Contents
- 1 What is the push or pull on an object known as …
- 1.1 Force – A Push Or A Pull
- 1.2 Force is Due to an Interaction
- 1.3 Force Has Magnitude as Well as Direction
- 1.4 Effects Of Force
- 1.5 1. A Force can Move a Stationary Object
- 1.6 2. A Force can Stop a Moving Object
- 1.7 3. A Force can Change the Speed of a Moving Object
- 1.8 4. A Force can Change the Direction of a Moving Object
- 1.9 5. A Force can Change the Shape and Size of an Object
Physics Topics can help us understand the behavior of the natural world around us.
What is the push or pull on an object known as …
When an athlete wants to throw a very heavy ball during shot put contest so that it may go as far as possible, he has to push the ball hard. We say that the athlete applies a large force to the ball to make it move through a large distance. When a person wants to draw water from a well, he has to pull the rope attached to the bucket full of water. We again say that the person applies a force to the rope attached to the bucket of water to make it move and come out of the well. And if, by chance, the sharp heel of a womans shoe falls on our foot, it hurts our foot too much and produces a lot of pain.
We say that the force of woman’s weight produces a large pressure on our foot because all her weight falls on a small area of our foot due to sharp heel. It is this large pressure on our foot which hurts the foot. On the other hand, if the broad heel of a man’s shoe falls on our foot, it hurts our foot much less. This is because due to broad heel, the force of man’s weight falls on a large area of our foot due to which the pressure produced on our foot is much less. In this Chapter we will discuss the various types of forces and the effects produced by these forces. We will also study the pressure exerted by solids, liquids and gases, including atmospheric pressure. Let us start by discussing force.
Force – A Push Or A Pull
When we want to open a door, we have to push the door handle with our hand. And when we want to close the door, we have to pull the door handle with our hand. This means that to move an object, it has either to be pushed or pulled. A push or pull on an object is called force (see Figure). The direction in which the object is pushed or pulled is called the direction of force. We open or close a door by applying force. When we push the door to open it, then we apply a force on the door in a direction away from us. And when we pull the door to close it, then we exert a force on the door in a direction towards us.
Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing. For example, a force is used when we push (kick) a football; a force is used when we pull a door; a force is used when we lift a box from the floor; a force is used when we stretch a rubber band; a force is used when we twist a wet cloth to squeeze out water, and a force is used when we press the brake pedal of a car. The fallen leaves of trees fly away with wind because the force of wind pushes them away. Even the roofs of some huts fly away during a storm because the force of strong winds pushes them away. And when we fly a kite, we can actually feel the force (or push) of the wind on it.
Before we go further, we should know the meaning of the term ‘interaction as used in physics. Interaction means ‘reciprocal action’. Interaction involves two objects. In interaction, each object acts in such a way as to have an effect on the other object. In most simple words, interaction means to act on each other.
Force is Due to an Interaction
An interaction of one object with another object results in a force between the two objects. In other words, a force arises due to the interaction between two objects. At least two objects must interact with each other for a force to come into play (and show its effect). If there is no interaction between two objects, no force can show its effect. This will become more clear from the following examples.
Suppose a man is standing behind a stationary car [see Figure], Since there is no interaction between the man and the car, no force acts on the car and hence the car does not move. Now, suppose the man pushes the car
with his hands due to which the car starts moving [see Figure], When the man pushes the car, there is an interaction between the man and the car. During this interaction, a force arises which acts on the car and makes it move in the direction of applied force. Please note that the man has to push the car to make it move. Here the two objects (or things) which are interacting for the force to come into play and show its effect are the ‘man and the ‘car’. In the above example of a stationary car and man only man is capable of applying force to the stationary car.
If both the objects are capable to applying force on each other, then the interaction between them can be of pushing’ or ‘pulling’. For example, the two boys shown in Figure are interacting and applying force on each other by pushing each other. On the other hand, the man and cow shown in Figure are also interacting but they are applying force on each other by pulling each other. In this case, the man is pulling cow, and the cow is also pulling the man.
