Momentum Flashcards
What is momentum?
Momentum is a measure of how easily an object may be brought to rest. An object with larger momentum will need a bigger force (acting for a longer time) to bring it to rest.
What unit is momentum in?
Kilogram metres per second (kg m/s) provided that the mass is in kg and the velocity in m/s.
What is newtons 2nd law?
The rate of change of momentum of an object was proportional to the size of the force acting on the object.
What is the equation for force (in terms of momentum)
Force = final momentum - initial momentum / time taken
Force = change in momentum / time
What does the conservation of momentum mean?
The momentum of bodies colliding is always conserved (i.e. the same before and after the event), providing that the only forces acting on the bodies is the force of one body on another and vice versa. In practice, other forces must be taken into account; i.e. Friction, air resistance and weight.
What must you take in account with momentum?
The direction of movement as it is a vector quantity: if one object is moving north and the other south, the sign of one must be negative. It does not matter which you choose to be negative but you must stick to that decision.
What is an elastic collision?
One in which momentum is conserved and no kinetic energy is lost
What is an inelastic collision?
Where colliding objects do not rebound at all, they stick together. Momentum is conserved but some of the kinetic energy is transferred into other forms e.g. sound or thermal
What are newton’s laws of motion?
- Unless a resultant force acts on an object it will continue to move in a straight line at a constant speed or remain at rest.
- The rate of change of momentum of a body is proportional to the force acting on it.
- For every action, there is an equal and opposite reaction.
Describe an experiment to demonstrate conservation of momentum:
On a levelled linear air track you should make one glider collide with another, it is simplest if the second glider is stationary. You measure the mass of both gliders. If you make the collision inelastic (so the gliders stick together) it is easier to measure the momentum after the collision. Measure the velocity of the moving glider before the collision and the velocity of the gliders after the collision has taken place. The experiment should be repeated but different masses and velocities.
What is the equation for momentum?
Momentum = mass x velocity
Equation to show increase in momentum:
Force x time = increase in momentum
What is a collision?
An event in which two moving objects exert forces on each other for a short period of time
Explain how momentum is used in safety
The equation force=change in momentum/time taken implies that when there is a momentum change e.g. when you land after a jump or when a vehicle stops suddenly in a crash, the force that is involved will be smaller if the time which the momentum change takes can be made longer
Examples of how momentum is used in safety?
- bending your legs when you land after a jump
- wearing a seatbelt in a car - the belt stretches slightly as you stop, increasing stopping time
- cars are fitted with air bags - the bags inflate automatically in the event of a crash and ‘cushion’ the impact, allowing your momentum to be reduced to zero more slowly
- cars are built with crumple zones at the front - they crumple when the car hits something head-on, increasing the stopping time
