MASS WEIGHT DENSITY

LESSON OBJECTIVESAt the end of the lesson, you should be able to:a) State that mass is a measure of the amount of

substance in a bodyb) State that a gravitational field is a region in which a

mass experiences a force due to gravitational attractionc) Define gravitational field strength, g, as gravitational

force per unit massd) Recall and apply the relationship weight = mass x

gravitational field strength to new situation or to solve related problems

e) Distinguish between mass and weightf) State that the mass of a body resists a change in the

state of rest or motion of the body (inertia)

HOW DO YOU MEASURE YOUR WEIGHT?

• Bathroom scale• kilograms (kg)

MASS • Mass is a measure of the amount of substance in a

body.

• SI Unit: kilogram (kg)

• Scalar

• Represented by m

• Mass does not change wherever the body is.

WEIGHT

• The weight of an object is the gravitational force, or gravity acting on it.

• SI Unit: newton (N)

• Vector

• Represented by W

WEIGHING MACHINE• Weighing machine measures weight but the

units is kg.So what does the weighing machine measure?

• Weighing machine measures WEIGHT.But it is calibrated to display the mass of the body.

• How do you calibrate the weighing machine?

FORMULA

Wm gWhat is

gravitational field

strength?

GRAVITATIONAL FIELD & ITS STRENGTH

• A gravitational field is a region in which a mass experiences force due to gravitational attraction.

• In a gravitational field, a mass experiences a force.

• Weight is also known as gravitational force.

Earth’s gravitational force acts towards the centre of Earth.

GRAVITATIONAL FIELD & ITS STRENGTH

• The strength of this field is strongest at the surface of Earth and gets weaker farther away.

say, 50 N >50 N

GRAVITATIONAL FIELD & ITS STRENGTH• Definition:

Gravitational field strength, g, is the gravitational force acting per unit mass.

• Gravitational – an effect due to gravityField – a region where force is experienceStrength – the magnitude of the force

• Actual gravitational field strength, g of Earth= 9.80 N/kg.In calculations however, we use 10 N/kg to make it simple.

ACTIVITY!Find Mr. Wong’s weight on these places!My mass = 100 kg

PlanetGravitational

field strength, g

Weight

Earth 10 N/kgMoon 1.6 N/kgJupiter 24.9 N/kgPluto 0.6 N/kg

ConclusionWeight is dependent on the gravitational field strength of the location. (direct proportion)

FUN FACT!

PlanetGravitational

field strength, g

Weight

Earth 10 N/kgMoon 1.6 N/kgJupiter 24.9 N/kgPluto 0.6 N/kg

The more massive the object, the greater the gravitational field strength!!!

Mass of these planets 5.97 × 1024 kg7.25 × 1022 kg1.90 × 1027 kg1.25 × 1022 kg

FUN FACT 2!!• Do you know that every object including yourself

also have some gravitational field strength?

• For a person of 60 kg, his gravitational field strength at 1 metres away ≈ 4 × 10-9

N/kg

• Maybe that’s how you attract someone!

Weight is measured by using spring balances.

DO YOU REMEMBER HOW IS WEIGHT MEASURED?

Extension Spring Balance

Compression Spring Balance

HOW TO MEASURE MASS?Beam balance with standard masses can be used to measure mass.Since the standard mass experiences the same g, this method can be used on any planet.

Beam Balance Electronic BalanceNowadays, the electronic beam is more commonly used to measure mass as it is more accurate and easier to use.

DIFFERENCE BETWEEN MASS & WEIGHTDifferenc

esMass Weight

The amount of substance in a body

The gravitational force acting of the body

Scalar Vector

kilograms (kg) newtons (N)

Independent of g Dependent on g

Measured with beam balance

Measured with spring balance

Definition

Scalar or Vector?

SI Unit

Dependence on gMeasuring tool

INERTIA• Mass is a measure of inertia

• The inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to its mass.

From at rest to moving; orMoving to rest.Changing direction

• The greater the mass, the more inertia an object has.

INERTIA IN ACTION

• A MRT train – large massTime is required to start moving a train.Large braking force is required to stop a moving train.

• Wild boarA wild boar is unable to change its direction quickly due to its mass

Wild Boar Drill: Zig-zag running pattern allows us to run away from it.

INERTIA IN ACTION• Oldest trick in the world!

http://youtu.be/-cM9S2AzU28

What is the reason the bottles did not move?The bottles have inertia which resists the change of motion.

Now watch the video again – Is it real or fake?

How do you think they manage to get it done?

INERTIA IN ACTION

Answer: