C4 - Force, Mass and Weight Flashcards
How is centre of mass / gravity calculated?
The plumb line and bob experiment
The centre of mass is a point through which any externally applied force produces straight line motion and no rotation.
What are the 6 key forces on free body force diagrams?
Weight (vertically downwards)
Friction (against motion)
Drag (against motion)
Tension (within rope or cable)
Upthrust (vertically upwards)
Normal (perpendicular to ground)
How is weight resolved into two parallel and perpendicular components?
Parallel:
- W sin ø
- Fx = mg sin ø
Perpendicular:
- W cos ø
- Fy = mg cos ø
What is drag?
A frictional force that opposes the motion of the object through the fluid.
It depends on the speed and area of the moving object.
What’s terminal velocity?
When the drag force is equal and opposite to its weight and there’s zero acceleration so speed is constant.
What’s a moment?
The turning affect of a force around a point.
The product of force and perpendicular distance from a pivot.
What’s a couple?
A pair of equal and opposite forces acting on a body but not in the same line / in opposite directions.
(Anti-parallel forces)
What’s torque?
The moment of a couple
Torque (of a couple) is the product of one of the forces of a couple and the perpendicular distance between forces.
How is a moment calculated?
Moment = Fx
How is force calculated?
F = am
Therefore:
W = mg
What is the principle of moments?
The sum of clockwise moments is equal to the sum of anti-clockwise moments when an object is in rotational equilibrium so the net moment is zero.
How is the perpendicular distance of a moment calculated?
By using trigonometry
How is torque of a couple calculated?
Torque = one of the forces * the perpendicular separation between the forces
= Fd
What is density?
How is it calculated?
The defined mass per unit volume of a substance.
p = m / v
How is pressure calculated?
P = F / A
Pressure is the normal force exerted per unit cross sectional area
How is the pressure of a vertical column of any liquid calculated?
From it’s weight and cross sectional area of its base
P = hpg Pressure = height of liquid column * density of fluid * acceleration of free fall (9.81)
What is upthrust?
The upward buoyant force exerted on a body immersed in a fluid.
How is upthrust calculated?
Upthrust = Axpg
= cross sectional area * distance * fluid density * acceleration of free fall
What is Archimedes’ principle?
Upthrust exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces.
Why would the weight of an object not act in the middle of the object?
Distribution of mass is not uniform.
If two people are holding an object with non-uniform distribution of mass and person B (further from the centre of mass) moves closer to the centre, what happens to forces on person A and B?
The turning effect of B is less therefore B needs a greater force to produce the same moment (in order to be in equilibrium)
What is a newton?
When 1kg mass has an acceleration of 1m/s
Describe, in terms of forces acting on the driver how wearing a seat belt and having an air bag in a car can help to protect the driver from injury in a collision:
Impact time increases
Force on the driver decreases
Prevents them colliding with the windscreen
Wider area of airbag (and seatbelt) reduces pressure
The air bag deflates quickly to prevent whiplash
What factors affect braking distance?
Condition of road - icy/wet etc
Condition or car/tyres
Mass of car
Speed of car
How is GPS used to locate the position of a carL
The distance of the car from different satellites is determined by using electromagnetic waves (radio and micro).
Trilateration used
Delay time from GPS to satellite determines distance/position of car.
Why might an object falling from a pulley have an acceleration less than acceleration of free fall / g?
There is tension in the wire which acts as a resistive force / there’s a vertical, opposing.
State why the equation F = ma cannot be applied to particles traveling at speeds very close to the speed of light?
The mass of particles increases as speed nears the speed of light
How does total force opposing the motion of a car change when it travels up a hill?
The car’s weight opposes motion / more work is done as the car gains GPE
What experiment did Galileo carry out which overturned Aristotle’s ideas of motion?
He dropped objects of different masses from the tower of Pisa.
He timed how long it took for them to hit the ground and found they both hit the ground at the same time.
Acceleration is the same for all objects. (Disproving Aristotle’s concept that heavier objects fall faster).
