Force and Pressure
Category : 8th Class
Learning Objectives
INTRODUCTION
It is our common experience that to move a stationary table, we need to push it or pull it. To stop a moving ball we have to apply some force opposite to its direction of motion. If the object is quite heavy it may not be set into motion by a force. A heavy object in motion may not be brought to rest on applying force on it. Thus based on these observations we can now define force.
FORCE
Force is defined as an external agent (push or pull) that changes or tends to change the state of rest or 01 uniform motion of a body.
Do You Know
Acceleration due to gravity on the surface of the earth \[(9.8m{{s}^{-2}})\] is 6 times the acceleration due to gravity on the surface of moon
FORCE
Force is defined as an external agent (push or pull) that changes or tends to change the state of rest or of uniform motion of a body.
Mathematical definition of force - Whenever force acts on a moving body it changes the speed of the body. Thus force produces acceleration (Positive or negative). Greater the force, greater is the acceleration. Also more the mass of the body, more is the force required to produce acceleration in it. Hence force is directly proportional to the mass and acceleration of the body.
\[\text{Force = mass}\times \text{acceleration}\]
\[\text{F}\,\text{= ma}\]
Thus, mathematically forces defined as the product of mass and acceleration.
When a body falls under the force of gravity. Then, the force exerted on the earth is equal to the weight of that body i.e. \[W=m\times g.\]
Where, g = acceleration due to gravity\[g=9.8\text{ }m/se{{c}^{2}}\] on the surface of the earth at mean sea level.
Units of force
The S.I. unit of force is newton (N)
1 newton force moves a body of mass 1 kg with an acceleration of \[1m/{{s}^{2}}.\]C.G.S. unit of force is dyne.
1 dyne force moves a body of mass 1 g with an acceleration of \[1\,cm{{s}^{-2}},\]
Relationship between newton and dyne
Using F = ma
\[1N=1\,kg\times 1m/{{s}^{2}}\]
\[=1000g\times 100\,cm/{{s}^{2}}\]
\[=100000gcm/{{s}^{2}}\]
Since 1 dyne = 1\[g\,cm/{{s}^{2}}\]
\[\therefore 1\,N\,={{10}^{5}}\,\text{dynes}\]
Do You Know
It is possible to have motion in the absence of force and to have force in absence of motion.
EFFECTS OF FORCE
Force can bring about the following effects:
Do You Know
There are four fundamental forces in nature the gravitational force, the electromagnetic force, the nuclear force and the weak force. Among these gravitational force is the weakest force.
TYPES OF FORCE
Force can be classified as follows:
Now, let us discuss these forces one by one.
(i) Contact forces - Forces which act only when two bodies are in contact with each other are called contact forces. Contact forces cannot act from a distance. For e.g., Force of friction acts only when two surfaces are in contact with each other. Push, pull, tension, force of friction and muscular force are examples of contact forces.
(a) Force of friction - An opposing force that acts when a body moves or tries to move over the surface of another body. Force of friction decreases the speed of a moving body.
(b) Muscular force - The force caused by the action of muscles in our body is known as muscular force. Lifting a load, pulling or pushing an object requires muscular force.
(ii) Non-contact forces -Forces applied by one body on another without being in actual contact are called non-contact forces. Non-contact forces act from a distance. For e.g. - An apple falls down due to the force of gravity acting on it from a distance. Force between magnets and magnetic materials (magnetic force), force between electric charges (electrostatic force) and nuclear force are all non-contact forces.
(a) Gravitational force - Force of attraction between any two masses is called gravitational force. For e.g.. All objects on Earth experience force of gravity due to earth. It is always attractive in nuture.
(b) Magnetic force - The force of attraction or repulsion between the poles of magnets is called magnetic force. Like poles repel each other and unlike poles attract each other.
(c) Electrostatic force -The force of attraction or repulsion between charged particles at rest is called electrostatic force. Like charges repel each other and unlike charges attract each other.
Do You Know
Tides are caused by the rotation of earth and the gravitational effects of the Moon and Sun.
Balanced and unbalanced condition of forces
When two or more forces acting on an object cancel each other's effect, then they are called balanced forces. If the forces on an object are balanced, then the net force on the object is zero. Thus the balanced forces do not cause any change in the object's state of rest or of uniform motion.
