Forces

Force is a basic concept in physics that describes any interaction that, when unopposed, will change the motion of an object. This encompasses things like causing objects to accelerate, decelerate, stay put, or alter course. In other words, force is basically a push or pull exerted on an object as a result of an interaction with another object.

The International System of Units (SI) unit for force is the Newton (N). The Newton was named in recognition of Sir Isaac Newton and his application of force in his laws of motion. The first of Newton's laws of motion states that a body will remain in its initial state at rest or uniform motion unless and until it is acted upon by a net resultant external force.

This concept of force comes under the chapter Laws of Motion which is an important chapter in Class 11 physics mechanics. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, VITEEE and more. Over the last ten years of the JEE Main exam (from 2013 to 2023), more than two questions have been asked on this concept. And for NEET one questions were asked from this concept.

Definition of Force

Force is defined as an effect which causes a body to change its state.

• Force on $1 \mathrm{~kg}$ mass in the presence of gravity ( $g=9.8 \mathrm{~m} / \mathrm{s}^2$ ) is $1 \mathrm{~kg}-\mathrm{f}=9.8 \mathrm{~N}$

Unit of Force

1. In SI unit- Newton(N)
2. In CGS- 1 dyne (1 newton = 100000 dyne)

• 1 Newton(N) is the force needed to accelerate an object with a mass of 1 kg at a rate of 1 m/s² (1 N = 1 kg . m/s²)

Types of Forces

After reading about force, now coming to the types of force, mainly three types.

a) Contact forces-

• Contact forces are due to direct physical contact between objects.

Types of contact forces-

• Tension
• Normal reaction
• Spring force
• Friction

b) Non-contact forces-

• These forces act without the necessity of physical contact between objects.

• They depend on the presence of a “field” in the region of space surrounding the body under consideration.

Types of non-contact forces-

• Gravitational force
• Electrostatic force
• Magnetic force

c) Weak forces-

• Vanderwaal force

d) Nuclear forces

$ \begin{aligned}
& F_{\text {nuclear }}>F_{\text {electro }}>F_{\text {gravitation }} \\
& F_e / F_g=10^{43} \\
& \text { Therefore, } F_e>>F_g
\end{aligned}$

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Solved Example Based on Force

Example 1: What is the push or pull on an object that causes it to accelerate called?

1) Mass

2) Force

3) Density

4) Speed

Solution:

The push or pull on an object that causes it to accelerate is called a force.

Hence, the answer is option (2).

Example 2: What is the unit of measure for force?

1) Amp

2) Seconds

3) Newton

4) Joule

Solution

One newton is defined as the force that is required to accelerate Unit of force in an SI system is Newton (N)a mass of $1 \mathrm{~kg}$ by $1 \mathrm{~m} / \mathrm{s}^2$ in the direction of applied force.

So, the answer is Newton.

Hence, the answer is option (3).

Example 3: A force $\vec{F}=(6 \hat{i}-8 \hat{j}+10 \hat{k}) N$ produces acceleration of $1 \mathrm{~m} / \mathrm{s}^2$ in a body. Calculate the mass of the body.

1) $10 \mathrm{~kg}$
2) $10 \sqrt{2} \mathrm{~kg}$
3) $5 \mathrm{~kg}$
4) $15 \mathrm{~kg}$

Solution:

Given:
Force, $(\vec{f})=(6 \hat{i}-8 \hat{j}+10 \hat{k}) N$
Acceleration, $(a)=1 \mathrm{~m} / \mathrm{s}^2$
Magnitude of the force is given as:
$
|\vec{F}|=\sqrt{(6)^2+(8)^2+(10)^2}=\sqrt{200}=10 \sqrt{2} N
$

As we know,
Force $=$ Mass $\times$ acceleration

$M=\frac{|\vec{F}|}{g}=\frac{10 \sqrt{2}}{1}=10 \sqrt{2} \mathrm{~kg}$

Hence, the answer is the option (2).

Example 4: A balloon has mass $10 \mathrm{~g}$ in the air. The air escapes from the balloon at a uniform rate with velocity $4.5 \mathrm{~cm} / \mathrm{s}$. If the balloon shrinks in 5 seconds completely. Then, the average force acting on that balloon will be (in dyne).

1)

2)

3)

4)

Solution:

$\begin{aligned}
& \mathrm{m}=\log =10^{-2} \mathrm{~kg} \\
& \mathrm{~F}_{\text {avg }}=\left(\frac{\Delta \mathrm{m}}{\Delta \mathrm{t}}\right) \mathrm{V} \\
& =\frac{10^{-2}}{5} \times 4.5 \times 10^{-2} \\
& =\frac{9}{10} \times 10^{-4} \\
& =9 \times 10^{-5} \mathrm{~N} \\
& \mathrm{~F}_{\text {avg }}=9 \text { dyne } \\
&
\end{aligned}$
Hence, the answer is the option (2).

Summary

In summary, force is one of the most central concepts in physics to drive the way objects influence one another and the changes in their motion. More precisely, it is defined as the interaction that can cause objects to accelerate, decelerate, change direction, or remain in place. Overall, force is the key concept underlying the physics of the physical world. Specifically, it is a vector quantity, meaning it has both magnitude and direction, which is important because it is critical for the accurate description of different interaction effects.