# 4 definitions by White Razor

Top Definition
Simple terms: The 'reason' that to why you have to use some sort of effort to move or stop something like an object.

The law/principle of inertia states that a body at rest will remain at rest and a body in motion will continue moving with a constant velocity unless acted upon by an external non zero force.

Don't confuse this with momentum
Due to inertia, a moving body in space (ie a vacuum) will not stop or deviate unless another non zero force acts on it (eg gravity)
by March 25, 2007
Defined as the change in position of an object (displacement) per unit of time. Velocity, unlike speed, takes into account the direction of travel relative to a base point.

v = s/t (where v and s are vectors)

Where (using SI units):
v is the velocity of the body in metres per second (ms^-1)
s is the displacement of the body in metres (m)
t is the time taken to travel from the initial point to the final point in seconds (s)
1. (One dimension) If you start at point A and travel directly north to point B which is ten metres away, and it takes you five seconds to get there, your velocity will be 2ms^-1 due north.

2. (One dimension) If you travel west for a ten seconds at 1ms^-1, take a break for twenty seconds and then travel for another ten seconds at 1ms^-1, your velocity for the trip will be .05ms^-1 due west.

3. (Two dimension) If you start at point A and travel north at 3kmh^-1 for two hours, and then instantaneously change direction and travel due east for four hours at a speed of 2kmh^-1 to point B, your velocity for the trip is 10kmh^-1 N53°E (53°T)

I think...
by March 25, 2007
The measure of the difficulty to bring a moving body to a halt.

The momentum of the body is equal to the product of the mass and the velocity of that body:

p = mv (where p and v are vectors)

Where (SI units used):
p is the momentum in Newton seconds (Ns or sec Newtons, sN)
m is the mass of the body in kilograms (kg)
v is the velocity of the body in metres per second (ms^-1)
1. A truck coming towards you at 100 ms^-1 will be hard to stop with your body because it has a lot of momentum.

2. A tenis ball thrown at you at .5 ms^-1 will be pretty easy to stop with your face because it has little momentum.
by March 25, 2007
Defined as the change in velocity over a period of time. Acceleration is a vector quantity and therefore is stated as a quantity with a direction.

a = (v-u)/t (where a, v and u are vectors)

Where (using SI units):
a = acceleration of the body in metres per second per second (ms^-2, or metres per second squared)
v = final velocity of the body in metres per second (ms^-1)
u = initial velocity of the body in metres per second (ms^-1)
t = time period between the initial and final velocity, in seconds (s)

An accelerating body can also be decelerating (ie, negative acceleration) or be at rest. Also, instantaneous acceleration is a bit different.
1) A car that is travelling at 2ms^-1 changes its speed to 6ms^-1 over a period of five seconds, and doesn't change direction. During that five seconds, it had an average acceleration of .8ms^-2 in initial direction.

2) A car travelling forwards at 2ms^-1 is suddenly put into reverse and five seconds later it is travelling 2ms^-2 backwards. It's average acceleration is .8ms^-2 backwards (-.8ms^-2 forwards). Note in this example, the car would have been temporarily at rest just when it changes direction. It is still accelerating (in negative direction, ie backwards, ie decelerating) during this period.
by April 10, 2007
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