physics notes chapter 3-7

7/30/2019 Physics Notes Chapter 3-7

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Chapter 4: Mass, Weight and Density

Mass and Weight1) Mass Weight

Definition Measure of the amountof matter in a body.

Amount of gravitationalforce acting on a body.

SI Unit Kilogram (kg) Newton (N)

Dependent on

Dependent on thenumber andcomposition of atoms

making up the body.

Dependent on the mass ofthe object and thegravitational field

strength.

Properties Mass has onlymagnitude, and isconstant (unaffected bygravitational fieldstrength).

Weight has bothmagnitude and direction(towards the centre ofEarth).

MeasuringInstrument

Beam balance,electronic balance

Spring balance,compression balance

Relationship

Weight of an object is directly proportional to itsmass.

Weight = Mass x Gravitational field strength

(W=mg)

Gravitational field strength

7) Gravitational field is the region where an object experiencesgravitational force.

8)

Gravitational field strength, g, is the gravitational force acting per unitmass on an object.

SI unit= N/kg.

9)

The gravitational force pulls objects to the centre of the Earth and gets

weaker with increasing altitude.


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10)

A 1kg object will experience a 10N gravitational force due to Earthsgravitational pull (gravity).


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Inertia

14

)

Inertia is the resistance of an object to change in its state of motion or

rest. (The larger the mass, the greater the inertia)

Note: Inertia is not a force.15)

The inertia of an object is directly proportional to is mass.

16)

Explain how seatbelts can prevent a driver from injury during asudden stop.

Initially, the driver is in motion. During a sudden stop, the driver willcontinue to move forward due to his inertia. Without seatbelts, the

driver will be thrown forward and crash into the windscreen. Seatbeltswill pull the driver back onto is seat and stop him from moving forward,thus preventing him from crashing into the windscreen and injury.

Density

17)

Density is the mass per unit volume of a substance. SI unit=kg/m3

18)

The density of a substance is dependent on the composition and

number of atoms making up the substance (mass). Metals have highdensities as the atoms are closely packed. The large number of atomsin 1kg the metal contributes to the higher mass and hence density,compared to gases where the molecules are spaced further apart.

19)

Use the kinetic particle theory to explain why solids havehigher densities than gases.

Solids have higher densities than gases as their particles are packed

closer together. The number of particles per unit mass in a solid ishigher than in gases. Of the same mass, solids have a lower volumehence a higher density; while gases have higher volumes hence alower density.


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Chapter 5: Turning Effect of Forces

Moment of a force about a point is the product of the force and the

perpendicular distance from the pivot to the line of action of the force.

Moments (Nm)= Force (N) x Distance travelled (m)

Conditions for object in equilibrium: 1) Fnet= 0

2) Net moments due to

external forces= 0

Principle of Moments

When a body is in equilibrium, the sum of clockwise moments about thebalanced point is equal to the sum of anticlockwise moments about thesame point (pivot).

Total clockwise moment = Total anticlockwise moment

When the clockwise moment is not equal to the anticlockwise moment,there is a resultant moment and the object will rotate in the direction ofresultant moment.

If there is no resultant moment, the object is balanced.

Centre of gravity

The centre of gravity (CG) of a body is the point through which the wholeweight of the object appears to act.

The CG of a regular object is at the centre.The CG of an irregular object is determined using a plumb line.

If a body is hanging freely at rest, its CG is always vertically below thepivot, thus the plumb line method works. It can only be used for flat,irregular objects.

Stability

Stability is a measure of the body's ability to return to its original positionafter being displaced slightly.

3 types of stability:

1. Stable equilibrium


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Object will return to original position after slight disturbance.

Line of action of the weight passes through the base area of theobject, the moment due to its weight will cause it to return to itsoriginal position.

2. Unstable equilibrium

Object will topple over after being tilted slightly.

If the line of action of the weight is outside the base area of theobject, the moment due to its weight will cause the object to topple.

3. Neutral equilibrium

Object remains in new position after being tilted slightly.

To increase the stability of a body, its base area shouldbe increased, and by lowering the centre of gravity.


