10/01/2012

Additional Physics Flashcards

 


 

Cards

Term

 

 

WHEN THINGS COLLIDE

Definition
When two things collide, the effect they have on each other depends on thier mass and initial velocity
Term

 

 

FORCE AND OBJECTS

Definition
Sir Isaac Newton realised that a force is needed to change the velocity of an object
Term

 

 

THE EFFECT OF A FORCE

Definition
Depends on the amount of force and the objects mass
Term

 

 

MOMENTUM

DEFINITION

Definition

Of a moving object;

= Mass x Velocity

 

p = mv

Term

 

 

FORCE NEEDED TO GIVE AN OBJECT A CERTAIN ACCELERATION

Definition

Force = mass x acceleration

 

F = ma

Term

 

 

WHERE DOES NEWTONS LAW OF MOTION NOT APPLY? 

Definition
Inside an atom (quantum physics), Speeds approaching the speed of light or in strong gravitational fields (Einstiens theory of relativity)
Term

 

 

MOMENTUM BASICS

Definition

Unit is kgms-1

Vector quantity - direction is same as the velocity

 

Term
Definition
Term

 

 

NEWTONS FIRST LAW OF MOTION

Definition

An object remains at rest or in uniform motion unless acted on by a force

(so basically a force is needed to change the motion of an object)

Term

 

 

CONSTANT MOMENTUM

Definition

This means there is no resultant force acting on it 

If the mass remains constant and the object has a constant momentum, the velocity will be constant; if the object lost/gained mass whilst moivng, velocity would change to keep momentum constant

Term

 

 

 NEWTONS SECOND LAW OF MOTION

Definition
The rate of change of momentum of an object is proportional to the resultant force on it. The resultant force is proportional to the change of momentum per second
Term

 

 

DERIVING F = ma

Definition

An object of mass is acted by a constant force of F.

Initial momentum = mu , Final momentum = mv

Force is proportional to change of momentum per second.


F≈mv-mu/t 

F≈ m(v-u)/t

a = v-u/t

≈ ma


F = kma 

K = constant of proportionality = 1

Term

 

 

F = v♦m/♦t

Definition
This form of Newtons second law is used when there is a continuous loss or gain of mass of an object
Term

 

 

IMPULSE

Definition

The impulse of a force is the the force x the time for which the force acts.

 

Impulse = Ft

Ft = ♦(mv)

This unit is usually Ns-1

Term

 

 

FORCE TIME GRAPHS

Definition

Rearranging Newtons second law gives:

F = mv - mu/t

The area under a force - time graph gives the change of momentum or impulse of the force

Term

 

 

IMPACT FORCES - STATIONARY

Definition
If something is initially stationary and the impact causes it to accelerate to v , the gain of momentum will be mv so the force of the impact will be F = change in momentum / contact time = F = mv / t
Term

 

 

IMPACT FORCES - CHANGING VELOCITY

Definition

If object is moving with velocity and the impact changes it's velocity to the change in momentum of the ball is mv-mu so impact force =

F = change of momentum/contact time = mv-mu/t

Term

 

 

SAFETY

Definition
Crumple zones, seatbelts, and airbags are all designed to lessen the impact force on people. This happens by increasing the time  which decreases the acceleration/deceleration which lowers the impact force
Term

 

 

FORCE - TIME GRAPHS FOR IMPACTS

Definition
Variation of impact forces can be recorded using a force sensor which is connected with long wires to an object. Equal and oppositte forces act on object so variation of force with time is displayed. Average force = change of momentum/contact time
Term

 

 

REBOUND IMPACTS

Definition
When a ball hits a wall it's momentum changes direction due to impact. Velocity and momentum are in the opposite direction if the ball hit the wall straight up; the momentum then has the opposite sign
Term

 

 

REBOUND IMPACTS - CHANGE SPEED

Definition

If the ball hits the wall with initial speed u, and rebound with speed v in the opposite direction then towards the wall the momentum would be +mu and away is -mv this means the impact force will be 

