FORCE AND FRICTION

FORCE AND FRICTION

revision

FORCE

Force is a push or a pull.

Living and non- living things can apply force.

 To apply a force over an object interaction between object and source of force is necessary.

 Whenever force act two object are involved. One which applied force, the other which receive force.

Direction and Magnitude of Force:

The measurement of strength and amount of force is called magnitude of force. Two or more forces on the same object can be applied in the same direction and opposite direction.

When two or more forces are applied in the same direction, then the total or net force is the addition of magnitude of both the forces.

Example: When two persons push the box in the same direction with the magnitude of forces of each 2 newton, then resultant force will be-Net force:-  2 newton + 2 newton = 4 newton (In the direction of applied force)

 When two forces are applied over an object in the opposite direction:

When two forces are applied over an object in the opposite directions, the total effective or net force is the difference of magnitude of two forces.
Example: When two persons push the box in the opposite direction with the magnitude of forces of each 2 newton, then resultant force will be-Net force:-  2 newton - 2 newton= 0 newton (Box will not move)

 If one person is applying a force of 6 unit in one direction and another person is applying a force of 8 unit in opposite direction,Then the resultant force: 8 newton - 6 newton = 2 newton (In the direction of higher magnitude force) In this case, force will act in the direction of higher magnitude of force.

Example 1

In a tug of war, a fat man pulls with a force of 100 N on a side, and a lean man pulls with 90 N on the other side. Determine the net force.

Example 2

A toy car is at rest, and a force of 70 N is applied to it. If the frictional force of 20 N, determine the net force.

Example 3

Michael pushes a 2kg box with 30N of force to the left. Annie pushes the same box with 40N of force to the right. What is the net force on the box?

Effect of Forces:-

1. Force can change the state of motion: An Object can be in two states.

(a) Rest State: When an object is not moving. This means a stationary object is called in the state of rest. For example –A ball and book kept over the ground and not moving.

(b) Motion State: A moving object is called in the state of motion. For example – a moving car, a moving ball, etc.A Force can move a stationary object. A force can speed up, decrease the speed and change the direction of a moving object. Force can stop a moving object. Finally we can say that force can change the state of motion

Force can change the shape of an object:

(1)When you apply force on an inflated balloon by pressing it using your hand from both sides, then the force of pressure changes the shape of balloon. (2) You can change the shape of rubber band and spring by stretching it in opposite direction.(3)You can change the shape of dough into bread by applying force with a rolling pin.So we can say that forces can change the shape of an object.

Types of Force:

1. Contact Force:

(a) For contact force, interaction between objects is necessary. Force that comes into action after the interaction between objects is called contact force. Contact force works on the point of contact. Example; pushing a car, hitting the ball, kicking a ball, etc.

(b) Muscular force, friction force are types of contact force.

Non- Contacting Force:

Force due to without interaction between two objects is called non-contact force. 

For example; a magnet can pull an iron nail from a distance.

 Magnetic force, Electrostatic force and Gravitational force are examples of non-contact force.

1. Contact Force:

For contact force, interaction between objects is necessary. Force that comes into action after the interaction between objects is called contact force. Contact force works on the point of contact. Example; pushing a car, hitting the ball, kicking a ball, etc.

Tension Force,

Normal Force,

Air Resistance Force, 

Applied Force, 

Spring Force,

Muscular force,

friction force are types of contact force.

Tension force:

The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

Normal force:

The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

Air resistance:

The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

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.

Spring force:

The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

 Muscular force:

(1) Force due to the action of muscles is called muscular force. We can say force resulting because of action of muscle is called muscular force.(2) Muscular force is applied only after interaction with the object.Examples:(a) When we push the object like school bag or lift the bucket of water we use the muscular force.(b) Animals also use of muscular force to carry out their task.

 Friction Force:

(1) The forces of friction arises between the surfaces of two objects

(2) The force of friction always acts on all moving object and its direction is always opposite to the direction of motion.

(3)Force of friction comes into action only after interaction between two objects, thus, it is a type of contact force.

