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| When two things collide, the effect they have on each other depends on thier mass and initial velocity |
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| Sir Isaac Newton realised that a force is needed to change the velocity of an object |
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| Depends on the amount of force and the objects mass |
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Definition
Of a moving object;
= Mass x Velocity
p = mv |
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FORCE NEEDED TO GIVE AN OBJECT A CERTAIN ACCELERATION |
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Definition
Force = mass x acceleration
F = ma |
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WHERE DOES NEWTONS LAW OF MOTION NOT APPLY? |
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Definition
| Inside an atom (quantum physics), Speeds approaching the speed of light or in strong gravitational fields (Einstiens theory of relativity) |
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Unit is kgms-1
Vector quantity - direction is same as the velocity
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NEWTONS FIRST LAW OF MOTION |
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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) |
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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 |
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NEWTONS SECOND LAW OF MOTION |
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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 |
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Definition
An object of mass m 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
F ≈ ma
F = kma
K = constant of proportionality = 1 |
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Definition
| This form of Newtons second law is used when there is a continuous loss or gain of mass of an object |
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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 |
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Term
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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 |
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IMPACT FORCES - STATIONARY |
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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 |
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IMPACT FORCES - CHANGING VELOCITY |
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Definition
If object is moving with velocity u and the impact changes it's velocity to v the change in momentum of the ball is mv-mu so impact force =
F = change of momentum/contact time = mv-mu/t |
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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 |
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FORCE - TIME GRAPHS FOR IMPACTS |
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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 |
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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 |
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Term
REBOUND IMPACTS - CHANGE SPEED |
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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 |
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IF THERE IS NO LOSS OF SPEED ON IMPACT |
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Definition
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Term
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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 |
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FINAL VELOCITY OF OBLIQUE IMPACTS |
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Definition
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CHANGE IN MOMENTUM OF OBLIQUE IMPACTS |
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Definition
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NEWTONS THIRD LAW OF MOTION |
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Definition
| When two objects interact, they exert equal and opposite forces on each other |
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THE PRINCIPLE OF CONSERVATION OF MOMENTUM |
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Definition
| For a system of interacting objects, the total momentum remains constant, provided no external force acts on the system. |
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CHANGE OF MOMENTUM OF TWO COLLIDING OBJECTS |
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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. |
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EQUAL AND OPPOSITE CHANGES IN MOMENTUM |
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Definition
| One object gains momentum and the other object loses momentum |
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Term
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Definition
| total final momentum = total initial momentum |
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COLLIDING OBJECTS TICKING TOGETHER |
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Definition
| They have the same final velocity |
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TESTING CONSERVATION OF MOMENTUM |
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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 = ma ua |
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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. |
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VECTOR NATURE OF MOMENTUM |
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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 + |
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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 |
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Term
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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 |
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Term
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Definition
| Lose kinetic energy. Colliding objects may stick together |
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PARTIALLY INELASTIC COLLISION |
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Definition
| Colliding objects move apart and have less kinetic energy after the collision than before. |
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KINETIC ENERGY AFTER/BEFORE A COLLISION |
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Definition
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KINETIC ENERGY IMMEDIATELY BEFORE IMPACT |
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Definition
| Loss of potential energy = mgH |
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KINETIC ENERGY IMMEDIATELY AFTER IMPACT |
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Definition
| Gain of potential energy = mgh |
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Definition
| h/H Gives the fraction of the inital energy that is recovered as kinetic energy after the collision. |
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Definition
| Always conserved in collisions. Energy is always conserved, but it may be transferred into other forms |
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Definition
| Two objects fly apart - recoil from each other with equal and opposite amounts of momentum -move away in opposite directions. |
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Definition
| Ratio of speed is the inverse to the mass ratio |
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