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Work

- Work is the product of the force on a body and the distance it moves in

the direction of the force- Work done = force x distance moved in the direction of the force- Work is done whenever energy is changed from one form into another.- SI unit is joule (J)- Work is a scalar quantity

Energy

- energy is defined as the ability to do work

- SI unit is joule (J)- Energy is a scalar quantity- kinetic energy is the energy possessed by an object due to itsmotion.- kinetic energy can be classified into

- kinetic energy= 1/2 mv2

- potential energy is the energy a body possesses due to its

position or state- potential energy can be classified into:

--gravitational potential energy: possessed by a body due to itsposition = mgh

-elastic potential energy: possessed by a body due to its strainedstate of being stretched or compressed

Eg. A ball of mass 500g is moving at a velocity of 5m/s. What isthe kinetic energy of the ball?

kinetic energy = 1/2 mv2 = 1/2 x 0.5 x 5 x 5 = 6.25 J

Eg. Billy has a mass of 40kg. He runs up a flight of 20 steps, eachof height 0.25m. Calculate his gain in gravitational potentialenergy

gain in gravitational potential energy = mgh = 40 x 10 x (20 x 0.25) =2000 J

Principle of Conservation of Energy:

Statesthat energy can neither be created not destroyed but can be

converted from one form into another with no change in its total amount.


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Power and efficiency

- Power is defined as the rate of work done.- Power = work done/time taken (P=W/t)- SI unit is watt (W)

- Efficiency is the ratio of useful output energy to the total input energyor the ratio of useful power to the total input power.

efficiency = (useful output energy / input energy) x 100%

Eg. A crane can lift a 200kg mass through a vertical height of 5min 4s. Calculatei. the power output of the motor driving the craneii. the efficiency of the motor if the power input is 5kW

i. power output = work done/time taken = (200 x 10 x 5)/4 = 2500Wii. efficiency of motor = (power output/power input) x 100% =(2500/5000) x 100% = 50%

Friction1. Static friction- related to objects which are not moving.- amount of force applied = amount of friction

2. Moving friction- applied force does not affect friction

- it can be affected by surface or sudden change in mass

Advantages of friction- enables walking- brakes of vehicles

Disadvantages- reduce efficiency of machinery- energy wasted as heat

Methods to reduce friction

- lubricants- ball bearings-----> so that moving parts are made smoother

Energy, Work, Power

Work Energy Power


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Definition: The constant maximum velocity reached by a body

falling through the atmosphere under the attraction of gravity.

When an object reaches terminal velocity, the force of gravity and

air resistance are balanced, the object falls at a constant speed and

doesnt accelerate.

Factors affected: Size, surface area, weight and nature of medium

where object is flying.

If an object is falling through a vacuum, there would be no air

resistance, thus acceleration is due to gravity alone.


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Pressure

- Pressure is force acting normally per unit area.

If the amount of applied force is the same, then

Larger area --> Lower pressure

Smaller area --> Higher pressure

Examples of Pressure

Skis have a large area to hold up the weight of the skier on the snow

Flat bottomed shoes are comfortable to wear due to reduced

pressure acting on our feet

A sharp knife can cut easily because the very high pressure under

the cutting surface is more than the object can withstand

Atmospheric Pressure

Atmospheric pressure exists because of MOLECULAR

BOMBARDMENT of energetic air molecules (from the air around us)

Under normal conditions, there are large numbers of air molecules

and these molecules move with high velocities. They make frequent

collisions with things around us

The pressure exerted by the air molecules is almost equivalent to

putting a 1 kg mass on an area of 1 cm2

Normal atmospheric pressure= 1 atm (about 1.013 x 105 pa or

101300 pa)

101300 Nm-2 = 10.13 Ncm-2 = 1.013 kgcm-2

Applications of atmospheric pressure

Drinking with a straw

Drawing a liquid into a syringe by withdrawing the plunger

Holding a rubber sucker on a smooth surface
https://sites.google.com/site/urbangeekclassroomsg/using-word-documents/pressure/Screen%20shot%202011-06-06%20at%20PM%2002.58.29.png?attredirects=0


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Removing dust with vacuum cleaner

Pressure due to a liquid column

The taller the liquid column (with narrow base), the larger the

amount of liquid contained, the greater the weight of the liquid to

exert pressure

The amount of pressure in the SAME liquid column is DIFFERENT at

DIFFERENT DEPTHS.

