1. Force and its types:

(i) A force can be described as a push or a pull on an object.

(ii) The Earth's gravitational field pulls all objects downwards. This force is called weight.

(iii) The Earth's surface exerts a force which counteracts an object's weight and keeps it from falling further downwards. This is reaction force.

(iii) When two objects slide over each other, friction acts against their motion. This is known as frictional force.

(iv) The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. This force comes into play even when the bodies are not in contact.

(v) The force of attraction between two opposite magnetic poles or the repulsion between two like magnetic poles is known as magnetic force.

(vi) Tension is a force that occurs when something like an elastic band or a rope is stretched. The force of tension pulls objects. The opposite is compression where an object exerts a force by being squashed, such as a spring.

(v) The upward force applied by the fluid on the object or the body when an object is put in or submerged in the fluid is known as upthrust.

2. Measurement of forces:

(i) The unit of force is a newton which is abbreviated to $N$ . As a result, a device which measures force is sometimes called a newton-meter.

(ii) A spring balance and a set of scales both measure the weight of an object. The scales convert their result into a mass according to the equation $F = m g .$

3. Representation of Forces:

(i) The forces are vector quantities which are represented by arrows. The direction of the force is represented by the direction of the arrow and the magnitude of the force is represented by the length of the arrow.

(ii) A free body diagram is a diagram of a chosen system in which we represent all the forces acting on it and thus calculate the net force. These diagrams are used to show the direction and magnitude of all the forces acting upon an object.

4. Work:

(i) In physics, work is the process of transferring energy to an object. Work is said to be done by a force when it causes displacement in a body in its own direction. It is a scalar quantity.

(ii) SI unit of work is joule $(J)$ . It is defined as the amount of work done when a force of $1 N$ causes a displacement of $1 m$ in its own direction.

(iii) The conditions to be satisfied for work to be done are

(a) a force should act on an object, and

(b) the object must be displaced.

(iv) The work done is zero when the displacement is perpendicular to the direction of the force.

5. Simple Machines:

(i) A first-class lever has the load on one side of the pivot and the effort force on the other side. The mechanical advantage of a first-class Iever will be directly correlated with the distance between the effort and the pivot. When the effort force is further away from the pivot, the lever can support a larger load, and so the mechanical advantage> 1; otherwise, when the effort force is close to the pivot, the mechanical advantage is <1.

(ii) A second-class lever has the effort and the load on the same side. In a second-class lever, the effort force is further from the pivot than the load, so the mechanical advantage > 1

(iii) The effort and the load in a third-class lever are on the same side of the pivot. The difference is that the effort is closer to the pivot than the load, and so the mechanical advantage of this Iever is < 1.

6. Newton's Laws of motion:

(i) Sir Isaac Newton ( $1642 - 1727$ ) made a systematic study of motion and proposed three laws of motion.

(ii) Newton’s first law of motion states that every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by some external force to change that state.

(iii) Momentum is the quantity of motion possessed by the body. It is the product of its mass ( $m$ ) and velocity ( $v$ ).

$p = m v$

Its SI unit is $kg m / s$ .

(iv) Newton’s second law of motion states that the rate of change of momentum of a body is directly proportional to the external force applied on the body and the change takes place in the direction of the applied force.

$F = \frac{Δ p}{Δ t} = m a$

(v) The force which acts on a body for a small duration of time, but produces a significant change in its momentum, is called impulse of force.

$Impulse of force = Force \times short interval of time = change in momentum$

$Impulse of force = F \times t = m (v - u)$

Its unit is newton second or $kg m / s$ .

(vi) Newton’s third law of motion states that for every action, there is always an equal and opposite reaction.

$F_{A B} = - F_{B A}$

7. Electric current and potential:

(i) Electric current is defined as the rate of flow of electric charge through any cross-section of a conductor in unit time. $Electric current (I) = \frac{Charge (q)}{Time (t)}$ . The SI unit of electric current is ampere $(A)$ .

(ii) Electric potential is defined as the amount of work done when a unit positive charge is moved from infinity to point. $Electric potential (V) = \frac{Work done (W)}{Charge moved (q)}$ . The SI unit of electric potential is volt $(V)$ .

(iii) Electric potential difference is defined as the work done per unit charge in moving a unit positive charge from one point to another point.

(iv) The work done by the cell in the moving unit positive charge through the whole circuit is called electromotive force.

8. Resistance and resistivity:

(i) At a given temperature resistance of a conductor depends on its (i) length $l$ , (ii) cross-section area $A$ and (iii) nature of the material of the conductor. It is found that $R \propto l$ and $R \propto \frac{1}{A}$ , Mathematically, $R = ρ \frac{l}{A}$ . Where, $ρ$ is the constant of proportionality called resistivity or specific resistance of the conductor.

(ii) Resistivity of a conductor is defined as the resistance of a conductor of unit length and unit area of cross-section.

(iii) The SI unit of resistivity is ohm-metre $(Ω - m)$ .

9. Combination of resistances:

(i) If $R_{1}, R_{2}$ and $R_{3}$ be the individual resistors joined in series, then the equivalent resistor $R_{S}$ is given by $R_{S} = R_{1} + R_{2} + R_{3}$ .

(ii) If $R_{1}, R_{2}$ and $R_{3}$ be the individual resistors joined in parallel, then the equivalent resistor $R_{P}$ is given by $\frac{1}{R_{P}} = \frac{1}{R_{1}} + \frac{1}{R_{2}} + \frac{1}{R_{3}}$ .

A screw is a long nail with a spiral (lever/screw) type of simple machine.

Important Questions on Functions

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