Forces and Interactions PDF

## What is Force? - Force is a push or a pull on a body. - It is commonly called force. - It changes the state of motion of a body or its direction or shape. - The direction in which a body is pushed or pulled is called the direction of the force. - The cause of the changes in motion of the body...

## What is Force? - Force is a push or a pull on a body. - It is commonly called force. - It changes the state of motion of a body or its direction or shape. - The direction in which a body is pushed or pulled is called the direction of the force. - The cause of the changes in motion of the body are the effects of force. ## Interaction Causes Force - For interaction to happen, at least two bodies are required-one on which the force is applied and the other that applies the force. - The two bodies must interact with each other for force to come into play. - In case there is no interaction between the two bodies, no force will come into play to show its effect. - In other words, force arises due to the interaction between the two bodies. ## Exploring Forces - More than one force acts on a body at the same time. - The effect produced by all the forces collectively is equivalent to a single force - referred to as the resultant force (or the net force). - It produces the same effect on the body as is produced when all the forces are collectively acting on the body. - When the resultant of all the forces acting on a body is zero, the forces are known as balanced forces. - Balanced forces cannot change the speed of a moving body or produce any motion in a stationary body. - However, balanced forces may change the shape and size of the body. - When the resultant of all the forces acting on a body is not zero, the forces are known as unbalanced forces. ## Identifying Actions - Actions can be grouped as pushing, pulling, lifting, or flying. ## Table: Description of the situation, Actions | S. No. | Description of the situation | pushing | pulling | lifting | |---|---|---|---|---| | 1. | shifting a stack of notebooks placed on a table | yes | | | | 2. | flying a kite | | yes | | | 3. | pressing the brake pedal of a car | yes | | | | 4. | stretching a rubber band | yes | | | | 5. | playing tug of war | yes | yes | | | 6. | typing using the letter keys on a keyboard of a desktop | yes | | | ## Conclusion The motion of an object can be classified as push or pull. ## Units of Force - The strength of force is expressed by its magnitude. - The force that a strong man will apply will be greater than that of a child. - This means the magnitude is not always the same. - The SI unit of force is newton, which is denoted by N - 1N = 100,000 dynes. - The CGS unit of force is called dyne. - 1N = 10^5 dynes. - The other units of force are kilogram force (kgf) and gram force (gf) - The force required to vertically lift a body of mass 1 kg is called kilogram force. - The unit, kilogram force, is denoted by kgf. - The force required to vertically lift a body of mass 1 g is called gram force. - The unit, gram force, is denoted by gf. - 1 kgf = 1000 gf. - Along with the magnitude of force, the direction in which it acts is also taken into account. Therefore, force has magnitude as well as direction - Therefore, force is a vector quantity. ## Solved Numericals 1. Two boys pull a rope from the two ends with a force equal to 200 N each. What is the resultant force acting on the rope? - Forces are acting along the same line but in opposite directions. - Resultant force = 200 N - 200 N = 0 - Thus, the resultant force acting on the rope is 0. 2. Two persons are pushing a heavy box in the same direction. They apply forces of magnitude 200 N and 150 N. What is the resultant force acting on the box? - Forces are acting along the same line in the same direction. - Resultant force = 200 N + 150 N = 350 N - Thus, the resultant force acting on the box is 350 N. 3. Two persons are pushing a heavy box from the opposite directions. They apply forces of magnitude 400 N and 220 N. What is the resultant force acting on the box? - Forces are acting along the same line but in the opposite directions. - Resultant force = 400 N - 220 N = 180 N - Thus, a resultant force of 180 N acts on the box along the direction of 400 N. ## Effects of Force - Though force cannot be seen, it can be judged by the effects it produces in different objects around us. - **Moves a stationary object** - For example, a ball at rest will begin to move when some push or force (like a kick) is applied to it. - Similarly, the force of an engine makes a stationary car or a bus move. - **Stops a moving object** - On application of force, a moving object can be stopped. - For example, a moving football stops after travelling some distance on the ground. - In this case, the force of friction between the ground and the football stops the moving football. - **Changes the speed of the moving object** - Force can change the speed of a moving object. - For example, when a moving bicycle is pushed from behind, the speed of the bicycle increases and it starts moving faster. - However, if the moving bicycle is pulled from behind, the speed of the bicycle decreases and it starts moving slower. - So, a push or pull (force) can affect the speed of a moving object. - **Changes the direction of motion** - Force can change the direction of motion of a moving object. - For example, during the game of cricket, the batsman changes the direction of the moving ball by striking it with the bat at a suitable angle. - In the game of carrom, when the striker rebounds after hitting an edge of the carrom board, its direction changes. - This happens because the edge of the carrom board exerts a force on the striker. ## Types of Forces - Forces can be classified into two types: contact forces and non-contact forces. - These are further classified into different types. ### Contact Forces - A force which can be exerted by a body on another body through physical contact is known as contact force. - Muscular force and frictional force are examples of contact forces. #### Muscular Force - The force exerted by the muscles of a body is known as muscular force. - Human beings exert muscular force to carry out various day-to-day activities such as jumping, climbing, pushing, lifting, kicking, running and walking. - Even animals exert muscular force to do work. - Various functions of the internal organs of our body are carried out due to the muscular force exerted by the muscles of the organs. - For example, our heart beats because of the muscular force produced by the cardiac muscles. #### Frictional Force - The force which