Forces In Action Flashcards
State the triangle of forces rule
If a body is in equilibrium under the action of three coplanar forces and these forces are represented in magnitude and direction by vectors drawn to scale and drawn in order, they will form a closed triangle
Define centre of gravity
The point at which the weight of an object is thought to act
Define a moment
Moment = force x perpendicular distance
Define a couple
A couple consists of two equal and opposite parallel forces acting at equal distances from a pivot
What are the properties of a couple?
- consists of two equal and opposite parallel force
- always tends to produce rotation
- cannot produce a resultant force
- cannot produce translational motion
State the principal of moments
When a body is in equilibrium, the sum of the clockwise moments about any point must equal the sum of the anti-clockwise moments about that point
What two conditions must be satisfied for a body to be in equilibrium?
a) the net (resultant) force in any direction is zero
I.e. no linear acceleration
b) the net (resultant) moment of all the forces about any axis is zero
I.e. there is no rotational acceleration
How do you calculate the total moment (or torque) of a couple?
The torque is equal to the magnitude of the force multiplied by the perpendicular distance between the forces
Define density
The density of a substance is defined as its mass per unit volume
Density = mass/volume
Define pressure
Pressure is defined as the force per unit area acting on a surface perpendicular to the surface
Pressure = force/area
What is the description and formula for the change in pressure in a liquid?
Pressure increases with depth in a fluid. It also depends on the density of the fluid and the gravitational field strength, but not on the area or volume of the fluid.
p = ρ x g x h
State the most common equation for net force
F = ma
where m is the mass of an object, and a is its acceleration
State the equation for weight of an object
W = mg
where m is the mass of the object, and g = 9.81
Explain terminal velocity for a vehicle
- the engine provides the driving force to move the vehicle
- as the car accelerates, the driving force remains constant but the air resistance increases
- eventually air resistance is equal and opposite to the driving force and the car moves with a maximum steady speed
When do frictional forces occur, and what are they referred to as?
An object travelling in a fluid (liquid or gas) will experience a resistive or a frictional force acting upon it. This is often referred to as drag.
The faster an object moves through a fluid the greater the drag.
