Question

Gravitation or gravity is a natural phenomenon by which all the things having mass are brought toward or gravitate toward one another, including stars and planets. Which of the following is the best example of gravitation or gravity?
A) A car hits a tree & motion stops.
B) A breeze blows and the sailboat moves.
C) A person drops a ball and it falls on the ground.
D) A book is pushed & it moves across the table.

Answer 1

To solve the question, we must know about the gravitational force. Also, find out how it acts on an object and in which direction. We have to find out what type of forces are acting on the objects in the options given. After finding the force in each case, we can reach the answer.

Complete answer:
When a car is moving, it attains a certain velocity. Let this velocity be $'v'$. Here, the car collides with a tree and after the collision, the car comes to rest at velocity, $v = 0$. We are considering here that the tree is massive and static as compared to the car. The acceleration of the car is given by:
$a = \dfrac{{v - 0}}{t}......(1)$
Where,
$a$ is the acceleration of the car.
$v$ is the initial velocity of the car.
$t$ is the time.
Using Newton’s second law of motion where force equals mass times the acceleration, $'a'$ we get,
$F = ma......(2)$
Where,
$F$ is the force.
$m$ is the mass of the car.
$a$ is the acceleration of the car.
Putting the value of acceleration in equation (2) we get,
$F = \dfrac{{mv}}{t}$
So, the car exerts this force on the tree but as the tree is massive and static, it exerts an equal and opposite force on the car. This situation uses Newton’s third law of motion.
Similarly, when the breeze blows, it has its kinetic energy. The breeze acts as an external force which through its interaction moves the sailboat which we consider was at rest before the breeze blows. Here, Newton’s first law of motion is used.
Now, for the third situation, let the book is in rest initially. So, the forces acting on it were balanced. But when we push the book, we provide an external force on it. This external force acts as an unbalanced force and as a result, the book moves. So, here Newton’s first law of motion is used. Also, as the book is in contact with the surface of the table, on moving the book. Force of friction also acts between them.
But, when a person drops a ball, the ball falls to the ground due to the gravitational force of gravity. As it falls freely, only gravity acts as an external force and it attains an acceleration due to gravity. According to the universal law of gravitation, every particle in the universe attracts every other particle with a force that is directly proportional to the product of the masses and inversely proportional to the square of the distance between them. Mathematically, it can be represented as:
$F \propto \dfrac{{{m_1} \times {m_2}}}{{{r^2}}}$
$\Rightarrow F = G.\dfrac{{{m_1} \times {m_2}}}{{{r^2}}}$
Where $F$ = Gravitational force in Newton
${m_1}$ and ${m_2}$ = masses of the bodies in kilograms
$r$ = distance between the masses in meters
$G$ = Gravitational constant
Therefore, both the ball and the earth attract each other, but as the mass of the earth is so huge the mass of the ball is negligible compared to it. As a result, we only observe the ball move downwards.
Hence, option (C) is correct.

Note:
The force of gravity always acts vertically downwards. So, whenever an object falls freely in the vertical direction, the gravitational force acts on it. If we look at the other forces mentioned in the options and find their direction, we can easily conclude that they provide motion only in the horizontal direction. Therefore, those motions cannot be caused by gravity.