Force Has Magnitude as Well as Direction
An adult man can apply large force on an object whereas a child can apply only a small force on an object. This means that a force can be larger or smaller than the other force. The strength of a force is expressed by its magnitude. The magnitude of a force is expressed in the SI unit of force called ‘newton’ (whose abbreviation is N). 1 newton is the force which can make an object of 1 kilogram mass to move at a speed of 1 metre per second. Along with the magnitude of force, the direction in which a force acts is also to be specified (or taken into account). This is because if the magnitude or direction of the applied force changes, the effect produced by force also changes. When two forces act on an object, then two cases arise: either the forces act in the same direction or the forces act in opposite directions.
(i) If the two forces applied to an object act in the same direction, then the resultant force acting on the object is equal to the sum of the two forces. In other words, when two forces act in the same direction, their effective magnitude increases. This will become clear from the following example. Suppose there is a heavy box which one man can move only by pushing it very hard. Now, if two men push this heavy box in the same direction, it becomes much easier to move the heavy box (see Figure). This is because when the two men apply their forces of push together in the same direction, the two forces add up to provide a much bigger force. And this bigger force can move the heavy box very easily.
(ii) If the two forces applied to an object act in the opposite directions, then the net force acting on the object is equal to the difference between the two forces. In other words, when the two forces act in opposite directions, their effective magnitude decreases. This will become more clear from the following example. Suppose there is a heavy box lying on the ground. Let the two men push this box from opposite directions (one from the left side and the other from the right side) (see Figure). Assuming that one of the men is stronger of the two and applies a larger pushing force than the other man, we can say that the box will move in that direction in which a larger force is applied by the stronger man. The box will, however, move very slowly in this case because the net force acting on the box in this case is equal to the difference in the magnitudes of the two forces applied by the two men. And this net force is small.
(iii) If the two forces applied to an object are equal in magnitude and act in opposite directions, then the net force acting on the object is zero (or nil). Since the net force acting on the object is zero (or nil) in this case, the object does not move at all, it remains in the same position. For example, if the two men push a heavy box from opposite directions by applying exactly equal forces, then the heavy box will not move at all. It will remain where it was. A familiar example of this case of equal forces acting in opposite directions is provided by the game called ‘tug of war’.
In this game, two (see Figure). Members of both the teams try to pull rope in their direction. When the two teams pull the rope equally hard (by applying equal forces), the rope does not move in any direction (see Figure). At this point of time, the two teams are applying equal but opposite forces to the rope due to which the net force acting on the rope is zero and hence the rope does not move. Actually, the equal and opposite forces applied on the rope cancel each other’s effect. Ultimately, the team which pulls the rope harder (by applying greater force) wins the game.
Effects Of Force
A force cannot be seen. A force can be judged only by the effects it can produce in various objects around us.
A force can produce the following effects:
- A force can move a stationary object.
- A force can stop a moving object.
- A force can change the speed of a moving object.
- A force can change the direction of a moving object.
- A force can change the shape (and size) of an object.
We will now give examples of all these effects produced by a force when it acts on objects.
1. A Force can Move a Stationary Object
Take a rubber ball and place it on a table top. Now gently push the ball along the surface of table. We will observe that the ball begins to move. Thus, a ball at rest (or stationary ball) begins to move when a force (of push) is applied to it. While taking penalty kick in a football match, the player applies a force on the stationary football. Before being hit, the football is at rest, and its speed is zero. The force applied by the player makes the football move towards the goal. Thus, if we kick a stationary football kept on the ground, then the football starts moving (see Figure).
In this case, the force of our foot makes the stationary football move. Similarly, the force of engine makes a stationary car to move. From these examples we conclude that a force can move a stationary object.
2. A Force can Stop a Moving Object
Take a rubber ball and place it on a table. Push the rubber ball gently so that it starts moving. Now, place your palm in front of the moving ball. We will observe that the moving ball comes to a stop. Actually, the palm held in the path of moving rubber ball applies a force to the moving ball. This force stops the moving ball. Thus, a moving ball stops when a force is applied to it. In a football match, when the goalkeeper dives or jumps up to save the goal, he applies a force to the moving football with his hands. This force applied by goalkeeper helps in stopping the moving football and saves a goal being scored (see Figure).
