Class 8 Notes | EduRev

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Push or Pull Forces
When you push against a wall the force that you exert is an example of a push force. When you pull a trolley car the force that you exert is an example of pull force.

Calculation of Pressure
For a force  applied normally on an area , the pressure  is given as

For example, for a force of  applied on an area of , the pressure is 
1. If force is not normal to the surface, only the components normal to the surface are considered.
2. For liquids, component of force on the surface is always normal to the surface.

Interactive nature of force
A force can be applied if the bodies which apply the force on each other interact. The interaction may or may not involve actual physical contact between the bodies but might actually interact from a distance. For example, like charges repel as the electric fields due to the two bodies interact.

Applications of pressure
Some applications of pressure are:
1. Sucking a drink with a straw.
2. Filling a syringe with a liquid.
3. Filling of ink in a fountain pen. 
4. Action of rubber suckers.

Force and its unit
Force is a push or a pull acting on an object which changes or tends to change the state of the object.
For example:
Actions, like opening the door, lifting a bag, kicking a ball, pulling a drawer, pushing a box are some of the tasks we do every day. All these actions result in the change of position of an object and for that, it requires force in the form of push or pull.

S.I unit of force is newton () or .

Change of pressure and observe its effect
 Class 8 Notes | EduRev
Take a rubber ball and make many holes in it with a needle. Fill the ball with water. Squeeze the ball with your hand. You will observed that water is coming with a great speed. Now slowly slowly if you decrease the applied pressure on the surface of the balloon, the speed of water will decrease. So more the pressure more forced induced towards the outside of the balloon. 
As we know pressure = , so more the contact area lesser will be the pressure exerted by the molecules.

Force changes direction of motion
When a ball hits against a wall, it bounces back in the opposite direction. So the force exerted by the wall on the ball changes its direction of motion.

Force changes shape of object
When a hammer strikes a hot rod, the rod bends. The force exerted by the hammer on the rod changes the shape of the rod.

Force can change the speed of an object
Blowing wind or wind storm can change the speed of moving car.

Force can change the direction of an object
A side wind can change the direction of a boat sailing in a sea.

Examples for effect of force
Force may bring the following change:
1) Can change the state of an object(rest to motion/ motion to rest):For example, pushing a heavy stone in order to move it.
2) May change the speed of an object if it is already moving.For example,catching a ball hit by a batsman.
3) May change the direction of motion of an object.For example,footballer kicks a   moving ball to change its direction.
4) May bring about a change in the shape of an object.For example, blowing air in balloon.
5) May cause some or all of these effects.

Difference between Contact and Non-Contact force
S.No.Contact ForceNon-Contact Force
1It acts when objects are in physical contact.Contact may be continuous or impulsive.In acts when there is no physical contact in objects.
2Muscular force,frictional force,spring force,normal force,tension force are few examples.Gravitational force,electrostatic force,magnetic force and electrochemical force are few examples.

Contact Forces
 A force that can cause or change the motion of an object by touching it is called Contact Force.
For example, muscular force,frictional force,spring force,tension force,air resistance force etc. 

Muscular force and its examples
Muscular force is the force applied using parts of the body like arms or legs. It is force that results due to the action of muscles, and is a contact force.
All physical activity like running, lifting weight etc. requires muscular force.

Tension Force
 Class 8 Notes | EduRev
Tension force is the force transmitted through a string, rope, cable or wire when it is tightly pulled by forces acting from opposite ends.
Example: In the given figure, the string applies a tension force on the block.

Non-contact forces
A non-contact force is any force applied to an object by another body without any contact.
For example, magnetic force, gravitational force and electrostatic force.

Non-contact forces and their examples
A non-contact force is a force applied on an object by another body without being in direct contact with the object.
  • Gravitational force
  • Electromagnetic force
  • Strong Nuclear force
  • Weak Nuclear force

Magnetic force and its examples
Moving charges and current carrying conductors create magnetic fields. Also magnets themselves have magnetic fields. When you keep two magnets close together a force is exerted between them which is a magnetic force. Iron filings placed around a magnet move because a magnetic force acts on them.

Electrostatic force
 Class 8 Notes | EduRev
Electrostatic force is the force between two charged particles. It is more when the amount of charge on bodies is more and when distance between the charges is small.
Similar charges repel each other while opposite charges attract each other.

Define and explain the characteristics of electromagnetic force
The electromagnetic force is a special force that affects everything in the universe because (like gravity) it has an infinite range. It has the ability to attract and repel charges. The electromagnetic force can be generated by three types of fields known as the electrostatic field, magnetostatic field, and the electromagnetic field.

Electromagnetic Force

Electromagnetic force is one of the four fundamental forces and exhibits electromagnetic fields such as magnetic fields, electric fields and light. It is the basic reason electrons bound to the nucleus and responsible for complete structure of nucleus.

It is the force exerted by the electromagnetic interaction of electrically charged or magnetically polarized particles or bodies.

Coulombs law: The force of attraction or repulsion between two charges is proportional to the product of magnitude of charges and inversely proportional to the square of distance between them acting along the line of joining the charges.

Lorentz Force: The fundamental force on an electric charge q moving with velocity v in a magnetic field B is called the magnetic Lorentz  force.

Characteristics of electromagnetic force
  1. These forces are both attractive as well as repulsive.
  2. They are central forces in nature.
  3. They obey the inverse square law.
  4. They are conservative forces in nature.
  5. These forces are due to the exchange of particles known as photons which carry no charge and have zero rest mass.
  6. They are  times stronger as compared to gravitational forces.

Pressure of liquid depends on gravitational force
The pressure of a liquid can be calculated by using a formula:

 pressure of a liquid
 height of the liquid column
 density of the liquid
 acceleration due to gravity

Natural effects of atmospheric pressure
Some natural effects of atmospheric pressure are:
1. As the temperature in a region is increased, it creates a region of low pressure. This causes air to flow from high pressure regions to low pressure regions. Hence, winds are caused due to a difference in atmospheric pressure.
2. Atmospheric pressure decreases with increase in height. This creates harsher environments in higher altitudes. The combined effect of low temperatures and pressure at high altitudes is reflected by a reduction in the density of forests.

Standard Atmospheric Pressure
Standard Atmosphere is mainly used as a reference value for the average atmospheric pressure at sea level. 1 standard atmosphere is defined as being exactly equal to 101,325 pascals.
Since the atmospheric pressure varies with changes in weather and altitude, it is convenient to standardise on a single value so that ratings, measurements and specifications can be compared. In particular 1 Standard Atmosphere is used by the Aviation industry as the reference standard for sea level pressure.

Experimental demonstration of atmospheric pressureFill a cup one-third with water. Cover the entire mouth with an index card. Holding the card in place, turn the cup upside down. Remove your hand from underneath.

Atmospheric pressure holds up the card and the water on it. 

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