2.3 - Momentum Flashcards
What is momentum
It’s a measure of an objects motion - it gives us an idea of what will be required to stop the object moving, best definition is mathematical
Momentum = mass x velocity
Is momentum a scalar or vector
Mass is a scalar and velocity is a vector - so this means it’s direction it’s very important
Momentum is a vector
How also can we consider momentum using Newton’s second law
It’s a measure of the accelerating force, and the time it is applied for -
Newton’s second law is..
The force is that needed to bring an object up to the speed it is moving at - or its the force required, and for how long, to bring a moving object to rest
How can Newton’s second law be written mathematically
F = ma
But the formula is only true as long as the mass remains constant
How did newton originally write his second law
The rate of change of momentum of a body is directly proportional to the resultant force applied to the body, and is in the same direction as the force
F = (mv-mu ) /t
Or f = triangle p/ triangle t
How can we investigate momentum change
You can investigate the rate of change of momentum in the school labatory
If you record the movement of a trolley over time as a force it makes from rest - you can find its velocity each second, you will be able to plot s graph of momentum against time - it should be a straight line as p = f x t - the gradient of this line will be equal to the accelerating force
Define momentum
P = mv
Mass x velocity
What does the conservation of linear momentum tell us
The principle tells us that if we calculate the momentum of each object before they collide - the sum of the total momentum accounting for direction will be the same as the sum total afterwards
What does the word linear refer to
Appears to remind us that this is all about objects moving in straight lines
What does the principle of momentum depend on
The condition that no external force acts on the objects in question - an external force would provide an additional acceleration, and the motion of the objects would not be dependent on the collision alone
Momentum is only conserved if no external force acts
Eg an external force of gravity means the conservation of momentum alone cannot be used to predict the motions after the collision
What do some people say when in a collision, two objects join together to become one and move off together
They are often said to coalesce
What happens with momentum when an object explodes
Velocity takes account for direction for all momentum cancels out - it had no momentum at the start so there’s none at the end
An explosion in physics might even be 2 trapeze artists let go of their hands and swing apart
How can we investigate the transfer of momentum
Using trolleys or sliders on an air track - by recording the movement of one trolley crashing into another, you can find the momentum of each one - calculate the sum of the total momenta before and after collision will allow you to verify the principle of conservation of linear momentum
Could also try an explosion in which the trolleys spring apart from a stationary position
I’m experiments we often find momentum not conserved in the measurements we make, with air track collisions, the measurements match very closely or exactly with conservation theory - minimal friction
Tell me about Newton’s third law
Conservation of momentum is directly responsible for Newton’s third law, told us for every force there is an equal and opposite force
F = (m(v -u) )/ t
Or F = dp/ dt
^ means a force changing momentum in one direction must be countered by an equal and opposite one to ensure overall momentum conserved
Eg the gravitational pull of the earth causes an apple to fall from a tree, the apple gains momentum towards the earth - to conserve momentum, earth must gain an equal and opposite momentum. This is then caused by an equal and opposite gravitational force on the earth from the apple - but the immense mass of the earth means acceleration is imperceptible to us
Define conservation of linear momentum
Means the vector sum of the momenta of all objects in a system is the same before and after any interaction (collision) between the objects