Factors affecting Friction:

(1) Friction is caused by the irregularities on the two surfaces in contact.(2) Even those surfaces which appear very smooth but microscopic view show those irregularities. When these surfaces come into contact, the irregularities of surfaces get interlocked. We have to apply force to remove the interlocking, a surface produces friction when it slides or rolls over another surface. (3) Friction force will increase with increase in weight. As weight increases irregularities between surfaces get interlocked tightly because of pressure, and the surface offers more resistance of friction.(4) Rougher surface offers more force of friction.

Types of Friction

• Static fricton

• Sliding friction

• Rolling friction

• Fluid friction

Static Friction

Static friction is the force which acts between two bodies when a force is applied on a body but there is no relative motion between the surfaces. 

Static friction opposes an impending motion.

Static friction is self adjusting force.

SLIDING FRICTION:

The Sliding Friction is friction which comes into play when surface of one object moves relative to surface of another object. 
It is also called as kinetic friction.
 It is that force that keeps a surface sliding with another surface.

Rolling Friction:

 Rolling friction comes into action when a surface rolls over another surface and resist its motion is called rolling friction.
 Rolling friction is smaller than sliding friction. Due to this roll anything over the ground is easier than to slide.

FLUID FRICTION:

Fluid friction is the resistance to an object's motion through a liquid or gas. 
When the motion is occurring in a liquid, it is referred to as viscous resistance. 
As an example, when an object falls through water, its speed reduces due to fluid friction acting on it.

Friction: A Necessary Evil

Friction is a necessary evil because it has both harmful and beneficial effects.
Examples: (1) Friction is necessary even a simple task like walking.
(2) It is difficult task to move on a wet muddy track, or wet marble floor. 
(3) Grooved soles give more friction to the ground which gives better grip when we walk. Shoes with worn out soles can be slippery.
(4) The tyres of vehicles have treads for better grip over road. It provides friction to the surface of road. When treads are worn out, the tyres need to be replaced with new one.
(5) Friction can also produce heat. 
(6)When a machine is operated, heat generated causes much wastage of energy.

Method of decreasing friction:

By polishing: The interlocking and projections are minimised by this method.
Ball-bearing:  As the rolling friction less than the sliding friction consequently in rotating machinery, the shafts are fixed on the ball bearing so that the friction can be reduced considerably. 
Lubricants: A lubricant is a substance (a solid or a liquid) which forms a thin layer between the two surfaces in contact. It also fills the depressions present in the surfaces of contact and reduces friction considerably. 
Streamline: The friction due to air is reduced by making the automobiles streamline.

Increase of friction:

(1) Friction is desirable in some situations
(2) Friction is necessary for us, therefore friction is increased to accomplish Some of important task.
Examples: (a) Treaded tyres of vehicle increase friction. Vehicle with treaded tyres provides mores friction which gives better grip with the road.
(b)  Soles of shoes are treaded to increase friction between sole and the ground.
(c)  Javelin thrower rubs some powder on his palm to get better grip on the javelin.
(d)  Kabaddi players rub their hands with soil for a better grip of their opponents.

Non- Contacting Force:

Force due to without interaction between two objects is called non-contact force. 
For example; a magnet can pull an iron nail from a distance.

 Magnetic force, Electrostatic force and Gravitational force are examples of non-contact force.

 Magnetic Force:

 A magnet can exert a force on another magnet without being in contact with it.
The force exerted by a magnet is an example of a non-contact force.


Example: Opposite pole of two magnets attract each other and same pole repel each other.

Electrostatic Force:

When a charged body exert a force on another charged or uncharged body is called electrostatic force.A charged body attracts an uncharged body.

 A positively charged body attracts a negatively charged body and repels a positively charged body without coming in contact, thus it is a non-contact force. 

Gravitational Force:

Gravitation force is exerted by earth, moon, sun and other planets to others.
 Earth and other planets attract all objects towards them. Since, earth attracts all objects even without coming in contact, thus gravitational force is a non-contact force.
Example: When we release an object from a height, it falls over the ground because of gravitational attraction of earth.

Pressure:

The force acting on per unit area of a surface is called pressure. We can say force per unit area is called pressure.  

