1.5 More examples of force interactions between objects involving gravity

A 'force' interaction is a pair of equal and opposite forces acting on two different objects e.g.

If you push down on the floor, the floor pushes back up on you.

The forces of you and the floor are equal and both objects experience a force.

This is an example of Newton's 3rd Law of motion which can be stated in various ways:

e.g. to every action there is an equal and opposite reaction,

whenever a force acts on one body, an equal and opposite force acts on some other body,

when two objects interact, the forces they exert on each other are equal in magnitude, but act in opposite in direction.

The two forces are called an interaction pair of forces and they must be of the same type and the same size but acting in opposite directions on the different objects.

On diagrams both forces will be shown by arrows indicating both the direction and magnitude of the vector quantities.

The solar system - non-contacting forces of gravity - objects in a gravitational field

When the moon is pulled towards the Earth by the Earth's gravitational field force, there is an equal and opposite force operating as the moon's gravitational field pulls the Earth towards!

If the forces were not equal, either the moon would drift away into space or collide with the Earth! Fortunately, its a good example of a gravitational non-contact force operating!

The same argument applies to explain the Earth orbiting the Sun, both bodies experience the same numerically equal force, but acting in opposite directions. The diagram below illustrates these two gravitational force situations.

Diagram to illustrate the gravitational field effects between several objects in our solar system

Two pairs of forces interacting on the same objects!

All objects standing motionless on the ground are examples of opposite contact forces operating.

The weight of the object acting as a downward force due to gravity is balanced by an upward push from the ground as the atoms are minutely compressed.

If the forces were not balanced, either the ground would sink or the object would be raised up!

BUT, take care with such descriptions, analysis of the above situation reveals some complications!

A bit more messy to analyse than you think!

Consider the flask of liquid standing motionless on a laboratory bench.

There are two sets of forces operating shown by the arrows of opposing direction, but the same length - same magnitude of force for each pair.

Both sets of forces are examples of Newton's 3rd Law, but don't mix the two up!

The normal contact force due to the weight of the object acting (pushing) down on the surface of the bench (F1) is balanced by the bench under minute compression pushing back up to an equal and opposite extent onto the flask (F2).

At the same time both the flask and the Earth (including the bench) are mutually attracting each other (F3 and F4) to an equal and opposite extent due to the non-contact force of gravity (this may seem confusing, but it makes no difference whether the objects are in contact or not, here gravity acts throughout everything!).

In the cases described so far there is no resultant force, everything is balanced.

If the forces were not balanced and there was some net resultant force, the object would move or be reshaped - something would change!

For stationary objects, if the resultant force acting on the object is zero the object is said to be in equilibrium (effectively means a state of balance).

See also 3. Calculating resultant forces using vector diagrams and work done calculations