Module 3.2 - Forces in Action Flashcards
What is the resultant force?
A single force that which has the same effect as the sum of all the forces acting on a body.
What does one newton mean?
The force that causes a mass of 1kg to have an acceleration of 1ms^-2.
How do you work out net force?
F = ma
How do you work out the weight of an object?
W = mg
Define tension
Force experienced by any rope, string, cable or wire that is being pulled, hung, rotated or supported
Define normal contact
Force that acts perpendicular to the point of contact of a body
Define upthrust
Upwards force that a liquid/gas exerts on a body floating on it.
Define friction
Force between two surfaces in contact, it opposes the motion at the point of contact.
Define drag
Frictional force experienced by an object travelling through a fluid. Opposite to motion.
What factors affect drag?
Density and Cross-sectional area
Describe the motion of objects falling in a uniform gravitational field in the presence of drag
> Initially, the object accelerates because the downwards gravitational force is greater than the resistive force
However, the drag increases as the object accelerates.
Eventually the drag becomes equal to the weight so the resultant force becomes zero - constant velocity
Define terminal velocity
The velocity at which an object’s drag equals to its accelerating force. Therefore there is no resultant force and no acceleration.
What is equilibrium?
When the resultant force is zero in the same plane
What is a triangle of forces?
Represents the direction and magnitude of three coplanar forces that are on an object in equilibrium.
What is a moment (Nm) of a force?
Force (N) x Perpendicular distance (m)
State the principle of moments
The sum of CLOCKWISE moments = the sum of ANTI-CLOCKWISE moments for the object to be in rotational equilibrium.
What is a couple?
When two equal and anti parallel forces act to produce a rotation - no linear motion occurs.
What is the moment/turning effect of a couple called?
A torque
Define the centre of mass
Single point at which ALL of the mass of an object can be assumed to be situated - usually the centre of objects with a constant density.
What is the centre of gravity?
Single point through which the entire weight of the object can be thought to act.
How do you work out density?
Density (kg m^-3) = mass (kg) ÷ volume (m^3)
How do you work out pressure for solids, liquids and gases?
p = F ÷ A
Why does pressure increase with depth in a fluid?
Due to the force exerted by the increased weight of the fluid above.
How do you work out the pressure of a fluid at depth?
p = depth x density of fluid x g
What is Archimedes’ principle?
UPTHRUST of an object in a fluid equals to the weight of the water displaced.
How do you work out the upthrust of a fluid?
Due to Archimedes principles where Upthrust = weight displaced:
Upthrust = pressure x area
= (depth x density x g) x area of object
