## 04 April 2015

### Fore Fiction and Pressure for VIII Physics-Notes and Evaluation

FORCE AND PRESSURE

What is force :

A force is a push or pulls acting on an object which changes or tends to change the state of the object.

In the international system of units (SI System), the unit of force is Newton (N) that is named after Sir Isaac Newton.

There are also other units like dyne, kilogram weight and pound.

Action of force and its effects: A force

a) can change the state or rest or motion

b) can change the shape of object
c) can change speed of a body
d) can change direction of motion

Contact and non-contact forces

Forces which act only when there is physical contact between two interacting objects are known as
Contact forces.

Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces.

What is the definition of balanced and unbalanced force?

When two equal forces acting on an object in opposite directions called balanced force.

When something does not move the forces are balanced, e.g. Gravity pulls a table down but that table is pushed up by the floor or the ground it stands on so doesn’t’ move.

An Unbalanced force is needed for something to change movement or change direction, e.g. a seesaw moving up and down because the forces are not equal.

Net force: - The sum of all the forces acting on a body is known as net force.

Types of forces

1. Muscular force: This is the force we can exert with our bodies by using our muscles, e.g. pull, push, kick etc. These are contact forces

2. Magnetic force : Magnets exert forces of attraction or repulsion on other magnets. An important feature of magnetic force is that it can act from a distance, and is therefore of a non-contact force.

3. Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. Electrostatic force is used to separate solid pollutant particles from smoke given out from factories.

4. Gravitational force : All objects in the universe exert a force on all other objects. This is called gravitational force. The  gravitational force exerted by the Earth on all the bodies on its surface is called gravity.

5. Frictional force : The fact that the rolling ball comes to rest after some time shows that there must be a force acting on it which tends to slow it down. This force seems to be more on rough surfaces than on smooth surfaces. The force acting against the relative motion of surfaces in contact is called frictional force or friction.

6. Tension Force:Strings, ropes and chains can only pull on things!  The force of pull supplied by strings, ropes or chains is called the tension force. The tension force is always directed along the length of the thing doing the pulling (string, rope, chain).

7. Spring Force‎: is the force exerted by a compressed or stretched spring upon any object that is attached to it.

8. Applied Force:  An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Mass and Weight :

The mass of an object is the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object.

The mass of an object (measured in kg) by Pan Balance and  will be the same no matter where in the universe that object is located.

On the other hand, the weight of an object (measured in Newton) by Spring balance and  will vary according to where in the universe the object is.

Type of friction:

Static friction:

The block remains at rest because a force of friction, equal but opposite to the applied pull, comes into action between the surfaces. Increase the force a little. The block des not move. This means that the force  of friction has increased to balance the pulling force on the block.

If the pulling force P is increased further, at a certain stage the block begins the move on the table. At this point the friction developed has reached the maximum value F for the two surfaces. This is called static friction.

Kinetic or sliding friction

Continue pulling the block with the spring balance, so that it slides at a steady speed. The reading on the spring balance is also steady and is slightly less than the static friction. This is a measure of the kinetic or sliding friction between the two surfaces.

Kinetic or sliding friction is less than static friction.

Rolling friction

Repeat the above procedure using an identical wooden block provided with wheels on either side. The reading on the spring balance when the block moves with a steady speed is much less than the sliding friction measured above.

Rolling friction is less than sliding friction.

Friction plays an important role in our daily life.

1) Without friction we would slip and fall every time we attempt to walk or run. There is very little
friction on a wet polished floor. That is why it is easy to slip on such a floor.

2) Friction causes nails and screws to hold on to walls.

3) It would not be possible to light a matchstick without friction between its head and the side of the
matchbox.

4) Cars and buses are able to run on roads because of friction between the tyres and the road.

5) Without friction writing on paper would be impossible as the tip of the pen will slip on paper.

6) It is because of friction between the brake ‘shoes’ and wheels that bicycles and automobiles stop
when brakes are applied.

Friction is a nuisance in some circumstances.

1) The heat produced in the moving parts of machinery due to friction results in wear and tear of the parts.

2) Forest fires are caused due to friction between branches of threes rubbing against each other.

3) Tyres of vehicles and soles of footwear wear out because of friction.

4) Energy is wasted in overcoming the force of friction.

Minimizing friction

1) By using a suitable lubricant, like oil (for light machinery) or grease (for heavy machinery). This
helps because fluid friction is less than solid friction

2) By using wheels and ball bearings.

