4 - Forces in Action Flashcards
What is Newton’s first law of motion?
A body will remain at rest or move with a constant velocity unless it is acted on by a net external resultant force.
What is Newton’s second law of motion?
The rate of change of momentum of an object is directly proportional to the resultant force acting on it.
resultant force ∝ rate of change of momentum
Fᵣₑₛ = Δp/t
If mass is constant:
resultant force ∝ acceleration
resultant force = mass x acceleration
Fᵣₑₛ = ma
What is Archimedes’ Principle?
Archimedes’ Principle states that the upthrust force is equal to
the weight of fluid displaced.
What is Newton’s third law of motion?
When body A exerts a force on body B, body B exerts an equal and opposite force along the same line on body A, and both forces are of the same type.
Although both forces are the same magnitude, they are acting on different bodies and are therefore having a different effect.
What is Terminal speed?
Fᵣₑₛ = (weight – drag) = ma
When weight = drag:
Fᵣₑₛ = 0 = ma
and so: a = 0
What is the moment of a force?
moment = force x perpendicular distance from the turning point to the line of action of the force
moment = F x
Unit: newton-metre (Nm)
What is the centre of mass?
The centre of mass of a body is the point through which a single force on the body has no turning effect.
What is the principle of moments?
When an object is in equilibrium (e.g. balanced):
total anticlockwise moments = total clockwise moments
What are the conditions required for equilibrium?
- The resultant force acting on the body must be zero.
- The resultant moment must be zero.
What is a couple (and torque)?
A couple is a pair of equal and opposite forces acting on a body, but not along the same line.
Torque is the total moment of a couple.
Torque = Fd
How do you calculate pressure at a depth in a fluid?
What is the formula for upthrust (in object in equilibrium)?
U = mg
(Upthrust = Weight of object)
How do you calculate the resultant force acting on an object immersed in a liquid?
Fₜₒₚ = Ahρg
Fᵦₒₜₜₒₘ = A(h + x)ρg
Fᵣₑₛ = U = A(h + x)ρg- Ahρg = Axρg
Where x is the difference in depth between the top and bottom of the object