Force = (-mv) - (+mu) / t

Term

 

 

IF THERE IS NO LOSS OF SPEED ON IMPACT

Definition
v = u so F = -2mu/t
Term

 

 

OBLIQUE IMPACTS

Definition
Initial direction of the ball is not perpendicular to the wall. Components of the velocity must be used. If the angle of which the impact and rebound are the same, no speed is lost. so u=v
Term

 

 

 

FINAL VELOCITY OF OBLIQUE IMPACTS

Definition
-ucosΘ
Term

 

 

 

CHANGE IN MOMENTUM OF OBLIQUE IMPACTS

Definition
-2mucosΘ
Term

 

 

 

NEWTONS THIRD LAW OF MOTION

Definition
When two objects interact, they exert equal and opposite forces on each other
Term

 

 

THE PRINCIPLE OF CONSERVATION OF MOMENTUM

Definition
For a system of interacting objects, the total momentum remains constant, provided no external force acts on the system.
Term

 

 

CHANGE OF MOMENTUM OF TWO COLLIDING OBJECTS

Definition
They collide then seperate.  Momentum of each object changes. Equal and opposite forces exerted on each other when in contact so equal and opposites change in momentum.
Term

 

 

EQUAL AND OPPOSITE CHANGES IN MOMENTUM

Definition
One object gains momentum and the other object loses momentum
Term

 

 

FINAL MOMENTUM

Definition
total final momentum = total initial momentum
Term

 

 

COLLIDING OBJECTS TICKING TOGETHER

Definition
They have the same final velocity
Term

 

 

TESTING CONSERVATION OF MOMENTUM

Definition

Mass of trolleys measured before the test. Trolley A is pushed towards B at a constant velocity. They both stick on impact. Computer records change in velocity. 

Measurements should show that

(mb+ma)V = mua

Term

 

 

HEAD COLLISIONS

Definition
If initial momentum of one object is exactly equal and opposite to that of the other object, the total momentum after the collision will be zero because they cancel out.
Term

 

 

VECTOR NATURE OF MOMENTUM

Definition
If the final momentum is in the other direction to the first object then it will be -. if it's in the same direction it will be +
Term

 

 

CRASH BARRIERS

Definition
Stop vehicles from entering the opposite carriageway. Strongest barrier are designed to ithstand the impact of a 38 tonne heavy good s vehicle at an angle of 20º and at 40mph
Term

 

 

ELASTIC

Definition

Elastic - loses no kinetic energy. e.g a ball will have the same amount of kinetic energy before the impact and after the impact.

NO LOSS OF KINETIC ENERGY

Term

 

 

INELASTIC COLLISIONS

Definition
Lose kinetic energy. Colliding objects may stick together
Term

 

 

PARTIALLY INELASTIC COLLISION

Definition
Colliding objects move apart and have less kinetic energy after the collision than before.
Term

 

 

KINETIC ENERGY AFTER/BEFORE A COLLISION

Definition
EK = 1/2 mv2
Term

 

 

KINETIC ENERGY IMMEDIATELY BEFORE IMPACT

Definition
Loss of potential energy = mgH
Term

 

 

KINETIC ENERGY IMMEDIATELY AFTER IMPACT 

Definition
Gain of potential energy = mgh
Term

 

 

 

HEIGHT RATIO

Definition
h/H Gives the fraction of the inital energy that is recovered as kinetic energy after the collision.
Term

 

 

MOMENTUM

Definition
Always conserved in collisions. Energy is always conserved, but it may be transferred into other forms 
Term

 

 

EXPLOSIONS

Definition
Two objects fly apart - recoil from each other with equal and opposite amounts of momentum -move away in opposite directions. 
Term

 

 

RATIO OF SPEED WITH MASS

Definition
Ratio of speed is the inverse to the mass ratio
Term
Definition