Student Question

What is Newton's Third Law of Motion?

Isaac Newton's third law of motion states that every action has an equal and opposite reaction.

This means that when an object exerts a force upon another object, that second object responds by exerting the same amount of force upon the first object. Every physical interaction results in two equal forces acting upon each other in opposite directions.

For example, if you were to drop a tennis ball onto a slab of concrete, the ball would exert a downward force on the concrete. At the same time, the concrete would exert the same amount of force in the opposite direction upon the ball. When the ball bounces after hitting the concrete, you are seeing the reactionary force acting upon it.

Not all examples of this law are obvious, yet you are subjected to it at all times. For instance, the chair you may be sitting on is reacting to the force of your weight pushing down on it by pushing back up against you. When you take a step, the same thing happens between you and the ground you walk on. A swimmer propels themselves through water by pushing water behind them, which then exerts the same force in the opposite direction to move them forward.

Approved by eNotes Editorial

What is Newton's Third Law?

I've always had a little trouble getting my head around this third law, the idea that every action has an equal and opposite reaction. I try to keep two examples in mind:

A book lying on a table is pushing down on the table, because it's being pulled downward by gravity. The table is pushing back, and its capacity for pushing back to the same extent that the book is pushing on it is what keeps the book from crashing through the table.

That example is everyday, but it's not very dynamic. Here's the second example that I like to keep in mind:

If an astronaut in space, just floating there, throws something forward -- say, a spare screwdriver -- the equal and opposing force will propel the astronaut backward. The force of the backward movement of the astronaut will equal the force of the forward throw of the screwdriver. (Why an astronaut would carry a spare screwdrive in space is another problem altogether!)

Those examples have always helped me. The website below, written for students, gives more discussion and examples.

Last Updated on

What is Newton's Third Law?

The way we most often hear Newton's Third Law stated is this: "any action has an equal and opposite reaction."

What this law is saying is that whenever two objects interact, there are two forces acting on those objects.  The forces that are acting have both size and direction.  The sizes of the forces must be the same.  In addition, the directions of the forces must be opposite.

So, for example, a person sitting in a chair is putting downward force on the chair.  But the chair is putting an equal force on the person.

Last Updated on

What is Newton's third law of motion?

Newton's third law of motion states that "for every action, there is an equal and opposite reaction'. Here, action and reaction are forces and are always coupled to each other, that is, if there is an action, there will always be a reaction. In simpler terms, if a body exerts a force on another body, the second body will also exert a force on the first one. Both the forces will be same in magnitude, but will act in opposite direction (from body A to body B and body B to body A). This means that a book lying on a table is exerting some downwards force on the table (action) and in response, the table is also exerting an upward force on the book (reaction) and that both these forces are equal in magnitude.

Hope this helps.

Last Updated on

What is Newton's second law?

Newton's second law of motion states that the rate of change of momentum of a body is proportional to the external force and takes place in the direction of the force.

Let a body of mass "m" having an initial velocity "u" be acted upon by a force "F" for time "t" so that its final velocity becomes v.

Then, according to Newton's law of motion, force is directly proportional to change of momentum/time.

It states that the acceleration produced in a body is:

1. Directly proportional to the force acting on it,

and

2. Inversely proportional to the mass of the body.

Last Updated on

What is Newton's third law?

Newton's third law states that for every action there is an equal and opposite reaction. This means that if a first object acts on a second object, the second object will essentially push back or react against the first object. For example, if a person runs into a stone wall at a high enough speed, they will be stopped and injured by the impact: that is the equal reaction of the matter of the wall to the action of running into it.

This reaction will differ depending on the first action. Running a huge truck into the wall or setting off a bomb next to it will cause a different reaction than a mere human running into it. However, the important point is that a reaction of some sort will happen. A bomb might cause the reaction of the stones of the wall splintering and flying in every direction, but a reaction there is. These reactions—these "forces"—are what keep the universe in motion.