Chapter 4 - Forces in Action Flashcards
What is the centre of gravity?
The centre of gravity of an object is a point where the entire weight of the object seems to act
What is the centre of mass?
The centre of mass of an object is a point where the entire mass of the object seems to be concentrated
What is unique about a uniform gravitational field?
The centre of mass is the same place as the centre of gravity
What is another definition of the centre of mass or gravity?
The centre of mass or centre of gravity of an object is the point through which a single force has no turning effect on the body
What happens when the line of action of force is applied to the centre point?
When an externally applied force is put through the centre point, it produces a straight line motion, but no rotation
Where does a freely suspended object come to rest?
It comes to rest with its centre of gravity vertically below the point of suspension
What makes an object more stable?
Objects with wide bases and low centre of mass are more stable
Where does the centre of mass lie?
Centre of mass will lay exactly where all lines of symmetry cross
What is a stable object?
A stable object will not fall over given a small push, and the centre of mass remains above the base
To which direction do objects fall?
Objects fall towards the side which the centre of mass lies past the pivot
How is the centre of mass found in a practical?
Suspending the body from a position near its edge by making a hole and sticking a rod through it Allow it to come to rest
Draw a line vertically down, using a plumb-line to guide
Repeat the process from another point
Centre of mass lies where the lines cross
What is a fluid?
A fluid is a substance that can flow including liquids and gases
What is drag?
Drag is a frictional force that opposes the motion of the objects
What does drag depend on?
Shape and CA of the object
Speed of the object
Density of fluid through which it travels
Shape and Speed are most important
What is a newton?
Force that will give a mass of 1kg an acceleration of 1ms-2
What is weight?
Weight of an object is defined as the gravitational force acting on an object
What is friction?
Friction is the force that arises when two surfaces rub against each other
What is up-thrust?
The upward buoyancy force acting on an object when it is in a fluid
What are the relationships of drag and its factors?
Drag is proportional to the square of speed
Drag increases as cross-sectional area increases
When is drag ignored?
The body’s mass is very small in relation its surface area
The body is falling at a considerable velocity
What happens when a body initially has a driving force on it?
Body accelerates
Velocity increases
Drag force increases
Resultant force increases
What happens when drag = driving force?
Resultant force decreases to 0
Acceleration decreases to 0
Terminal velocity = Constant Speed
What is the moment?
Moment is the turning effect of a force
Moment = Force x Perpendicular Distance
What does the principle of moments state?
For a body to be in equilibrium, total clockwise moments = total anticlockwise moments
What is the support force?
The support force acts where a body is in contact with a surface and balanced horizontally on a pivot, acting upwards at the pivot
What does the magnitude of the support force equal on one pivot?
S = W1+ W2+ W3
How do you calculate the moment around 2 pivots?
Taking moments about one pivot means including the moment of the support force form the second pivot
How does the centre of mass influence the value of the support force?
If the pivot is closer to the centre of mass, greater moment value
What does the supporting force for two pivots equal?
S1+ S2 = W
Weight is from the CoM
What is the formula for the supporting force of two pivots?
Sy = Wdx / D
Sx = Wdy / D
‘x’ and ‘y’ are opposite pivots
What is a couple?
A couple is a pair of forces of equal size which act parallel to each other but in opposite direction
What does the size of torque depend on?
The size of torque depends on the size of the force and the distance between them
How are coupled forces different to a single applied force?
Coupled forces do not cause a linear resultant force but does cause a turning effect called torque. Applying a single force causes a rotational and translational motion
What is the formula for torque?
Torque = Force x Distance
Nm = F x D
What are the factors required for a triangle of forces?
Have arrows that run head to tail
Be closed
Forces be in equilibrium
What is the formula for density?
Density = Mass / Volume
What is mass measured using?
Measuring scale
What is volume measured using?
Displacement cans
Measuring Cylinders
Ruler
Micrometer
What is the formula for pressure?
Pressure = Force / Area
What is pressure in a liquid determined by?
Pressure at a given depth is determined by the weight of the water above it
What is the formula for pressure in a liquid?
Pressure = Density of Liquid x Gravitational Field Strength x Depth
Why does upthrust come about?
Results from the pressure difference that exists between the top and the bottom of the object
What is the formula for upthrust?
Upthrust = Density of Liquid x Gravitational Field Strength x Submerged Depth x Surface Area
