JEE Important Chapter - Laws of Motion

The chapter on laws of motion starts with the basic revision of all the concepts we studied in our lower grades to build the connection between the advanced parts of the laws of motion. In laws of motion, we have discussed Newton's laws of motion, Inertia, derivation of the second law of motion, conservation law, etc…. 

While deriving the second law of motion we study the relation between the force and momentum, $\vec {F}=\dfrac {d \vec {P}}{dt}$. This relation further leads us to study the conservation of linear momentum which helps us in solving most of the problems related to motion. In the chapter laws of motion, we also studied the force and types of forces along with examples. 

We got introduced to a few most important concepts: friction and laws of friction along with the concept of tension. We studied how to determine the forces acting on the body with the help of free body diagrams.

In this chapter students also get to learn about the circular motion where we study an object in circular motion experiencing the different forces (i.e., Centrifugal and Centripetal force). Students will also study the “frames of reference”, a very interesting concept of classical mechanics.

Now, let us move on to the important concepts and formulae related to JEE and JEE main exams along with a few solved examples.

JEE Main Physics Chapter-wise Solutions 2022-23 

Important Topics of Laws of Motion

• Newton’s Law of Motion Formula

• Linear Momentum

• Conservation of Linear Momentum

• Types of Forces

• Friction and types

• Laws of Friction

• Resultant force

• Tension

• Simple pulley system

Important Concepts of Laws of Motion

List of Important Formulae

Solved Examples 

1. An object of mass 0.2 kg is thrown vertically upwards. Calculate the magnitude of the net force acting on the body and direction, 

1. During its upward motion

2. During its downward motion

3. At the highest point at which the object is considered to be at rest. Will your answers change if the object was thrown at an angle of 45° in the horizontal direction and ignoring the air resistance?

Sol:

Given, 

Mass of the object=m=0.2 kg

Assume value of acceleration due to gravity=g=10 m/s2

1. During upward direction

When the object is thrown upward, the acceleration due to gravity g is acting downward, therefore, the net force is acting in a downward direction.

${F_{net}}=m g= 0.2\times 10= 2N$

2. During downward motion also the net force and direction remain the same i.e, F= 2N and the direction of force acting is downwards

3. The object will not be at rest when it reaches the highest point but it will have the horizontal component of velocity. Therefore, the direction and magnitude of the net force on the object will not change even if it is thrown at 45°. 

Key point: Here only acceleration that acts will be acceleration due to gravity since the object was subjected to free fall.

2. A body of mass 10 kg is acted upon by two perpendicular forces 10 N and 12 N. Calculate the magnitude and direction of the acceleration of the body.

Sol:

Given,

The mass of the object=m=10 kg

Forces acting on the body F1= 10 N and F2=12 N

Now we are asked to determine the magnitude and direction of the body. As a first step, we must calculate the resultant force acting on the body.

$\vec {F_{net}}=\sqrt{{{F_1}^2}+{{F_2}^2}+2 {F_1}{F_2} \cos \theta (\theta=90^o)}$

$\vec {F_{net}}=\sqrt{{10^2}+{12^2}}=15.62\simeq17$

The acceleration of the body,

$\vec {a}=\dfrac {\vec {F_{net}}}{m}=\dfrac {17}{10}=1.7 m/{s^2}$ in the same direction as the resultant force.

The direction of the acceleration is

$\tan \beta =\dfrac {10}{12}=0.8333$

$\beta =\tan^{-1}(0.833)= 39.7^o $ With 12 N force.

Key point: Formula for resultant force when two forces acting perpendicular to each other is $\vec {F_{net}}=\sqrt{{{F_1}^2}+{{F_2}^2}}$.

Previous Year Questions from JEE Paper

1. A block of mass slides along a floor while a force of magnitude F is applied to it at an angle $\theta$ as shown in the figure. The coefficient of kinetic friction is ${\mu_k}$. Then, the block's acceleration 'a' is given by : (g is the acceleration due to gravity) (March -2021)

Sol:

Given, 

A block of mass ‘m’ is moving with the force F. We are asked to determine the acceleration of the block. This can be easily solved with the help of FBD. Let us draw the FBD for the object.

From FBD we write:

$N+F \sin \theta=mg$

$\Rightarrow N=mg-F \sin \theta$

$F\cos \theta-{f_k}=ma$

$\Rightarrow F\cos \theta-{\mu_k}N=ma$

$\Rightarrow F\cos \theta-{\mu_k}(mg-F \sin \theta)=ma$

$\Rightarrow a=\dfrac {F}{m} \cos \theta -{\mu_k}(g-\dfrac {F}{m} \sin \theta)$

Trick: We can solve these problems just with the help of FBD, FBD should have all the forces acting on the body.

2. A block of mass 5 kg is (i) pushed in case (A) and (ii) pulled in case (B), by force F=20 N, making an angle of 300 with the horizontal, as shown in the figures. The coefficient of friction between the floor is $\mu = 0.2$. The difference between the accelerations of the block in case (B) and case (A) will be: (JEE-2019)

Sol: 

Given,

Mass of the block=m=5 kg

The magnitude of the force acting on the block=F=20 N

Case A:

In case A, the block is pushed over the surface.

Free body diagram of the block is

From FBD we get,

${N_1}=mg+F \cos {30^o}$

${N_1}=50+ 20 (\dfrac {1}{2})=60 N$

Then the net acceleration in case A

$m {a_1}= F \cos {30^o}-{f}=20 \dfrac {\sqrt {3}}{2}-(0.2\times 60)=10 \sqrt{3}-12$

${a_1}=\dfrac {10 \sqrt{3}-12}{5}$

Case B:

From FBD, net force

Here,

$N+F \sin {30^o}=mg$

$N=mg-F \sin {30^o}$

${F_{net}}=F \cos {30^o}-{f}=10 \sqrt {3}-\mu N$

${F_{net}}=10 \sqrt {3}-\mu (mg-F \sin (30^o))=10\sqrt{3}-8$

$m{a_2}=10\sqrt{3}-8$

${a_2}=\dfrac {10\sqrt{3}-8}{5}$

Therefore,

${a_2}-{a_1}=0.8 m/{s^2}$

Trick: Resolving the given vector into components is important in order to solve the above example. 

Practice Questions

1. A block of mass 2kg rests on a rough inclined plane making an angle of 30° with the horizontal. The coefficient of static friction between the block and the plane is 0.7. The frictional force on the block is (Ans: 9.8 N)

2. A car is moving on a circular horizontal track of a radius of 10m with a constant speed of 10 m/s. A plumb bob is suspended from the roof of the car by a light rigid rod. The angle made by the rod with the track is (g = 10 m/s2)  (Ans: 450)

Conclusion

In this article we have provided important information regarding the chapter laws of motion such as important concepts, formulae, etc.. Students should work on more solved examples and newton's laws examples for securing good grades in the JEE exams. 

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