4. Forces (3.2) Flashcards
Force equation (the one assuming constant mass)
F = ma
Tension
The force within a stretched object
Friction
The force that opposes motion
Drag
The force that opposes motion in a fluid
Normal contact force
The force which acts at 90° to where 2 objects touch
Upthrust
The upward buoyancy on an object in a fluid
What is the unit for force?
What is the this unit in base SI units?
N
kg m s^-2
What is a pair of forces?
The force of X on Y
The force of Y on X
Does centre of mass depend on the gravitational field?
No
What things lead to high stability of an object? (2)
- low centre of masss
- wide base
What is the line of action in a force diagram?
The line downward below the centre of mass.
What is the formula for the distance of centre of mass (from the point of reference) of two masses of m1 and m2 which are, respectively, a distance of d1 and d2 away from your point of reference?
d = (m1d1 + m2d2)/(m1+m2)
What four factors affect drag in a fluid?
- SA
- particle density in fluid
- roughness/texture of object
- speed of object
What happens to the drag if SA increases?
Increases
What happens to drag if fluid density increases?
Increases
What happens to drag if roughness increases?
Increases
What happens to drag when speed increases?
Increases
What are the two most important factors that affect an object’s drag (F)?
Speed and SA
F is proportional to v^2
F is proportional to A
What is terminal velocity?
The velocity at which there is 0 acceleration.
The drag force is equal and opposite to its weight.
Read up on the PAG where you drop the ball bearing into viscous liquid to determine terminal velocity.
What is the formula for the moment of a force?
and unit?
M = Fx
Nm - apparently it shouldn’t be J
What is the formula fortorque?
Torque = Fd where d = distance between both points/diameter of the imaginary circle
What is the principle of moments?
For a body in rotational equilibrium the sum of anti-clockwise moments about any point is equal to the sum of clockwise moments about that same point.
Do you know how to experimentally determine the centre of gravity?
If so, make a flash card about it.
What is upthrust? (in terms of pressure diff)
The diff in pressure from the bottom of a submerged object and the top of the object
What is the upthrust (F) of an object equal to?
F = mass of water displaced * g
or,
F = Weight of water displaced
Density formula
Rho = m/V
Pressure formula for all states
P = F/A
Pressure formula for fluids
P = h * Rho * g
What is upthrust (F) in terms of SA (A), vertical length (x), density (Rho), and gravitational constant (g)?
F = AxRho*g
Archimedes’ Principle
The upthrust exerted on a body immersed in water is equal to the weight of the fluid displaced.
