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 9

Definition of density

Answer 9

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 10

Density in a vacuum

Answer 10

0

Question 11

Density in water

Answer 11

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

Question 12

Equation for density

Answer 12

Mass / Volume

Question 13

Density’s of solid

Answer 13

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

Question 14

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

Answer 14

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

Question 15

How do you calculate the density of an alloy

Answer 15

Alloys have different masses therefore different densities

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

Question 16

Define pressure

Answer 16

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

Question 17

What is fluid pressure?

Answer 17

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 18

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

Answer 18

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

Question 19

Equation for pressure

Answer 19

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

Pa = N m⁻²

Question 20

How does the pressure in fluid vary?

Answer 20

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 21

Equation for change in pressure

Answer 21

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 22

In what conditions does an object float?

Answer 22

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

Question 23

In what conditions does an object sink?

Answer 23

If weight is larger than upthrust
Weight > upthrust

(the resultant force is acting downwards)

Question 24

Archimedes principle

Answer 24

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

Question 25

How does the pressure in the atmosphere vary?

Answer 25

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

Question 26

What is a couple?

Answer 26

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 27

What is the torque of a couple?

Answer 27

The rotation/turning force or moment produced by a couple

Question 28

Example of a couple

Answer 28

A steering wheel or turning a spanner

Question 29

How to calculate the moment of a couple/torque

Answer 29

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 31

What is tension?

Answer 31

The force within a stretched cable or rope

Question 32

What is upthrust?

Answer 32

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

Question 33

What is a normal contact/reaction force?

Answer 33

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

Question 34

What is the centre of mass?

Answer 34

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 35

What is the centre of gravity?

Answer 35

Where the objects total weight will act

Question 36

What is the line of action?

Answer 36

A line ACTING FROM the centre of mass representing the weight

Question 37

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 37

The object is in stable equilibrium it will not fall over

Question 38

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

Answer 38

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 39

What factors affect the stability of an object? Why?

Answer 39

- 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 40

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

Answer 40

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

Question 41

Where do you find the line of action?

Answer 41

Directly down from a pivot

Question 42

Density of wood

Answer 42

200 - 800 kg m⁻³

Question 43

Density of metal

Answer 43

5000 - 10000 kg m⁻³

Question 44

Density of fresh water

Answer 44

1000 kg m⁻³

Question 45

Density of air

Answer 45

1.3 kg m⁻³