      Pressure = Force / Area

From above formula, we can say that with same applied force, Pressure is indirectly proportional to the area, thus pressure decreases with increase in area and increases with decrease in area.

1. Calculate the pressure produced by a force of 800 N acting on an area of 2.0 m².

2. The elephant weighs 20,000 N stands on one foot of area 1000 cm². How much pressure would it exert on the ground?

3. The pressure of a gas contained in a cylinder with a movable piston is 300 Pa. The area of the piston is 0.5 m². Calculate the force that is exerted on the piston.

4. Find the thrust required to exert a pressure of 50000 pascals on an area of 0.05m² ?

5. Find the area of a body which experiences a pressure of 50000 Pa by a thrust of 100 N?

6. The base of a container measures 15 cm × 20 cm. It is placed on a table top. If the weight of the container is 60N, what is the pressure exerted by the container on the table top?

Pressure exerted by Liquid and Gas:

==> Liquid and gas can also exert pressure on inner walls of the container in which they are kept.

==> When air is filled, a rubber balloon gets inflated from all sides. This happens due to air or gas exerts pressure over inner walls of the balloon.

==> Water starts leaking if there is a pore in the bottle. This happens due to water exerts pressure over the walls of the bottle.

Atmospheric Pressure:

Our atmosphere is made of different types of gases. Since gases exert pressure, thus Pressure exerted by air is called atmospheric pressure.

The weight of air in a column of the height of the atmosphere and area 10 cm × 10 cm is as large as 1000 kg. 

A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure.

Multiple Choice

Which force is being applied by a vendor when he is pushing the pushcart?

• Muscular force

• Magnetic force

• Friction

• Electrostatic force

Multiple Choice

What will happen to the speed of a car when the driver applies brakes?

• Speed increases

• Speed decreases

• No change in speed

• Change in direction

Multiple Choice

Which force always opposes motion?

• Magnetic force

• Electrostatic force

• Friction

• Muscular force

Multiple Choice

When your friend pushes your bike from behind, what will happen to the speed of the bike?

• Speed decreases

• Speed increases

• No change

• Any of the above

Multiple Choice

Which of the following possess gravitation?

• Earth

• Venus

• Saturn

• All of the above

Multiple Choice

How much weight of air is always over your head?

• 1000 kg

• 100 kg

• 10 kg

• 1 kg

Multiple Choice

Which force causes a charged balloon to attract another balloon?

• Muscular force

• Electrostatic force

• Magnetic force

• Gravitational force

Multiple Choice

A dropper to fill ink works because of which of the following?

• Friction

• Magnetic force

• Air pressure

• Electrostatic force

Multiple Choice

What is the SI unit of force?

• newton

• Galileo

• Einstein

• Faraday

Multiple Choice

Which animal can easily walk in desert?

• Lion

• Zebra

• Giraffe

• Camel

Multiple Choice

The pressure which is exerted by air around us is known as

• force

• atmospheric pressure

• muscular force

• friction

Multiple Choice

The force involved in falling of an apple from a tree is

• magnetic force

• electrostatic force

• contact force

• gravitational force

Multiple Choice

Pressure is

• Area/Force on which it acts

• Force/Area on which it acts

• Volume/Force on which it acts

• Force/Volume on which it acts

Multiple Choice

Which of the following produces least friction?

• Sliding friction

• Rolling friction

• Composite friction

• Static friction

Multiple Choice

The shape of an aeroplane is like a

• dog

• bird

• car

• all of these

Multiple Choice

It is difficult to walk on ice because

• pressure is high

• pressure is low

• friction is high

• friction is low

Multiple Choice

Friction can be increased by

• making the surfaces smooth

• lubricating the surfaces

• using ball bearings

• making the surfaces rough

Multiple Choice

Which one of these characteristics does a smooth surface has?

• Less frictional force

• More frictional force

• Sometimes less and sometimes more force

• All of above

Multiple Choice

How is friction due to air reduced?

• Streamlining

• Lubrication

• By using ball bearings

• By polishing

Multiple Choice

Friction can be increased by ___________

• Using air cushion

• Lubricants

• Using sand

• Using ball bearings

Time

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