Momentum + Moments Flashcards
What is centre of mass/gravity
the point on an object where we can assume all of the weight is concentrated
https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2021/november/AQA-84632H-QP-NOV21.PDF
question 7.3
Describe how the force on the pedal causes a moment about the rear axle.
(the force on the pedal) causes
a moment about the pedal axle
which causes a force on the
chain (which causes a moment
about the rear axle)
State the equation that links momentum, mass and velocity
momentum = mass × velocity
p = m v
momentum, p, in kilograms metre per second, kg m/s
mass, m, in kilograms, kg
velocity, v, in metres per second, m/s
State the conservation of momentum
In a closed system, the total momentum before an event is equal to the total momentum after the event
When does a change in momentum occur
When a force acts on an object that is moving, or able to move, a
change in momentum occurs.
The formula that links force, mass, change in velocity and chane in time together
F =m ∆ v / ∆ t
What is m∆ v
m∆ v = change in momentum
What does this equation mean
F =m ∆ v / ∆ t
force equals the rate of change of momentum.
Explain how to get the equation
F =m ∆ v / ∆ t
The equations F = m × a and a =
v − u / t
combine to give the equation F =
m ∆ v /∆ t
If an object is not moving what is its momentum
If an object is not moving then its momentum is zero.
p = mv
if not moving then v = 0 therefore momentum = 0
All moving objects have ________
all moving objects have momentum
A car has a mass of 1000kg and is not moving. What is the momentum of the car?
The car is not moving at all
Therefore its momentum must be 0
Explain how momentum is conserved in the example of a van moving towards a stationary car
The van moves towards the stationary car
Because the van is moving, it has momentum in the forward direction
The momentum of the van is its mass multiplied by its velocity
The stationary car has no momentum as it is not moving
Now the van collides with the car.
Both the van and the car move together at a lower velocity than the initial velocity of the van.
The total momentum of the van and the car together is now the same as the initial momentum of the van by itself.
The total momentum before the collision is the same as the total momentum after the collision. Momentum has been conserved
There is a cannon containing a cannon ball. The cannon is fired Explain how momentum is conserved in the example.
Before the cannon fires, both the cannon and the cannon ball are not moving.
So the total momentum before firing = 0
When the cannon fires, the cannon ball is moving at a very large velocity. So the cannonball has momentum in the forward direction
At the same time, the cannon recoils (moves backwards)
This means that the cannon has momentum acting in the backwards direction
The backwards momentum of the cannon equals the forwards momentum of the cannon ball
This means that the total momentum after firing = 0
The total momentum before the firing the cannon is the same as the total momentum after the firing the cannon. Momentum has been conserved
A child with a mass of 40kg is stationary on a skateboard which has a mass of 1.5kg
If the child jumps off the skateboard with a velocity of 2m/s, calculate the velocity of which the skateboard moves
momentum before = 0kg m/s
momentum before = momentum after
0 = (40 x 2) + (1.5 x -v)
0 = 80 - 1.5v
1.5v = 80
v= 80/1.5 = 53.3m/s
Explain in detail how an air bag reduces the injury to a driver, when a collision occurs
F = ∆p / t
The change in momentum, when a collision occurs, is the same with or without the airbag
When a collision occurs, the airbag slowly changes shape. As a result, this increases the time taken for the collision to occur.
As a result, decreasing the force that will act on the driver, which reduces the injury to the driver
Explain in detail how a waring a seat belt reduces the injury to a driver, when a collision occurs
F = ∆p / t
The change in momentum, when a collision occurs, is the same with or without the seat belt
When a collision occurs, the seat belt will stretch/slowly changes shape. As a result, this increases the time taken for the collision to occur.
As a result, decreasing the force that will act on the driver, which reduces the injury to the driver
State some safety measures in a car
Seat belts
Air bags
Crumple zones
What is the momentum of the hammer just before it hits the nail
m = 0.75 kg
v = 15N
p = mv
p = 0.75 x 15
p = 11.25 kg m/s downward
Explain why it is easier to use a metal hammer to hit the nail into the wall and not a rubber mallet
F = ∆p / t
The change in momentum, when a collision occurs, is the same with the metal hammer and with the rubber mallet
When a collision occurs (and the metal hammer hits the nail), the surface that comes into contact with the nail does not change shape. As a result, this decreases the time taken for the collision to occur.