Example: Two students pushing a table in opposite directions, each applying an equal amount of force, form a pair of balanced forces. Similarly in a tug of war if two teams pull the rope with equal force, then the rope stays in same place.
When two or more forces acting on a body do not cancel the effect of each other, they are called unbalanced forces. Unbalanced forces always cause change in the state of rest or of motion.
Example: A ball hit by a bat, someone moving the furniture etc. are due to unbalanced forces.
Do You Know
Birds achieve thrust during flight by flapping their wings.
Camels have broad fest, so that area of contact is high and pressure is less and they are able to walk on sand without sinking in it.
THURST AND PRESSURE
A force which acts normally on a surface is called thrust.
Force acting normally (perpendicular) i.e., thrust per unit area is called pressure.
If F is the force acting on area A, then
\[\text{Pressure}\,\text{=}\frac{\text{Thrust}}{\text{Area}}\text{P=}\frac{\text{F}}{\text{A}}\]
Thus smaller is the area on which the given force acts, the greater is the impact of force i.e. pressure and vice-versa.
(Remember: Force acting at an angle does not exert pressure on the surface)
Unit of Pressure
The S.I. unit of pressure is Pascal (Pa). Pascal can be defined using the equation.
\[P=\frac{F}{A}\]
\[\text{1Pa}\,\text{=}\frac{\text{1N}}{\text{1}{{\text{m}}^{\text{2}}}}\]
When a force of IN acts normally on a surface of area 1 m2, then the pressure acting on the surface is called 7 Pascal.
Example based on pressure - area relation
(1) The bags and suitcases are provided with broad handles so that less pressure is exerted on our hands or shoulders while carrying them because more the area, less will be the pressure.
(2) Railway lines are laid on large sized wooden or concrete tracks so that the normal force due to the weight of the train spreads over a greater area thereby reducing the pressure on the ground. This prevents the yielding of ground.
(3) Dams are made thicker at the bottom in order to withstand the high-pressure of water at the bottom.
(4) Pins and nails are pointed at one end to have least area of contact with the surface. Due to this on pressing, the pins exert high pressure on the surface and hence easily penetrate the surface.
Do You Know
Water pressure 10,000m below the surface is equivalent to seven elephants standing on a dinner plate
PRESSURE IN FLUIDS
All liquids and gases are regarded as fluids as they can flow.
A fluid has weight and hence on account of its weight, it exerts pressure on the walls of the container. "Moreover, unlike solids a fluid exerts pressure in all directions.
For instance if two holes are drilled in a bottle filled with water, it starts pouring out. This shows that water exerts pressure on the walls i.e., in all directions of the bottle or container.
Factors affecting pressure in fluids
(1) Pressure in fluids is directly proportional to its height or depth in the container. Thus pressure at various points but same height will be equal.
For e.g. If 3 holes in a bottle of water are drilled at the same level from its base water will come out of all the three holes with equal pressure.
Also pressure below the liquid surface goes on increasing with depth.
(2) Pressure depends directly on the density\['\text{ }\!\!\rho\!\!\text{ }'\]of the fluid — A denser fluid exerts greater pressure.
(3) Pressure in a fluid is directly proportional to the acceleration due to gravity 'g'
Thus, pressure \[\text{P}=\text{h }\!\!\rho\!\!\text{ g}\]
Applications of pressure in fluids
Do You Know
The sudden fall in atmospheric pressure produces the possibility of a storm.
ATMOSPHERIC PRESSURE
The thrust due to the air column on a unit surface area of the earth is called atmospheric pressure.
Atmospheric pressure is measured by a barometer. A barometer using a liquidise known as liquid barometer. If a gas (air) is used, then it is known as aneroid barometer.
Atmospheric pressure on the sea level is\[\text{1}\text{.013}\times \text{1}{{\text{0}}^{5}}\text{Pa}\].
Atmospheric pressure varies with the variation in height, season, temperature etc.
Do You Know
The pressure of the air on the outside of your body is balanced by the pressure of fluids inside.
Applications of atmospheric pressure
Two holes are made in a sealed oil cane. Through one hole the atmospheric pressure acts which forces the oil to come out from the other hole.
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