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Physics - Forces and Motion Part 2 Section 1 (Force & Momentum (A…
Physics - Forces and Motion Part 2 Section 1
Momentum
Momentum depends on mass and velocity.
Momentum = mass x velocity
Symbols: p = m x v
Units: Kgm/s = Kg x m/s
Example: 70Kg person, jogging at 5 m/s
p = m x v
= 70 x 5
= 350 Kgm/s
Example 2: How fast is the same person going if they have a momentum of 210 Kgm/s?
v = p ÷ m
210 ÷ 70
= 3 m/s
Momentum 2
Momentum has both magnitude and direction. (so it is a vector.)
It's direction is the same as its velocity
The faster the rugby player runs, the greater his momentum is.
Force & Momentum
A force will cause the velocity of an object to change and therefore also it's momentum.
The greater the force the faster the momentum changes.
Force = Momentum change ÷ time taken for the change
Force is in Newtons (N)
Change is in Kilogram metres per second (Kgm/s)
Time is in seconds (s)
= 3200 x 5
Momentum is conserved in any collision or explosion provided no external forces act on the colliding or exploding bodies.
= 16,000 Kgm/s
Example 1: A van has a mass of 3200Kg. Just before the collision, the van was moving at 5 m/s and the car was stationary.
Momentum = mass x velocity
16,000 Kgm/s = total mass x velocity
16,000 ÷ 2 = total mass
= total mass x 2 m/s
Total mass = 8000
mass of car + 3200 = 8000
mass of car = 8000-3200
= 4800 Kg
Force & Momentum 2
If the momentum is conserved in this collision, the total momentum afterwards will be the same.
Total Momentum before collision = Total Momentum after collision.
If the car is stationary then the momentum is 0.
Explosions
Before an explosions the total momentum is 0.
As momentum is conserved, the total momentum afterwards must also be 0.
This means that the different parts of the exploding body must move off in opposite directions.
An artillery gun of mass 1500Kg fires a shell of mass 20Kg at a velocity of 150 m/s, calculate the recoil velocity of the gun.
p = m xv
= 20 x 150
= 3000 Kgm/s
-> momentum of shell, <- momentum of gun
p = - 3000 Kgm/s
Centre of Mass
Also called centre of gravity, the centre of mass is the point at which all of the mass of the body can be assumed to be concentrated.
Centre of Gravity
Where there are two lines of symmetry the centre of gravity is where the two lines cross.
If suspended, a body will come to rest with the COG directly below the point of suspension.
The centre of gravity of a symmetrical body is along the axis of symmetry.
Method: Finding the centre of mass
Hang a plumb line from the point of suspension.
Using the plumb line as a reference draw a vertical line on the card.
Suspend the card from one of these holes,
Repeat for the other holes
Pierce the card in at least two places.
The centre of mass is where the lines cross on the card.
Centre of Mass
A body is stable as long as the line of action of it's weight lies inside the base of the body.
If this is not the case there will be a resultant moment and the body tend to topple.
Turning Effect of a Force
Also known as the 'moment' of a force.
The moment of a force about any point is defined as as:
Force x perpendicular distance from the turning point to the line of action of the force.
Moment = f x d
(Nm) = (n) x (m)
Moment can be either CLOCKWISE or ANTICLOCKWISE
A moment is the turning effect of a force.
The moment of a force is equal to the force multiplied by the perpendicular distance between the line of action of the force and the turning point.
Principal of Moments
When an object is not turning (e.g. balanced). The total Clockwise moment is equals the total anticlockwise moment.
If the ruler is balanced then:
CLOCKWISE MOMENT = ANTICLOCKWISE MOMENT
Changing shape
Force can change the shape of an object.
A stretching force puts an object such as a wire or spring under tension.
A squashing force puts an object under compression.
Force can change: Shape, Speed, Direction
Mass
Mass is the amount of matter
Matter = stuff
Mass is in Kg
Weight
Weight is the force due to gravity acting on mass.
W = m x g
Weight = mass x gravitational field strength
Hooks Law
A practical to measure the extension of a spring when forces are added gives results like this:
Hooks Law states that the extension of a spring is proportional to the force used to stretch the spring.
'Proportional' means that if the forces is doubled then the extension also doubles.
The line on a graph of force against extension will be a straight AND go through the origin.
