ICSE Class 9 Physics Chapter 03 Laws of Motion

Laws Of Motion

Syllabus

(i) Contact and non-contact forces; C.G.S. and S.I. units.

Scope - Examples of contact forces (frictional force, normal reaction force, tension force as applied through strings and force exerted during collision) and non-contact forces (gravitational, electric and magnetic). General properties of non-contact forces. C.G.S. and S.I. units of force and their relation. Gravitational unit.

(ii) Newton's first law of motion (qualitative discussion), introduction of the idea of inertia, mass and force.

Scope - Newton's first law; qualitative discussion, definitions of inertia and force from first law; examples of inertia as illustration of first law (Inertial mass not included).

(iii) Newton's second law of motion (including F = ma); weight and mass.

Scope - Detailed study of the second law. Linear momentum p = mv; change in momentum Δp = Δ(mv) = mΔv for mass remaining constant, rate of change of momentum Δp/Δt = mΔ/Δt = ma {or \[\frac{p_2 - p_1}{t} = \frac{mv - mu}{t} = \frac{m(v-u)}{t} = ma\]}. Simple numerical problems combining F = Δp/Δt = ma and equations of motion. Units of force-only CGS and SI.

(iv) Newton's third law of motion (qualitative discussion only), simple examples.

Scope - Statement with qualitative discussion, examples of action-reaction pairs (F_BA and F_AB), action and reaction always act on different bodies.

(v) Gravitation

Scope - Universal law of gravitation (statement and equation) and its importance. Gravity, acceleration due to gravity, weight and mass, weight as force of gravity, comparison of mass and weight; gravitational units of force, simple numerical problems (problems on variation of gravity excluded).

A - Contact And Non-Contact Forces

3.1 Force

We are familiar that a force when applied on a body can produce the following two main effects:

(1) It can change the state of rest or of motion of the body i.e., it can produce motion in the body.

Examples: The push exerted by a broom moves the trash. A ball lying on the ground moves when it is kicked. The pull exerted by a horse moves a cart. The pull exerted by a steam engine moves a train. The force due to gravity (or the earth's pull) makes an apple fall. A fielder on the ground stops a moving ball by applying force with his hands.

When force is applied on the pedal by a cyclist, the speed of the cycle increases. A freely falling object continuously gains speed due to the earth's pull acting along its direction of motion. The speed of a moving vehicle is slowed down by applying the brakes. A stone tied to one end of a string, whirling at a constant speed in a horizontal circle, changes its direction of motion continuously due to the force of tension in the string (which acts normal to the direction of motion of stone). In cricket, tennis and badminton, the direction of motion and the speed of the ball (or cock) is changed by hitting it in the direction other than its direction of motion. A player applies force with a hockey stick to change the speed and direction of motion of the ball.

(2) It can change the size or shape of the body i.e., it can change the dimensions of the body.

Examples: By loading a spring hanging from a rigid support, the length of the spring increases. By hammering a small piece of silver sheet, a big thin foil is made (here the force increases the surface area). The steam pushing out from a pressure cooker occupies a large volume in the atmosphere. On pressing a piece of rubber, its shape changes, if a fold is made, we see that the string moves slightly upwards with a jerk due to tension in the string acting upwards and then falls downwards due to its own weight.

Note: A force when applied on a rigid object does not change the inter-spacing between its constituent particles and therefore it does not change the dimensions of the object, but causes only the motion in it. On the other hand, a force when applied on a non-rigid object, changes the inter-spacing between its constituent particles and therefore causes a change in its dimensions and can also produce motion in it. Thus

A force is that physical cause which changes (or tends to change) either the size or the shape or the state of rest or of motion of the body.

Kinds of forces: From the point of view of application, the forces are classified in two categories, namely, (i) the contact forces and (ii) the non-contact forces.

3.2 Contact Forces

The forces which are applied on bodies by making a physical contact, are called the contact forces.

These forces are produced and experienced when a body comes in contact with another body.

Examples: (1) The force of friction (frictional force), (2) normal reaction force, (3) Force of tension exerted by a string, (4) Force exerted by a spring, (5) Force exerted on two bodies during collision, etc.

(1) Frictional force: When a body slides (or rolls) over a rough surface, a force starts acting on the body in a direction opposite to the motion of the body, along the surface in contact. This is called the frictional force or the force of friction. In Fig. 3.1, when a book placed on the table top is pushed to the right, a frictional force acts on the book towards the left. This force resists the motion of the book on the table top.