The greater the depth, the greater the weight of the liquid above it,

the greater the pressure

The pressure in a liquid depends on the HEIGHT of the liquid

The amount of pressure increases with DEPTH

2 cases of liquid pressure

1. With atmospheric pressure

p = p0 + gh

Pressure at bottom = atmospheric pressure + pressure due to liquid

column In this case, when the container is open, there is

atmospheric pressure acting on the liquid as well.

2. Without atmospheric pressure

p = gh


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Pressure at bottom = pressure due to liquid column only In this

case, when the container is closed, air is removed (vacuum), so

there is no atmospheric pressure.

Factors affecting pressure in a liquid

1.Density of liquid

2.Depth of liquid

3. Gravitational acceleration

When it is at equilibrium, pressure must be the same at any point along

the same depth (h).

Note: pressure does not depend on the shape of the liquid column.

Measurement of pressure

Simple Mercury Barometer

- Used to measure atmospheric pressure

How to construct

A thick-walled glass tube (about 1m long) is filled with mercury

completely

The open end of the tube is covered with a finger and inverted

Place the inverted tube in a trough of mercury
https://sites.google.com/site/urbangeekclassroomsg/using-word-documents/pressure/Screen%20shot%202011-06-06%20at%20PM%2003.08.59.png?attredirects=0https://sites.google.com/site/urbangeekclassroomsg/using-word-documents/pressure/Screen%20shot%202011-06-06%20at%20PM%2003.05.56.png?attredirects=0


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Observation: The height of the mercury column found to be about

760mm or 76cm

Atmospheric pressure = 1 atm or 760 mmHg or 76 cmHg

Reasons for using mercury in a barometer

Mercury does not wet glass

Mercury has a high density

Manometer

- Used to measure gas pressure

How to construct

The manometer consists of a U-tube containing a column of liquid

The liquid can be mercury, water or oil
https://sites.google.com/site/urbangeekclassroomsg/using-word-documents/pressure/2.jpg?attredirects=0https://sites.google.com/site/urbangeekclassroomsg/using-word-documents/pressure/baro.jpg?attredirects=0


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How to measure?

When both arms are open, same atmospheric pressure is exerted on

the liquid surfaces (same horizontal level)

To measure the pressure of a gas, left side is connected to a gas

supply

The gas exerts pressure on the surface at L. The gas pressure must

be greater than atmospheric pressure to cause the right side to rise

Pressure at L given by p = p0 + gh

Hydraulic System

Pressure can be transmitted throughout a liquid in hydraulic presses

When a small force is applied to the smaller piston, pressure is

exerted on the liquid

This pressure is transmitted in the liquid (oil) and is the sameeverywhere within the oil. Thus the pressure at the bigger piston

must also be p.

Since area at the bigger piston is bigger, force must also be greater

A small force applied to the smaller piston can lift a greater load

on the bigger piston
https://sites.google.com/site/urbangeekclassroomsg/using-word-documents/pressure/1.jpg?attredirects=0


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Additional Notes

Pressure is the force acting normal or perpendicularly per unit area

SI unit: Pascal (Pa) or N/m2

Pressur Solid Liquid Gas


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e in:

Equatio

n

Pressure =

Force/Area

Pressure = hpg

h = depth of liquid(m)

p = density of

liquid (kg/m3)

g = gravitational

field strength

The air

surrounding us

exerts a pressurein all directions

which is about 105

Pa.

Remark

s

This formula

can only

be used for

solids.

- A liquid exerts

pressure because

of its weight.

- Liquid pressure

acts equally in all

directions. This is

because particles

of the water can

flow and wrap

around the object.

- A barometer is

used to measure

pressure. It

consists of an

inverted

tube in a dish of

mercury. The

space above the

mercury in the

tube is vacuum.

- Liquid mercury is

used as its density

is very high and ashorter barometer

can be used to

show

atmospheric

pressure.

- An object can be

bent/sucked in due

to the production

of vacuumand due to the

difference in

pressure; the

atmospheric

pressure will press

on the object.

physics notes chapter 3-7

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