The role of Gravity and Gravitational Force

Introduction:

The gravitational pull binds all celestial objects together and causes them to move along a fixed trajectory.

Activity:

When a small object is tied and whirled around the object moves in a circular path and changes its direction of motion at every point. We see that the force which is causing it to move in a circular path is always directed toward the center of the path.

The force which is pulling the object toward the center of the circular path is known as the centripetal force. The tension in the string will provide the necessary centripetal force. When the string breaks, the object moves along the straight line tangentially to the circular path.

Similarly, Newton concluded that the Moon revolves around the earth in a circular orbit, and at each point of its orbit, the moon is attracted by the Earth. Instead of moving in a straight line.

The necessary centripetal force is provided by the gravitational force exerted by the Earth.

What Keeps Objects in Orbit?

An orbit is a path one object takes around another object—planets orbit around the Sun, and moons orbit around their planets. Planets are held in their orbits by the force of gravity between each planet and the Sun.

If gravity were the only force acting on a planet, the planet would be pulled into the Sun.

What prevents this from happening?

The Moon orbits the Earth, while Earth and the other planets revolve around the Sun.

Why does not the Sun’s gravity drag the Earth? (Or the Moon into Earth)?

Activity:

To answer the query, imagine kicking a ball off the ground at an angle.

When you kick it slowly, it curves and returns to the ground.

The ball travels longer before reaching the ground when you kick it faster. The route of the ball would follow the curvature of Earth if you could kick it fast enough. Instead of falling back to Earth, the ball would launch into orbit. At 8,000 m/s, an object will orbit the Earth.

All objects have a property called inertia, which is the tendency of a moving object to keep moving in a straight line.

Inertia and Gravitational Force:

The interplay of inertial (an object’s forward motion in space) and gravitational forces generates an orbit. A planet would fly off into space in a straight path without inertia, which causes the planet to move in a direction at a right angle to the pull of gravity.

The planet’s orbit is a result of the interaction between the Sun’s gravitational pull and the planet’s inertia.

As a space vehicle orbits Earth, members of the crew float in the cabin, and they feel weightlessness.

Why aren’t the crew or other objects inside the space vehicle being pulled toward Earth by gravity?

Like all objects in orbit, the pull of gravity on the space vehicle is balanced by its forward motion. If the space vehicle speeds up, its forward motion would overcome the pull of Earth’s gravity. The vehicle would pull out of that orbit and move further away from Earth and if it is made slow the space vehicle will fall back on the earth. At a particular speed, it starts revolving around the Earth.

Planets in the solar system act very much like space vehicles. As the planets orbit the Sun, they tend to fall toward it. But at the same time, their forward motion tends to make them move away from it. The effect of these two motions makes the planets move in a nearly circular orbit called an ellipse.

When the Earth is closest to the Sun, it is 147,098,074 km away. When Earth is furthest from the Sun, it is 152,097,701 km away. This 5-million-kilometer difference shows that Earth’s orbit is an ellipse and not a perfect circle.

Question:

What would happen if the gravitational force between the moon and the Earth disappears someday?

Answer:

The moon flies away in the tangential direction in a straight line undergoing a uniform motion.

This is like the case of breakage of the string tied to the stone wherein the stone flies away in a straight line tangentially in a uniform motion.