(2) Normal reaction force: When a body is placed on a surface, the body exerts a force downwards, equal to its weight, on the surface, but the body does not move (or fall) because the surface exerts an equal and opposite force on the body normal to the surface which is called the normal reaction force. For example, in Fig. 3.2, when you hold a block on your palm, the block exerts a force due to its weight downwards on your palm and you have to exert a reaction force upwards on the block normal to the palm to keep the block in position. Similarly in Fig. 3.1, the book exerts a force (= weight) on the table top downwards and the table top exerts an equal reaction force upwards normal to the top of the table.

(3) Tension force as applied through strings: When a body is suspended by a string attached to a rigid support, the body, due to its weight W, pulls the string in its stretched condition pulls the body upwards by a force which balances the weight of the body. This force developed in the string is called tension (or the force of tension) T. Fig. 3.3 shows the two forces which are equal and opposite in

(4) Force exerted by a spring: Consider a spring with its one end kept fixed [Fig. 3.4(a)]. If its other end is either stretched [Fig. 3.4(b)] or compressed [Fig. 3.4(c)], the spring exerts a force F opposite to the direction of displacement of its free end, the magnitude of this force is directly proportional to the magnitude of displacement i.e., its elongation or compression. This force is called restoring force. A spring-balance works on this principle.

Similarly a horizontal spring with two objects A and B attached at its two ends in its normal form, does not exert any force on the object attached at its ends [Fig. 3.5(a)]. But if the spring is compressed, it pushes away each object with a restoring force F at its ends [Fig. 3.5(b)], while if the spring is stretched, it pulls in each object with a restoring force F at its ends [Fig. 3.5(c)]. In each case, the spring has a tendency to come back to its original form.

(5) Force exerted during collision: When two bodies collide, they push each other. As a result, equal and opposite forces act on each body. These forces are the force of action and force of reaction. In Fig. 3.6, a body B while in motion, collides with a moving body A and exerts a force F_AB on the body A which is called the force of action. At the same instant, the body A also exerts an equal and opposite force of reaction F_BA on the body B. As a result of these forces, the two bodies move apart after the collision.

3.3 Non-Contact Forces

The forces experienced by bodies even without being physically touched, are called the non-contact forces or the forces at a distance.

Examples: (1) Gravitational force, (2) Electrostatic force and (3) Magnetic force.

(1) Gravitational force: In universe, each particle attracts the other particle due to its mass. This force of attraction between them is called the gravitational force. The earth also, because of its mass, attracts all other masses around it. The force on a body due to earth's attraction is called the force of gravity or the weight of the body. It causes motion in the body towards the earth (i.e., downwards) if the body is free to move. Thus, it is the force due to gravity that makes a body fall, when released from a height. The body also attracts the earth by an equal force, but no motion is caused in the earth because of its huge mass.

Examples: (i) A ball placed on a table starts rolling down when the table is tilted. (ii) If a body is thrown up in air, it goes up, reaches to a height and then returns to the ground. (iii) A coin falls down when it is released at a height.

(2) Electrostatic force: Two like charges repel, while two unlike charges attract each other. The force between the charges is called the electrostatic force. This force acts between the charged objects even when they are separated.

Example: When a comb is rubbed on dry hairs, it gets charged. If this comb is brought near the small bits of paper, opposite charges are induced on the bits of paper and they begin to move towards the comb. The motion of paper bits is due to the electrostatic force of attraction exerted between the unlike charges on the comb and the paper bits.

(3) Magnetic force: Two like magnetic poles repel, while two unlike magnetic poles attract each other. The force between the magnetic poles is called the magnetic force. This force acts even when the magnetic poles are at a separation.

Example: When a pole of a magnet is brought near a small iron nail (without touching it), an opposite polarity is induced on the nail and it moves towards the magnet. The motion of nail is due to the magnetic force of attraction exerted between the unlike poles on the magnet and the nail.

General character of non-contact forces -

(i) The gravitational force is always of attractive nature, while the electrostatic force and the magnetic force can be either attractive or repulsive.

(ii) The magnitude of non-contact forces on the two bodies depends on the distance of separation between them. It decreases with the increase in separation and increases as the separation decreases. It varies inversely as the square of distance of separation i.e., on doubling the separation, the force becomes one-fourth.

Teacher's Note

Understanding contact and non-contact forces helps us explain everyday phenomena like why objects fall, how magnets work, and why friction affects the motion of vehicles on roads.

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