opposes the motion of one body over the surface of another body is known as frictional force. - This force acts between the two surfaces which are in direct contact with each other. - For example, a ball moving on the ground automatically slows down and finally stops because of the frictional force between the ball and the ground. - The frictional force (or simply friction) always acts on all moving and stationary bodies. - The direction of frictional force is always opposite to the direction of motion. ### Non-Contact Forces - Forces that do not involve physical contact with the bodies on which they act are called non-contact forces. - A non-contact force is also known as action-at-a-distance force. #### Magnetic Force - The force exerted by a magnet is known as magnetic force. - The magnet exerts a magnetic force on objects made of iron, cobalt and nickel. - When a magnet is brought near some iron pins, they get attracted towards the magnet (Fig. 8.8) even when the magnet is not in physical contact with the pins. - So, the magnetic force can be said to be a non-contact attracting force. - A magnet also exerts force on another magnet placed near it. - The magnetic force between two magnets placed near one another can be attractive or repulsive. - It depends upon the poles facing one another. - We already know that like poles (north-north or south-south) repel each other and unlike poles (north-south) attract each other. #### Electrostatic Force - An electrically charged object can exert force on an uncharged object. - This force is known as electrostatic force. #### Gravitational Force - The force with which the Earth pulls objects towards its centre is known as gravitational force or simply gravity. - This force of gravity causes all objects to fall towards the Earth. - The gravitational force of the Earth acts on all objects without establishing any physical contact. - Therefore, gravitational force is an example of non-contact force. - The force of gravity acts on all the objects irrespective of where they are - in the air, water or on the ground. ## Pressure - Often, an object exerts force on another object by making contact with it. - You may have experienced that a sharp knife is more effective in cutting fruits and vegetables than a blunt knife. - In case of the blunt knife, the area of the cutting edge is wide. - Therefore, the force exerted by you has to act on a larger area and its impact is reduced. - This makes cutting difficult - If the area of the cutting edge is narrow, as in the case of a sharp knife, your force acts on a very small area with greater impact and cutting the fruit becomes easy. - To understand this better, we have to understand the concept of pressure first. - Pressure is defined as the perpendicular force acting on a unit area of an object. - Pressure (P) = Force (F) / Area (A) ## Effects of Pressure - The concept of pressure helps us understand many observations in our daily lives. * The rear wheels of a tractor are broader when compared to the front wheels. - Broad tyres greatly reduce the pressure on the soil and the tractor easily moves through the soft soil of the field. * All tools that help us cut and pierce, such as knife, saw and needle, have sharp edges. - This is because sharp edges occupy less area as compared to blunt edges. - This makes the tools exert greater pressure with only a small force. * A school bag has wide straps made of thick cloth so that the weight of the bag is distributed over a large area. - This produces less pressure on the shoulders and the child can carry the bag comfortably. * Railway lines are supported by wooden sleepers underneath. - The wooden sleepers distribute the pressure over a larger area of the ground and ensure that the railway lines do not sink into the ground under the pressure of the train. ## Pressure Exerted By Fluids - All liquids exert pressure on the bottom and the walls of the container in which they are kept. - When some liquid is poured into a vessel, the weight of the liquid presses down on the base of the vessel and exerts pressure on the container. - The pressure exerted by a liquid is given by the formula: - Pressure = Force/Area - Here, force is the weight of the liquid and area is the area of the base of the vessel. ## Measuring Pressure - Pressure is measured using a simple device called a manometer. - A common simple manometer consists of a U-shaped glass tube filled with a liquid. - It is a simple pressure gauge and it measures differences in pressure exerted at the two ends of the glass tube. - When both ends of the tube are open, the liquid is at the same height in each leg. - When pressure is applied to one leg, the liquid is forced down in that leg and moves up in the other. - The difference in heights 'h' which is the sum of the readings above and below zero, indicates the pressure. ## Pressure Exerted By Gases - Gases are made up of molecules which move around in all directions. - These fast-moving molecules collide against each other and against the walls of the container. - So, they exert force on the walls of the container. - This force exerted on the walls gives rise to air pressure. - Air pressure is measured with a device called a barometer. ## Atmospheric Pressure - We live on the surface of the Earth. - There is atmosphere all around us. - The atmosphere extends up to 1000 km above the surface of the Earth and has a total weight of about 4.5 x 10^18 kg (that is, four and five tenths million billion tonnes). - This ocean of air is pressing against everything. - The atmosphere exerts pressure on us from all sides due to its weight. - The weight of the ocean of air per unit area is a measure of the atmospheric pressure or air pressure. - We can demonstrate air pressure by performing Activity 10. ## Key Words * **atmospheric pressure:** the pressure exerted by the layer of air (atmosphere) around the Earth * **balanced force:** when the resultant of all the forces acting on a body is zero * **force:** a push or a pull that can be used to change the speed, direction or shape of a body * **manometer:** the apparatus used for measuring pressure * **pressure:** the force acting perpendicularly per unit area of the surface * **resultant force:** the single force which is equal to the effect of all the forces acting on a body * **spring balance:** the device used for measuring the force acting on a body ## Concept Recall * **muscular** * **frictional** * **magnetic** * **electrostatic** * **types** * **contact** * **non-contact** ## Types of Force * **liquids** * **measured by** * **manometer** #### Force and Pressure * **pressure**