Chapter 10 Pressure, Gravity, Moments

What is Density?

Mass per unit Volume

p=m/V

unit: kg m^-3

What is Pressure?

Pressure is Force per unit Area

P=F/A

Unit: Pascal (Pa)

Conversion of Units

State Archimedes’ Principle

When an object is completely or partially immersed in a fluid it experiences an upthrust equal in magnitude to the weight of the fluid displaced

To Demonstrate The Principle of Archimedes

A stone which weighs 10N in air is immersed in an overflow can which was full to the brim

The stone now weights 7N and the weight of the fluid displaced is 3N

State the Law of Flotation

The weight of a floating body is equal to the weight of the fluid it displaces

What device is used to measure the density of a liquid

Hydrometer

Real life applications of a Hydrometer

Hydrometers are used to find:

1. The percentage of alchohol in beers, wines and sprites

2. The density of sulfuric acid in a lead acid battery

3. The percentage of fat in milk

State Boyles Law

Boyles law states that at constant temperature the volume of a fixed mass of gas is inversely proportional to its pressure

Verification of Boyle’s Law

• The tube is filled with air. 

• Note the pressure of the gas from the pressure-gauge and the volume from the graduated scale. 

• Turn the screw to decrease the volume and increase the pressure.  

• Note the new readings and repeat to get about seven readings. 

• Draw a graph of pressure against 1/volume. This should result in a straight line through the origin. 

Graph for Boyles Law

Why is it necessary for the column of air to have a uniform diameter?

So that V is proportional to l

Why might temperature change during the experiment?

Change in pressure increases molecule collisions and could increase temperature

How do you ensure temperature stays the same throughout?

Wait to make readings after changing volume

Why is there no atmosphere on the moon?

The gravitational force is too weak to sustain an atmosphere

Why does the Moon not fall down to Earth?

The gravitational force is too weak / Tangential velocity

Define Moment of a force

Moment = Force X perpendicular distance

M = (F)(⟂d)

Unit: Newton Metre (Nm)

What is a Lever?

A lever is any rigid object free to turn about a fixed point called a fulcrum

State the laws of Equilibrium

• The vector sum of the forces in any direction is zero

• The vector sum of the moments about any point is zero

What are co-planer forces?

Forces acting on a body in the same plane

What is a Couple?

Two parallel forces with the same magnitude acting in opposite directions is called a couple

What is the moment of a couple?

The moment of a couple is known as Torque

Torque = Force x Distance

Investigation of the Laws of Equilibrium for a set of Co-planer Forces

Find the centre of gravity on the metre stick by balancing it on a pivot

Note the weight of the metre stick using an electronic balance

Set up as shown and make sure the metre stick is horizontal

Record the weights and readings on Newton Metres

How was the centre of Gravity found?

Balance metre stick on a pivot or finger

How did student know the system was in equilibrium?

System not accelerating, remains horizontal

Why is it important to have forces applied vertically to the metre stick?

So that distance to each force is a perpendicular distance

Why is it necessary to find the centre of gravity?

So you can differentiate between clockwise and anticlockwise moments

Why could the centre of gravity be sightly off the 50cm mark?

Stick could be dented or chipped

How was weight found?

Weighing scale / Electronic Balance

How were vertical forces applied?

Weights for downward forces
Newton Balances for upward forces

Explain how the results verify the laws of Equilibrium

Net Force = 0

Net Moments = 0