The chapter Laws of Motion carries an average weightage of \(5\)–\(8\) marks, making it one of the most important topics in Physics.
A strong understanding of this chapter helps students clearly explain how objects move, how forces act, and how Newton’s laws apply in everyday life. This topic also forms the foundation for numericals involving momentum and the relationship between mass and weight.
To score well in this unit, it is essential to understand all three of Newton’s Laws, solve problems based on momentum, and be able to differentiate concepts such as mass vs. weight and balanced vs. unbalanced forces.
Below is the question pattern expected from this chapter:
Question Distribution:
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Part I: (\(1\) mark) – One question
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Part II: (\(2\) marks) – One question
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Part III or IV: (\(4\) or \(7\) marks) – One detailed question based on Newton’s laws or momentum
Learning Outcomes:
- Newton’s First Law (Law of Inertia): Understand the meaning of inertia and how it relates to the mass of a body.
- Newton’s Second Law: Learn how force, mass, and acceleration are related. Solve numerical problems involving momentum and impulse.
- Newton’s Third Law: Apply “action–reaction” principles to daily situations like walking, swimming, rocket propulsion, and jumping.
- Momentum and impulse: Define momentum and understand that the rate of change of momentum produces force.
- Mass and weight: Clearly differentiate between mass (constant) and weight (varies with gravity).
Exam tips:
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Learn Newton’s laws
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Practice momentum sums
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Remember mass–weight
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Revise key examples
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Check unit conversions
Statics - Branch of mechanics that studies the bodies at static equilibrium under the action of forces.
Dynamics - Branch of mechanics that studies the bodies at motion under the action of forces.
Dynamics is further classified as kinematics, kinetics.
Galileo Galilee proposed the following concepts about forces, motion and inertia of bodies:
The essential property of a body to resist any change in its state of rest or the state of uniform motion, unless influenced by an external unbalanced force, is known as ‘Inertia’.
Types of inertia:
- Inertia of rest
- Inertia of motion
- Inertia of direction
The multiplication of mass and velocity of a moving body provides the magnitude of linear momentum. Linear momentum is a vector quantity. It acts in the direction of the velocity of the body.
\(p\ =\ mv\), The unit of momentum in the S.I. system is \(kg\ ms^{-1}\)
Newton’s First Law:
It states that every body remains in its state of rest or the state of uniform motion along a straight line unless some external force acts upon it.
| Action of forces | Resultant force |
| Parallel forces are acting in the same direction |
\(F_{net}\ =\ F_1\ +\ F_2\) |
| Parallel unequal forces are acting in opposite directions |
\(F_{net}\ =\ F_1\ −\ F_2\) \(if\ F_1\ >\ F_2\)
\(F_{net}\ =\ F_2\ −\ F_1\) \(if\ F_2\ >\ F_1\) \(F_{net}\) is directed along the greater force
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Parallel equal forces are acting in opposite
directions in the same line of action \(F_{1}=F_{2}\) |
\(F_{net}\ =\ F_1\ −\ F_2\) \(F_1\ =\ F_2\), \(F_{net}\ =\ 0\)F net =F 1 −F 2 (F 1 =F 2 ) F net |
The axis of the fixed edge about which the door is rotated is called the ‘Axis of rotation’.
Attach one end of a rod to the floor or wall and tangentially apply a force at the other end. The rod will be turned about the fixed point is known as the ‘Point of rotation’.
Moment of the force:
The turning or rotating effect of a force about a fixed axis as the moment of the force about that point or torque (τ).
It is measured by the multiplication of the force (F) and the perpendicular distance (d) between the fixed axis and the line of action of the force. Torque is a vector quantity. The unit is \(Nm\).
\(τ\ =\ F \times d\)
Couple: Two equal and unlike parallel forces acting on two different points simultaneously form a couple.
Principle of Moments:
The principle of moments can be expressed as follows:
\(Moment\ in\ clockwise\ direction\ =\ Moment\ in\ anticlockwise\ direction\)
\(Moment\ =\ Force\ \times\ distance\)
\(F_2\ \times\ d_2\ =\ F_1\ \times\ d_1\)
Force:
Force is a push or pull that leads to change in the state of the object. The SI unit of force is newton (N), and in the C.G.S. system, its unit is dyne.
\(1\ N\ =\ 10^{5} dyne\)
\(1 kgf\ =\ 1 kg\ \times\ 9.8\ ms^{−2}\ =\ 9.8\ N\)
\(1\ gf\ =\ 1 g\ \times\ 980\ cms^{−2}\ =\ 980 dyne\)
\(1\ kgf\ =\ 1000 g\ \times\ 980\ cms^{−2}\ =\ 980×10^{3}\ dyne\)
An object is considered to be in a state of rest when there is no change in its position with respect to time.
An object is considered to be in the state of motion when its position changes with respect to time. From small atoms and molecules to larger stars and galaxies, everything is in motion.
Newton’s second law of motion:
According to Newton’s second law of motion, “the force acting on an object is directly proportional to the rate of change of linear momentum of the object, and the change in momentum takes place in the direction of the force”.
\(F\ \propto\ \frac{m\ \times\ (v−u)}{t}\)
\(F\ =\ \frac{k\ m\ \times\ (v−u)}{t}\)
Where \(k\) is the constant of proportionality.
\(F\ =\ ma\),
\(a\ =\ \frac{v\ -\ u}{t}\)
A large force that acts on a body for a very short period of time is called an 'Impulsive force'.The SI unit of impulse is the newton−second (Ns) and it is a vector quantity.
\(J\ =\ \Delta p\)
\(Impulse\ =\ Force\ \times\ time\)
Newton's third law:
It states that 'for every action, there is an equal and opposite reaction. They always act on two different bodies'.
Law of conservation of momentum:
In the absence of unbalanced external force, the sum of momenta of the two objects before and after the collision will be equal. Or, the total momentum of the two objects is unchanged or conserved by the collision.
\(m_A\ u_A\ +\ m_B\ u_B\ =\ m_A\ v_A\ +\ m_B\ v_B\)
Rocket propulsion:
An equal and opposite reaction force is generated in the combustion chamber to balance the momentum, making the rocket project forward.
Newton’s universal law of gravitation:
This law states that every particle of matter in this universe attracts every other particle with force.
\(F\ =\ \frac{GmM}{r^2}\)
The SI unit is \(Nm^{2}kg{-2}\), the value of G is \(6.674\ \times\ Nm^{2}kg{-2}\)
Acceleration due to gravity:
\(g\ =\ \frac{GM}{r^{2}}\)
Acceleration due to gravity is inversely proportional to radius.
Mass:
Mass is the fundamental property of a body. The mass of a body is defined as the quantity of matter contained in the body.
Weight:
The gravitational force exerted on a body due to the earth's gravity alone is called the weight of a body. Weight is a vector quantity.
The weight of a body changes from one place to another on the earth's surface because it depends on the acceleration due to the gravity of the Earth (g). The weight of a body is higher at the poles than at the equatorial region.
Apparent weight:
The weight you feel to possess during up and down motion is not the same as your actual weight.
The weight that the body acquires due to the action of gravity and other external forces acting on the body is called apparent weight.