Therefore, increasing the force that will act on the nail, which makes it easier for the nail to go into the wall
What is a moment
A moment is produced when a force acts on an object which has a pivot which causes the object to turn/rotate
Moment equation in words and symbols
Moment = Force x distance
distance is the perpendicular distance from the line of action of the force to the pivot.
Moment = Nm or Ncm
F = Newtons
d = m or cm
Apart from the magnitude of the moment what must also be stated
The direction of the moment must be stated as being clockwise or anti-clockwise e.g. 50Nm clockwise
How can an object be balanced
An object can be balanced if the pivot is placed in the centre of a regular object
In terms of moments, describe what is happening when the object is balanced
If an object is balanced, the total clockwise moment about a pivot
equals the total anticlockwise moment about that pivot
Explain why when objects are placed down on a surface, they don’t topple over
The centre of mass/gravity is the point on an object where we can assume all of the weight is concentrated.
If the pivot is placed at this point, no moment is produced, therefore the object will be in equilibrium
Explain why objects fall over
The centre of mass/gravity is the point on an object where we can assume all of the weight is concentrated.
An object will fall over when its centre of mass falls outside its base. This will produce a moment causing the object to fall (turn). -
The edge acts as a pivot. A moment is produced, causing the object to rotate/fall
State ways to make an object more stable
Lower the objects centre of gravity
Widen the area of the object’s base
Explain in detail, why a door handle is placed as far away from the hinges as possible
M = Fd
The moment required to open the door is the same wherever the handle is placed
If the door handle is placed as dar away from the hinges as possible, then it means the disance from the line of action of the force to the pivot (the hinges) is greater.
As a result, less force is required produce the moment to open the door, making it easier to open
Explain how a wheelbarrow allows a person to lift a large load that they would otherwise be unable to lift without the wheelbarrow?
see book
The Load is producing a moment around the pivot which is clockwise
The force applied is producing an anticlockwise moment
The clockwise moment = anticlockwise moment
Load x X = Force x Y
y is greater than x therefore a smaller force is required to move a large load
What are gears uses for
Gears use moments to transmit turning forces from one place to another
What is dependent on a gear to make the turning for bigger or smaller
Depending on the radius of the gear, the turning force can be made bigger or smaller
Features of gears
2 gears are connected together
The gear has radius, r - Gear A
Another gear connected, has radius, 2r - Gear B
The first gear is driven by the engine, this gear is connected to the bigger gear which is connected to the wheels
When would this type of gear be used
This is when the car is taking off - speed is not required but force
- They turn in opposite directions
2.The second gear has twice the radius therefore twice the moment. Therefore more turning force is produced but at a lower speed (of rotation)
M = Fd
Gear A rotates twice every time Gear B rotates once.
This means that the work done by the two gears is the same
Features of gears
2 gears are connected together
The gear has , has radius, 2r - Gear A
Another gear connected, r - Gear B
The first gear is driven by the engine, this gear is connected to the bigger gear which is connected to the wheels
When would this type of gear be used
This is used when a car is in motion and wants the wheels to rotate faster, so that the car moves faster.
The second gear has half the radius, therefore half the turning force (moment).
Therefore the turning force is reduced, but the speed of rotation is quicker
Gear B rotates twice every time Gear A rotates once.
This means that the work done by the two gears is the same
https://www.exampaperspractice.co.uk/wp-content/uploads/Moments-Leavers-and-Gears-1.pdf
Question 1
In which position will the weight of the camera cause the largest moment about the pivot
Give the reason for your answer
A
perpendicular distance between the camera and pivot is
greatest
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Question 2
The see saw is going to rotate clockwise
This is because the total anticlockwise moment is 600Nm
The clockwise moment of the girl is also 600Nm
However the weight of the seesaw is also producing an additional clockwise moment (since its centre of mass is 0.25m from the pivot.)
This causes the clockwise moment to be greater than the anti-clockwise moment which causes the see saw to rotate clockwise
