Friction : What Causes It ?

8/14/2019 Friction : What Causes It ?

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Friction : What causes it ?To any layman, frictional force is always related to surface roughness. This is indeedthe truth. Two rough surfaces sliding on each other, will definitely produce some kindof "resistance". The following diagram shows the microscopic side view of a roughsurface sliding over another rough surface.

When surface A moves over surface B, the sharp pointed parts of both surfaces willdefinitely hit each other, resulting in a resistance which of course is known as friction.Polishing both surfaces reduce the sharp pointed parts and hence reduces friction.

Why is a wet floor slippery ?A wet floor has a layer of water on it.

When surface A moves above surface B, the water cushions off the impact of surfaceA's weight on B. The sharp pointed parts exert less force on surface B and hencereduces friction.

Floating and Sinking : Its Relation to Density


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It is not uncommon to hear someone says that oil floats in water because it is

lighter. This is a seriously wrong statement. Whether a substance floats or sinks in

water or any other fluid depends on its density. If its density is lower than the fluid

density, then it floats.

The density of ice is 0.92 g/cm 3, lower than the density of water which is 1 g/cm 3.

Hence ice floats in water. A cupro nickel coin, with a density of 8.35 g/cm 3 thus

easily sinks in water. A ship, with its big mass however is able to float in water

because its massive volume helps to lower its density less than 1g/cm 3.

So the correct statement to say is that " oil floats in water because its density is less

than the density of water ".

Photo of a ship stranded on a beach.What makes a very heavy ship like

this able to float in water?

Inertia and MassWhy is it easier to stop a boat than a supertanker?From a speed of 72 km/h ( 20m/s), a small boat can be easily stopped in severalmetres. However, a supertanker with the same speed can only be stopped at a muchfarther distance, probably several kilometres.

This exhibits the basic characteristic of inertia ( the reluctance of any object tochange its state of motion), which increases with mass. A supertanker which has amuch superior mass needs more distance to be stopped. Why is this so?

The answer can be found if we look at it in term of kinetic energy. The kineticenergy of a moving object is given by 1/2 x mass x speed 2. The bigger the mass, thebigger is the kinetic energy.

Let say, the small boat has a mass of 9000 kg. At a speed of 72 km/h ( 20 m/s ), itskinetic energy is then

= 1/2 x 9000 x 20 2

= 1.8 x 10 6 J

At the same speed, for a supertanker with a mass of 9 x 10 7 kg, thekinetic energy is

= 1/2 x 9 x 10 7 x 20 2

= 1.8 x 10 10 J


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This is 10,000 times more kinetic energy than the boat. The amount of energyneeded to stop the supertanker is thus 10,000 times more than to stop the boat.That is why, inertia increases with mass.

Why Mass is Constant and Weight Changes

GCE O -Level Physics

Why is mass always constant ?

The mass of an object is always constant. Whether it is at the Equator or the North Pole,

or even on Moon, it remains the same. Why is this so ? The answer is actually very

simple. Just go back to the definition of mass. It is defined as the quantity"(number) of matter in an object. ( Note : matter is anything made of atoms and molecules. Matter is

anything that has a mass. )

Lets refer to the object shown below.

If each circle represents a matter, then this object has 18 matter. Bring it to the North Poleor to the top of Mount Everest ; or even to Moon, the quantity (number ) of matter it has isstill 18. In short, mass will always remain constant.

Why weight changes according to location ?

The weight of an object changes according to location. At the equator a person is lighter than at the North Pole. On the Moon, it is much less ! Why is this so ? Weight is defined as the gravitational force acting on an object. Since this force changesaccording to location, so does weight. Since Earth is spherical in shape, the North Pole isnearer to the centre of the Earth compared to the Equator . The gravitational force at theNorth Pole is hence a little bit stronger than at the Equator. Therefore any object will havea higher weight at the North Pole than at the Equator. On Moon, the gravitational force is
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much less than on Earth. An object will have much less weight on Moon then. ( Goodnews for the overweight person? ) Since weight W = m g ( m = mass; g = gravitational force ), it is not difficult to seewhy weight changes when g changes. The following table shows the weight of a 1 kgobject at the Equator, North Pole and Moon.

Location g ( N / kg ) Weight ( Newton )

Equator 9.83 9.83

North Pole 9.78 9.78

Moon 1.6 1.6

Mass related formula in O-Level Physics

For quick reference, in O-Level Physics, the following topics are related to mass.

Quantity/ Topic Relation

Weight, W W = mass x gravitational force W = m g

Beam balance instrument use to measure mass

Inertia property of an object to maintain its

state of motion.

It increases with mass.

Density, = mass / volume = m / V

Centre of mass the point through which the whole

weight seems to act. An object is

more stable if the centre of mass is

lower

Force, F F = mass x acceleration F = m a

Work ( against

gravity ), W

Work = weight x height W = m g h

Potential energy, E E = mass x gravitational force x

height

E = m g h

Heat gain / lost; H H = mass x specific heat capacity x

temperature change

H = m c

Specific Latent Heat, L = heat / mass L = H/m


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If the two are dropped on Moon, then both will definitely fall atthe same time since the moon is a vacuum.

What happens to the period of apendulum on Moon?

If a pendulum is brought to Moon, its period T, will be longer. Why is this so?To explain this, we have to look at the formula for the period of a pendulum, i.e. ;

The formula shows that besides the pendulum's length L, gravitational acceleration,g also affects the period T.

Since the gravitational acceleration g, on Moon is much less than on Earth( about 6 times lower ) the period of a pendulum on Moon is thus higher/ longer.

For example, for L = 0.4m, the period on Earth ( where g = 9.81 m/s 2 ) is

For the same length, on Moon ( where g = 1.63 m/s 2 ), the period is now


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Friction : What Causes It ?