FORCE
Force
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1.1 General Introduction
We have studied which science phenomena(or a concept) used for Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling which are used to done certain tasks (or actions) . Each of these actions usually results in some kind of change in the motion of an object. In this chapter we will also analyses the speed ,direction and other parameters of a body whose activities such as how is the cycle able to move forward?, how a fielder stopping a ball, what makes the bullock cart to be in motion.
1.2 Motion
If a body changes its position as time passes, it is said to be in motion (or moving). But when do we say that it is not changes its position as time passes, it is said to be in rest. for example – a man moves when he walks along a road which is in motion and a tree which is fixed at a place which is in rest.
Do
you Know ? Rest and motion are relative terms. A book kept on the table is at
rest with respect to the table. But you know that Earth is rotating. The
objects kept on earth must also be moving with the earth.
How the objects move?
for all examples in para1.1
The central answer (or perfect answer) to all these types of questions is a physical quantity called force.
NOTE- The force is also known as the cause of the motion.
1.3 FORCE
When we want to open a door or want to close the door,
We have to push or pull handle with our hand .this means that to move a body (or an object), it has either to be pushed or pulled. A push or pull on a body is called a force. It changes the position of a body or tends to change the position of a body. the unit of force in SI system is Newton and in CGS system is dyne (1newton=105 dyne)
FORCE = MASS×ACCELERATION
or, F=m×a
1.4 Net force (or resultant force)
In real situation, more than one forces act on one body.
For example- Poona is pushing a heavy box which only moves when pushing hard the box. It cannot sufficient by poons’s push (or force applied by Poona) to move the box. So he asks her friend to help in pushing the heavy box. Now, it is easier to move it. Can you explain why? It is easier to move it because poona and her friend both are push hardly the heavy box and the force acts on the box is the sum of Poona and her friend forces.
EXAMPLE 1: Ram is pushing a heavy box having wheels (looking like a cart) with a force of 40N.he finds it is very heavy and is unable to move it faster. He calls his friend Pawan to help him. Pawan applies a force of 35N Along with ram in the same direction, what is the total force acting on the heavy box?
= force applied by ram + force applied by Pawan =40N+35N=70N |
The total force applied on the heavy box (or an object) is called the resultant force.
Suppose a car at rest. It can be pushed by a group of two or three persons. If the car is pushed then it starts moving. Its speed or direction of motion depends upon the force and the direction of application of force.
If the net force acts in the direction of motion, the speed of an object moving with constant speed also increase. If the net force acts in a direction opposite to the motion, then it either slows down the object or brings it to a rest or it may change the direction of motion.
1.4 State of motion
The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed. An object may be at rest or in motion; both are its states of motion.
1.4 Effects of forces
A force cannot be seen directly but these are used in our everyday actions like pushing, pulling lifting, stretching, twisting and pressing. A force can be judged only by the effects which it can produce in various bodies around us. A force can produce the following effects:
1. It change the state of motion i.e., object may be at rest (or state of zero speed) or in motion. A force can make a stationary body to move or force can stop a moving body.
2. It can change the speed of a moving body. For example- a horse by exerting a force on a cart, pull it from rest and subsequently exerting a larger force, the horse makes the cart move with a larger speed.
3. It can change the direction of a moving body. 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.
4. It can change the shape (and size) of a body. For example- if we hold a rubber ball between our palms against each other, the ball no longer remains round but gets oblong.
Problems for practice
Question-1: What do you understand by term ‘force’? What is its S.I unit?
Question-2: What happen when two force acts in opposite direction on an object?
Question-3: State two actions which involve both push and pull.
Question-4: Give one real life example in which two or more men exert a force on an object so that the resulting force on an object is zero.
Ouestion-5: Give two examples to verify that a force changes the shape of a body.
Ouestion-6: Sachin told her friend to roll a ball towards her, then sachin push it in another direction other than where it is coming from. Explain the effects of force on the ball?
Ouestion-7: what force would need to produce an acceleration of 4m/s2 in a ball of mass 7kg?
Ouestion-8: In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case. (a) Squeezing a piece of lemon between the fingers to extract its juice. (b) Taking out paste from a toothpaste tube. (c) A load suspended from a spring while its other end is on a hook fixed to a wall. (d) An athlete making a high jump to clear the bar at a certain height.
Ouestion-9: Continuous application of force is necessary to maintain the motion of an object. Is it true?
Higher Order Thinking Questions
1. If you push a heavy box which is at rest, you must exert some force to start its motion. However once the box is sliding you apply a lesser force to maintain that motion. Why?
2. A cricket ball of mass ‘m’ is thrown upward with some initial speed. If the air resistance is neglected, what forces are acting on the ball when it reaches (a) half its maximum height and (b) its maximum height?
1.5 magnitude and direction of force
A force could be larger or smaller than the other or equal to each other. The strength of a force is usually expressed by its magnitude (i.e. the magnitude of force is the sum of all the forces that are acting on an object). The direction of a force is as important as its magnitude. We represent the direction of force, using ‘arrows’
1.6 Positive and negative force
The Force on an object is said to be negative if the force and displacement of the object is acting in the opposite direction with respect to a reference direction. However, the magnitude of a force cannot be negative. The Force on an object is said to be positive if the force and displacement of object are in same direction respect to a reference direction. a point used to describe the position of an object is known as reference direction or reference point. A positive force represent repulsion (e.g. between two like charges) and a negative force represents attraction (e.g. between two unlike charge).
1.5 Types of forces
There are different types of forces which are categorized into two groups i.e. contact forces and non-contact forces.
1.6 Contact forces
Contact forces are often when two objects are in the actual contact with each other. For example: lift a bucket of water by holding it, Pressing tube to come out of the toothpaste.
Let’s discuss the different kinds of contact forces.
1.7 i) Muscular force: the force which results due to the action of muscles is called muscular force. both human beings and animals exert muscular force to do some work, e.g. We use muscular force in our daily life activities like walking, jumping, climbing, stretching etc.
1.8 ii) frictional force:- a force that arises due to contact between two surface and oppose the motion of an object is called frictional force for example, if we slide or try to
REMANING CONENT AVAILABLE IN THE WEBSITE IN THE PREVIOUS POST.
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