What is Momentum
Momentum is an often used phrase in sports. A squad that has the momentum is on the move and is going to require some work to stop. A squad that has a lot of momentum is truly on the move and is going to be hard to stop. Momentum is a physics word; it refers to the amount of motion that an item possesses. A sports squad that is on the move has the momentum. If an item is in motion (on the move) then it has momentum.
Momentum may be described as "mass in motion."
All things have mass; hence if an object is moving, then it has momentum - it has its mass in motion. The quantity of momentum that an item possesses is dependant upon two variables: how much stuff is moving and how quickly the thing is going. Momentum relies upon the variables mass and velocity. In terms of an equation, the momentum of an object is equal to the mass of the object times the velocity of the object.
Momentum = mass x velocity
In physics, the sign for the quantity momentum is the lower case p. Thus, the preceding equation may be represented as
p = m • v
where m is the mass and v is the velocity. The equation indicates that momentum is directly proportional to an object's mass and directly proportional to the object's velocity.
The units for momentum would be mass units times velocity units. The standard metric unit of momentum is the kg•m/s. While the kg•m/s is the standard metric unit of momentum, there are a range of additional units that are valid (though not traditional) quantities of momentum. Examples include kg•mi/hr, kg•km/hr, and g•cm/s. In each of these cases, a mass unit is multiplied by a velocity unit to give a momentum unit. This is compatible with the equation for momentum.
Momentum as a Vector Quantity
Momentum is a vector quantity.
As stated in a previous section, a vector quantity is a quantity that is completely characterised by both magnitude and direction.
To adequately explain the momentum of a 5-kg bowling ball going westward at 2 m/s, you must include information about both the size and the direction of the bowling ball.
It is not enough to mention that the ball has 10 kg•m/s of momentum; the momentum of the ball is not completely explained until information about its direction is supplied.
The direction of the momentum vector is the same as the direction of the velocity of the ball.
In a previous unit, it was mentioned that the direction of the velocity vector is the same as the direction that an item is travelling.
If the bowling ball is going westward, then its momentum may be completely defined by stating that it is 10 kg•m/s, westward.
As a vector quantity, the momentum of an object is completely characterized by both magnitude and direction.
The Momentum Equation as a Guide to Thinking
From the definition of momentum, it becomes evident that an item has a big momentum if both its mass and its velocity are large.
Both factors are of equal significance in determining the momentum of an item.
Consider a Mack truck and a roller skate travelling down the street at the same pace.
The considerably greater mass of the Mack truck gives it a considerably greater momentum.
Yet if the Mack truck were at rest, then the momentum of the least large roller skate would be the biggest. The momentum of any object that is at rest is 0.
Objects at rest do not have momentum - they do not have any "mass in motion."
Both variables - mass and velocity - are significant in comparing the momentum of two objects.
The momentum equation may allow us to think about how a change in one of the two variables could alter the momentum of an item.
Consider a 0.5-kg physics cart loaded with one 0.5-kg brick and travelling at a speed of 2.0 m/s.
The overall mass of laden cart is 1.0 kg and its momentum is 2.0 kg•m/s.
If the cart was instead loaded with three 0.5-kg bricks, then the total mass of the laden cart would be 2.0 kg and its momentum would be 4.0 kg•m/s.
A doubling of the mass results in a doubling of the momentum.
Similarly, if the 2.0-kg cart had a velocity of 8.0 m/s (instead of 2.0 m/s), then the cart would have a momentum of 16.0 kg•m/s (instead of 4.0 kg•m/s).
A quadrupling in velocity results in a quadrupling of the momentum.
These two examples explain how the equation p = m•v works as a "guide to thinking" and not just a "plug-and-chug prescription for algebraic problem-solving."
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