Chapter 4 - Forces in Action

Question 1

What is the centre of gravity?

Answer 1

The centre of gravity of an object is a point where the entire weight of the object seems to act

Question 2

What is the centre of mass?

Answer 2

The centre of mass of an object is a point where the entire mass of the object seems to be concentrated

Question 3

What is unique about a uniform gravitational field?

Answer 3

The centre of mass is the same place as the centre of gravity

Question 4

What is another definition of the centre of mass or gravity?

Answer 4

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

Question 5

What happens when the line of action of force is applied to the centre point?

Answer 5

When an externally applied force is put through the centre point, it produces a straight line motion, but no rotation

Question 6

Where does a freely suspended object come to rest?

Answer 6

It comes to rest with its centre of gravity vertically below the point of suspension

Question 7

What makes an object more stable?

Answer 7

Objects with wide bases and low centre of mass are more stable

Question 8

Where does the centre of mass lie?

Answer 8

Centre of mass will lay exactly where all lines of symmetry cross

Question 9

What is a stable object?

Answer 9

A stable object will not fall over given a small push, and the centre of mass remains above the base

Question 10

To which direction do objects fall?

Answer 10

Objects fall towards the side which the centre of mass lies past the pivot

Question 11

How is the centre of mass found in a practical?

Answer 11

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

Question 12

What is a fluid?

Answer 12

A fluid is a substance that can flow including liquids and gases

Question 13

What is drag?

Answer 13

Drag is a frictional force that opposes the motion of the objects

Question 14

What does drag depend on?

Answer 14

Shape and CA of the object
Speed of the object
Density of fluid through which it travels

Shape and Speed are most important

Question 15

What is a newton?

Answer 15

Force that will give a mass of 1kg an acceleration of 1ms^-2

Question 16

What is weight?

Answer 16

Weight of an object is defined as the gravitational force acting on an object

Question 17

What is friction?

Answer 17

Friction is the force that arises when two surfaces rub against each other

Question 18

What is up-thrust?

Answer 18

The upward buoyancy force acting on an object when it is in a fluid

Question 19

What are the relationships of drag and its factors?

Answer 19

Drag is proportional to the square of speed
Drag increases as cross-sectional area increases

Question 20

When is drag not ignored?

Answer 20

The body’s mass is very small in relation its surface area
The body is falling at a considerable velocity

Question 21

What happens when a body initially has a driving force on it?

Answer 21

Body accelerates
Velocity increases
Drag force increases
Resultant force increases

Question 22

What happens when drag = driving force?

Answer 22

Resultant force decreases to 0
Acceleration decreases to 0
Terminal velocity = Constant Speed

Question 23

What is the moment?

Answer 23

Moment is the turning effect of a force

Moment = Force x Perpendicular Distance

Question 24

What does the principle of moments state?

Answer 24

For a body to be in equilibrium, total clockwise moments = total anticlockwise moments

Question 25

What is the support force?

Answer 25

The support force acts where a body is in contact with a surface and balanced horizontally on a pivot, acting upwards at the pivot

Question 26

What does the magnitude of the support force equal on one pivot?

Answer 26

S = W₁+ W₂+ W₃ W = Weight of object

Question 27

How do you calculate the moment around 2 pivots?

Answer 27

Taking moments about one pivot means including the moment of the support force form the second pivot

Question 28

How does the centre of mass influence the value of the support force?

Answer 28

If the pivot is closer to the centre of mass, greater moment value

Question 29

What does the supporting force for two pivots equal?

Answer 29

S₁+ S₂ = W Weight is from the CoM

Question 30

What is the formula for the supporting force of two pivots?

Answer 30

S_y = Wd_x / D S_x = Wd_y / D 'x' and 'y' are opposite pivots 'd' is perpendicular distance to one pivot 'D' is distance between pivots

Question 31

What is a couple?

Answer 31

A couple is a pair of forces of equal size which act parallel to each other but in opposite direction

Question 32

What does the size of torque depend on?

Answer 32

The size of torque depends on the size of the force and the distance between them

Question 33

How are coupled forces different to a single applied force?

Answer 33

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

Question 34

What is the formula for torque?

Answer 34

Torque = Force x Distance Nm = F x D

Question 35

What are the factors required for a triangle of forces?

Answer 35

Have arrows that run head to tail Be closed Forces be in equilibrium

Question 36

What is the formula for density?

Answer 36

Density = Mass / Volume

Question 37

What is mass measured using?

Answer 37

Measuring scale

Question 38

What is volume measured using?

Answer 38

Displacement cans Measuring Cylinders Ruler Micrometer

Question 39

What is the formula for pressure?

Answer 39

Pressure = Force / Area

Question 40

What is pressure in a liquid determined by?

Answer 40

Pressure at a given depth is determined by the weight of the water above it

Question 41

What is the formula for pressure in a liquid?

Answer 41

Pressure = Density of Liquid x Gravitational Field Strength x Depth

Question 42

Why does upthrust come about?

Answer 42

Results from the pressure difference that exists between the top and the bottom of the object

Question 43

What is the formula for upthrust?

Answer 43

Upthrust = Density of Liquid x Gravitational Field Strength x Submerged Depth x Surface Area

Question 44

What is torque?

Answer 44

The moment created by a couple