Thus, the stopping of a moving football by a goalkeeper demonstrates that a force can stop a moving object. A cricket ball moving on the ground stops automatically after some time. In this case, the force of friction of ground stops the moving cricket ball. Similarly, the force of brakes can stop a moving car. From all these examples we conclude that a force can stop a moving object.
3. A Force can Change the Speed of a Moving Object
Suppose we are moving on a bicycle at a certain speed. Now, if someone pushes the moving bicycle from behind, then the speed of bicycle increases and it will move faster. On the other hand, if someone pulls the moving bicycle from behind, then the speed of bicycle decreases and it will move slower. Thus, a push or pull can change the speed of a moving bicycle. But a push or pull is called force. So, we can say that a force can change the speed of a moving bicycle (or any other moving object). If the force is applied in the direction of motion of the object, its speed increases. On the other hand, if the force is applied in the direction opposite to the direction of motion of an object, then its speed decreases.
When a ball is dropped from a height, its speed goes on increasing. The speed of a falling object (like a ball) increases because the earth applies a pulling force on it which is called the force of gravity. It is the force of gravity of the earth which pulls a falling object towards its centre and increases its speed. On the other hand, when a ball is thrown upwards, then its speed goes on decreasing. This is because the earth applies a pulling force of gravity on the ball in the downward direction (opposite to the motion of the ball).
When a hockey player hits a moving ball, the speed of ball increases (see Figure). When we pedal the bicycle faster, then the speed of bicycle increases. And when we apply brakes to the moving bicycle, then the speed of bicycle decreases. From all these examples we conclude that a force can change the speed of a moving object.
4. A Force can Change the Direction of a Moving Object
In a cricket match, when a moving cricket ball is hit by a bat, then the direction of cricket ball changes and it goes in another direction (see Figure). In this case, the force exerted by bat changes the direction of a moving cricket ball. In the game of carrom, when we take a rebound, then the direction of striker changes. This is because the edge of the carrom board exerts a force on the striker. If we blow air from our mouth on the smoke rising up from a burning agarbatti, then the direction of motion of smoke changes. In this case, the force exerted by the blowing air changes the direction of moving smoke. From these examples we conclude that a force can change the direction of motion of a moving object.
Before we go further, we should understand the meaning of the term ‘state of motion’. The state of motion of an object is described by its speed and the direction of motion. A change in either the speed of an object, or the direction of its motion, or both, is called a change in its state of motion. A force can change the state of motion of an object.
5. A Force can Change the Shape and Size of an Object
If we take a light spring and pull it at both the ends with our hands, then the shape and size of the spring changes (see Figure).
In this case, the force of our hands changes the shape and size of the spring (The turns of the spring become farther apart and its length increases). Here are some more examples in which a force changes the shape (or size) of an object. The shape of dough (kneaded wet flour) changes on pressing with a rolling pin (belan) to make chapatis. When we press the dough with a rolling pin, we apply force. So, we can say that the shape of dough changes on applying force. The shape of kneaded wet clay changes when a potter converts it into pots of different shapes and sizes.
This happens because the potter applies force on the kneaded wet clay. The shape of a toothpaste tube (or an ointment tube) changes when we squeeze it because we apply force while squeezing it. When we hammer a piece of aluminium metal, its shape changes and an aluminium sheet is formed. This change in shape occurs because we apply force while hammering. When we sit on a sofa with springs, then the springs of the sofa get compressed and their shape and size changes. This happens because our weight applies a force on the springs and compresses them.
Similarly, the shape of a sponge, tomato, balloon or tennis ball changes on pressing. And the shape and size of a rubber band changes on stretching. The shape and size of a balloon changes when it is filled with air (or water) because the weight of air (or water) exerts force on the walls of the balloon from inside. From these examples, we conclude that a force can change the shape and size of an object. We will now discuss the various types of forces.