Module 3: C7 - Laws Of Motion And Momentum Flashcards
What is Momentum
Momentum is a property of objects with mass and velocity.
It is a vector quantity with the same direction as the velocity of the object.
The units are kgms–1. (This can also be expressed as Ns).
Equation for Momentum
Momentum = Mass x Velocity
p = mv
What is the rate of change of momentum proportional to
The rate of change of momentum of an object is proportional to the resultant force acting on the object.
Equation for Force with constant Masses
F = mΔv / t
F = ma
Equation for Change in Momentum
Δp = mv-mu
Example Question:
A rugby ball of mass 0.5 kg is kicked from stationary to a velocity of 8 m/s. The kicker’s foot is in contact with ball for 0.1 seconds. What force does the kicker use?
Ft = mv-mu
0.1F = (0.5x8) - (0.5x0)
0.1F = 4
F = 40N
Example Question:
A tennis ball of mass 250g strikes a surface with a velocity of 15m/s. It rebounds directly backwards with a velocity of 7m/s. What is the change in momentum of the ball?
(0.25x15) - - (0.25x7) 5.5kgms^-1
What is the Principle of Conservation of Linear Momentum?
The total linear momentum of a system of interacting bodies is constant, providing no external forces act.
This applies to collisions, where objects move together and hit one other, and to explosions, where objects fly apart from one another after initially being at rest.
How does the idea of the Conservation of Momentum work?
If two objects collide or interact, the forces acting on each one will be the same size but in opposite directions. The same is true for the change in momentum of each object.
This means that the momentum lost by one of the objects will be gained by the other object. Therefore, whenever two objects collide or interact, momentum is conserved.
Example Question:
Consider the nuclear decay of Americium-241:
If the new neptunium atom (of mass 237) moves away at a speed of 5x105 ms-1 what was the velocity of the alpha particle?
0 = 237 x 5x10^5 - 4V
4v = 237x5x10^5
v = -29625000
v = -3x10^7
What happens in Perfectly Elastic collisions
In this type of collision the objects separate after the collision.
- Momentum is conserved i.e. the momentum before the collision is equal to the momentum after.
- Total energy in the collision is conserved
- Kinetic energy is conserved i.e. the kinetic energy before the collision is equal to the kinetic energy after
What happens in Inelastic collisions
In this type of collision the objects stick together after the collision.
- Momentum is conserved i.e. the momentum before the collision is equal to the momentum after.
- Total energy in the collision is conserved
- Kinetic energy is not conserved however.
What is Newton’s First Law of Motion
An object will remain at rest or continue to move with constant velocity unless acted upon by a resultant force.
If an object’s velocity changes, then you know a resultant force must be acting on the object. Remember that velocity is a vector quantity, so an object’s velocity changes if it’s speed and/or direction changes.
What is Newton’s Third Law of Motion
When two objects interact, they exert equal and opposite forces on each other.
When two objects interact, the pair of forces produced will always be equal and opposite. The forces acting on the interacting objects are always the same type.
How could you Investigate Momentum
There are several ways to investigate momentum in the laboratory.
A linear air track is ideal because a cushion of air minimises the friction between the gliders and track, but trolleys and a horizontal runway would also work. The velocity of each object is determined with a motion sensor and a laptop, light gates, and a digital timer (or a stopwatch) to measure the time taken over a known distance.