Use of wheels between surface moving over each other reduces friction. Ball bearings have small balls of steel between steel surfaces. Because of the balls the steel surfaces can easily moves over each other.

3) By making the rubbing surfaces smooth by polishing them.

4) Friction due to air (air resistance) or water is reduced by using streamlined shapes in aeroplanes or ships.

A streamlined shape is narrow in front and broader at the back. Birds and aquatic animals have streamlined shapes which held them in flying or swimming.

Increasing friction

1) By making the moving surfaces rough, e.g. tyres have designs and patterns with grooves on the surface to increase resistance with the road. This prevents slipping of the tyres on a wet road.

2) Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is them easier to walk on the ground.

3) To increase friction, spikes are provided in the soles of shoes used by players and athletes.

Pressure :

Pressure is defined as the force  acting on a unit area.  Pressure = Force/ Area

If the force increases the pressure increases.

If the area over which the force acts decreases, the pressure increases

Unit of pressure

The SI unit of pressure is the pascal (Pa). It is the pressure exerted by a force of 1 N acting over an area of 1 m2.

Thus 1Pa = 1N/m2

Solved Problem

Q.1: A liquid gives force of 100N over an area of 2m2. What is the pressure?
Solution: Force = 100N  Area = 2m2         Pressure = 100N / 2m2 = 50 N/m2  Pressure = 50 N/m2

Q. A liquid’s force is acting over an area of 4m2. If the pressure is 25 N/m2, what is the force?
Solution : F = P x A = 25 N/m2     x 4m2 = 100N

Everyday examples

@  When you cut an apple with a knife, the shaper the knife, the smaller is the area of contact of the knife with the apple. Thus a sharper knife exerts a greater pressure making it easier to cut the apple.

@ It is easier to pierce a piece of cardboard with a pin if the pin has a sharp point, as the area of contact is  then smaller.

@ School bags and shopping bags have broad belts or straps as handles. Narrow string-like handles cause severe pain in the hand because the weight of the bag acts on a small area and so the pressure is
considerably higher.

Q Porters wear turbans when they have to carry heavy loads on their heads, to increase the area of
contact. This reduces the pressure on the head.

Q Lorries and trucks carrying heavy loads have 8 tyres instead of four, and the tyres are broader. This
increases the area of contact with the ground, this reducing the pressure exerted on the ground.

Q War tanks move on caterpillar tracks which are broad and chain-like and cover the wheel. This

causes large increase in the area of contact with the ground. The pressure on the round reduces so much that tanks can even move on soft wet ground without sinking.

Atmospheric Pressure

The Earth is surrounded by air and this covering of air is known as the atmosphere.

The atmospheric air exists to height of about 1000 km. But beyond 100 km the air is very thin. The force exerted by the atmosphere on unit area is called atmospheric pressure.

Liquid Pressure

a) Water exerts pressure downwards
b) Water exerts pressure in the upward direction
c) Water exerts pressure in all directions on the sides. This is called lateral pressure.
d) Liquid pressure increases with depth.

Pressure at the bottom of the ocean is very high. Deep sea divers wear steel diving suits to prevent their bodies from begin crushed by the tremendous pressure of water exerted on all sides.

Submarines are built of hard thick sheets of metal to withstand the high pressure under water. Fro the
same reason dams are broader at the base than at the top.

Measuring liquid pressure – the manometer

An instrument called a monometer is used to measure liquid pressure.
@ A liquid exerts pressure in all directions (downward, upward and sideways).
@The pressure is the same in all directions at the same depth.
@  Pressure increases with depth.

The pressure exerted by water in the oceans increases with depth. Very deep down the pressure is enough to crush the human body. That is why deep-sea divers wear special suits, which can withstand such high pressures.

1. What is meant by force?

2. What changes can a force produce to the motion of an object?

3. What is elastic force? Give two examples.

4. How is pressure related to the area over which a force acts?

5. What is the relation between pascal and N/m2?

6. If you wear pointed heels in one leg and slippers on the other leg, which leg exerts greater pressure
on the ground?

7. What is atmospheric pressure?

8. Explain how it is possible to drink a liquid by using a straw.

9. Why are dams over rivers made broader at the bottom than at the top?

10. Differentiate between mass and weight. What are the SI units?

1. Explain contact and non-contact forces. Give two examples for each.