Forces & Equilibrium

Question 1

Two conditions required for equilibrium

Answer 1

• Net force (in any direction) = 0
• Net moment = 0

Vectors must be drawn tip to tail so they form a closed shape

Question 2

What does smooth slope refer to?

Answer 2

When an object is smooth, it means that there is no friction

Question 3

What is a moment?

Answer 3

A turning force produced when a force is applied to an object at distance from the pivot

Question 4

How do you calculate a moment?

Answer 4

Moment = Force x perpendicular distance from the pivot

M = fd

Question 5

What is the principle of moments for equilibrium?

Answer 5

Sum of clockwise moments = Sum of anticlockwise moments

(at any point)

Question 6

How would you calculate the force in an anticlockwise direction about a pivot in equilibrium?

Answer 6

ACW (F1 x d) = CW (F x d)
F1 = CW/d

Question 7

Definition of density

Answer 7

The mass of a substance in a unit volume (kgm*3)

How much mass of a material that is in a certain volume?
(How spread out the particles are in an object)

Question 8

Density in a vacuum

Answer 8

0

Question 9

Density in water

Answer 9

Density above 1000 kg m⁻³ (density of water) = sink
Density below 1000 kg m⁻³ (density of water) = float

Question 10

Equation for density

Answer 10

Mass / Volume

Question 11

Density of a specific material

Answer 11

Every same material has the same density, regardless of the object dimension

Question 12

How do you calculate the density of an irregular shaped object?

Answer 12

• Determine the volume using water displacement (how much the level rises in ml = volume)

Question 13

How do you calculate the density of an alloy

Answer 13

Alloys have different masses therefore different densities

Find the mass x volume of each separate metal then calculate the density with the totals

Question 14

Define pressure

Answer 14

When a substance exerts a force over a particular area
Solid and fluid pressure

Question 15

What is fluid pressure?

Answer 15

Liquid or gas

The pressure exerted when the particles of fluid collide with the surface/container walls

The pressure in fluids causes forces at right angles to any surface

Question 16

In what direction to particles bounce of the walls of a container

Answer 16

The pressure in fluids causes a forces from the particles at right angles to any surface

Question 17

Equation for pressure

Answer 17

Force (at a right angle to a surface)
/
Area of the surface

Pa = N m⁻²

Question 18

How does the pressure in fluid vary?

Answer 18

Pressure increases with the depth of a fluid

E.g deep sea divers are under a lot of pressure (there’s lots of weight/ more particles pushing down above them) - weight acts down

Question 19

Equation for change in pressure

Answer 19

Changing in depth x density of liquid x strength of gravity
(Displacement x density x 9.81)

Or

P = Weight / Area
P = pgh - density x gravity x depth

Question 20

In what conditions does an object float?

Answer 20

Upthrust = Weight
If the forces are balanced on the vertical plane (no resultant force)

Question 21

In what conditions does an object sink?

Answer 21

If weight is larger than upthrust
Weight > upthrust

(the resultant force is acting downwards)

Question 22

Archimedes principle

Answer 22

For an object to float, the upthrust of the object must be equal to the weight of the fluid it displaces

Question 23

How does the pressure in the atmosphere vary?

Answer 23

It varies with altitude
(Decreases with height) - fewer particles above creating a weight pushing down)

Question 24

What is a couple?

Answer 24

A pair of forces acting on a body that of equal magnitude and acting opposite directions. They act in parallel to each other. (Turning Force)

• A couple has no resultant force as the pair cancelled each other out, meaning it turns around a fixed pivot

Question 25

What is the torque of a couple?

Answer 25

The rotation/turning force or moment produced by a couple

Question 26

Example of a couple

Answer 26

A steering wheel or turning a spanner

Question 27

How to calculate the moment of a couple/torque

Answer 27

Force x perpendicular distance between the line of action of the two forces M = F x d (Torque = 2X the individual moment of one of the forces)

Question 28

What is tension?

Answer 28

The force within a stretched cable or rope

Question 29

What is upthrust?

Answer 29

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

Question 30

What is a normal contact/reaction force?

Answer 30

A force arising when one object rests against another (always acts perpendicular to the surface)

Question 31

What is the centre of mass?

Answer 31

Where we assume all of the objects mass to be concentrated (usually the centre of the object) If the centre of mass is at the centre of an object, the forces on either side of it are balanced (equilibrium)

Question 32

What is the centre of gravity?

Answer 32

Where the objects total weight will act

Question 33

What is the line of action?

Answer 33

A line ACTING FROM the centre of mass representing the weight

Question 34

What does it mean if the line of action is acting from the centre of mass and is inside of the base of an object?

Answer 34

The object is in stable equilibrium it will not fall over

Question 35

What happens if the line of action is acting from the centre of mass and is outside of the base of the object?

Answer 35

The object is unstable and will fall over (this happens when the object is tilted) The forces are not balanced and THERE IS A RESULTANT MOMENT ACTING ON THE OBJECT

Question 36

What factors affect the stability of an object? Why?

Answer 36

- Position of centre of mass The lower the centre of mass/the centre of gravity the more stable an object will be - Width of an object The wider the base of the object the more stable it will be Why? It is less likely that the line of action falls outside of the base and produces a resultant moment if the object is wider or the centre of mass is lower down

Question 37

Examples of how factors of stability of an object are used in real life

Answer 37

Sport cars have a low centre of mass/gravity and a wide base

Question 38

Where do you find the line of action?

Answer 38

Directly down from a pivot

Question 39

Density of wood

Answer 39

200 - 800 kg m⁻³

Question 40

Density of metal

Answer 40

5000 - 10000 kg m⁻³

Question 41

Density of fresh water

Answer 41

1000 kg m⁻³

Question 42

Density of air

Answer 42

1.3 kg m⁻³