SI units : Kgms-1
Momentum is a vector and so direction/sign must be considered
For the example below, both objects are moving in the opposite direction with the same speed. So one object has a positive velocity and the other one has a negative velocity
So when calculating momentum for each object, the sign of the velocity must be considered
Notice that one velocity is negative as it's moving in the opposite direction
To find the total momentum we just need to add both of them .
So really the sum is zero
Usually this refers to only one object
Change is the opposite of Sum
This is only for one object
For example a mass of 1kg moving at 2ms-1 rebounds with a velocity of 1ms-1
Calculating the change in momentum
This can be applied for the above example.
As there is now a change in direction, the final Momentum is negative. This is because, the sign of the velocity is also negative(opposite)
So we just add it, why?
So now it's more negative so the change is in the opposite direction and greater in magnitude. However, "change" does not usually include the sign.
The sign is very important as it shows the BALL experiences a force in the opposite direction - this means for the change of momentum to be negative a resultant force must be in the opposite direction
And the ball exerts an equal and opposite force on the wall when rebounding
We already know the definition of force
Another word for change in momentum is Impulse and it is same as:
Just remember it like a push. When someone keeps pushing you for some time. Greater the push(force) and time. The more you fall!
So we can calculate the sum of the momentum of a system by adding the momentums together(considering the signs also)
This sum should then equal to the momentum after the collision
That's why we found, how to calculate the total momentum of a system
This is actually important so remember this!
We apply this law when making sure that the total momentum before and after the collision is the same
So we need to distinguish the difference between an elastic and inelastic collision:
There is an easier way to identify an elastic collision
To calculate relative speed, you should use a bit of logic. When two objects are moving in the same direction, then the relative speed is the difference. If they are moving in opposite direction, the relative speed is the sum of two velocities.
Remember that K.E is a scalar and so we just take the magnitude of the K.E of an object(excluding any signs)
As we discus what it is, we will see how to identify the pairs
For example below, the gravitational force between planents are of the same type ,same magnitude and etc. Always remember the features above
They are very similar things however, applied in different situations
You must always sayperpendicular!
So you can either find the component of the force which is perpendicular to the distance (resolving), which is the easiest way - we followed the same method in energy but, in this the force must be perpendicular
Remember that moment is actually a vector and it has a direction - clockwise or anticlockwise
Like the above one -
The clockwise moment:
The anticlockwise moment:
As we know that the clockwise and anticlockwise momentum is equal we can:
So we can find the missing forces or missing distances
This can be used to do questions which state that the system is in equilibrium but, let's first see what center of gravity means!
Center of mass and gravity are two different things but, can be the same if the object is uniform and the same material all the way. This is used in gravitational fields(point masses)
So usually the center of gravity is the point where the weight of the beam is considered to act on
So you need to remember that the weight of the beam can be represented by a resultant force and this also must be considered in moment calculations
The two forces acting opposite to each other is called a couple
They have these characteristics:
So torque only gives turning effect, where as moment can give linear acceleration unless it's stopped by a pivot
The resultant force in a torque is zero but, not in equilibrium as it has a net moment or torque
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