4 Forces in Action Flashcards
Describe Newton’s Second Law
Force is directly proportional to the rate of change of momentum IN THE SAME DIRECTION.
Describe weight
Weight of an object is the gravitational force acting on an object. F=ma ∴ W=mg
What is the centre of mass
The resultant gravitational force on the object, total weight acting through a singular point.
Practical - finding the centre of gravity
Freely suspended object comes to rest with centre of mass vertically below point of suspension. Plumb-line - plumb-bob comes to rest, string is vertical. Where these intersect - centre of gravity.
6 Forces acting on an object
Weight, friction, drag (air resistance), tension, upthrust, normal contact force
Force parallel to the slope?
Fx=mg sinθ
(Wsinθ)
Force perpendicular to the slope?
Fy=mg cosθ
(Wcosθ)
Define drag force
Drag is a frictional force that opposes the motion of an object.
What affects the magnitude of a drag force?
SPEED of an object
cross-sectional AREA
- large area increase drag
- drag force ∝ speed^2
Define terminal velocity
Drag force on object is equal and opposite to its weight. Zero acceleration, constant velocity.
Define moment
The moment of a force is the turning effect of a force about some axis.
Moment=force X perpendicular distance
Moments - how to find perpendicular distance?
x=d cosθ
Principle of moments?
For a body in ROTATIONAL equilibrium the sum of the anticlockwise moments is equal to the sum of the clockwise moments
Define couple?
Two forces equal and parallel and along different lines
Define torque?
Moment of a couple is a torque.
Torque of a couple= one of the forces X perpendicular separation between forces
Density equation?
Mass per unit volume ρ=m/v (kg m^-3)
Pressure equation and definition?
Normal force exterted per unit cross-sectional area. p=F/A
1Pa= N m^-2
Pressure in FLUIDS equation?
p=hρg
Derive equation for pressure in FLUIDS
pressure at base=weight/area
W=mass * g (mass=density X volume)
W= ρV * g (volume=Ah)
W= ρ * Ah * g
P =(ρAh*g)/A
P=hρg
What is pressure proportional to?
p∝h
- water pressure increases with depth
ρ shows denser liquids will exert greater pressure
Define Upthrust
The upward force exerted by a fluid that opposes the weight of a partially or fully immersed object.
Equation of upthrust
force at the top: hρgA
force at bottom: (h+𝑥)ρgA -hρgA = 𝑥ρgA
∴ upthrust=A𝑥ρg
State Archimedes’ principle
The upthrust exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
