Back To CoursePhysics 101: Intro to Physics
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To begin, forces always act in pairs and always act in opposite directions. When you push on an object, the object pushes back with an equal force. Think of a pile of books on a table. The weight of the books exerts a downward force on the table. This is the action force. The table exerts an equal upward force on the books. This is the reaction force. Note that the two forces act on different objects. The action force acts on the table, and the reaction force acts on the books.
Newton's Third Law states that forces always act in pairs. Consider an example of a boy playing with a dog's toy and what it illustrates. There is a force from the boy on the dog's toy, and there is a force from the dog's toy on the boy. These two forces create an interaction pair. Forces always come in pairs similar to this example. Consider the boy (A) as one system and the toy (B) as another. What forces act on each of the two systems? Picture the boy pulling on a toy and the toy being pulled from the boy. You can see that each system exerts a force on the other. The two forces - F(A on B) and F(B on A) - are the forces of interaction between the two. Notice the symmetry in: A on B and B on A.
The forces F(A on B)and F(B on A) are an interaction pair, which is a set of two forces that are in opposite directions, have equal magnitudes and act on different objects. Sometimes, an interaction pair is called an action-reaction pair. This might suggest that one causes the other; however, this is not true. For example, the force of the boy pulling on the toy doesn't cause the toy to pull on the boy. The two forces either exist together or not at all.
There can never be a single force acting alone. Forces only come in action-reaction pairs. Think carefully about propelling a skateboard with your foot. Your foot presses backward against the ground. The force acts on the ground. However, you move, so a force must act on you, too. Why do you move? What force acts on you? You move because the action force of your foot against the ground creates a reaction force of the ground against your foot. You 'feel' the ground because you sense the reaction force pressing on your foot. The reaction force is what makes you move because it acts on you.
Newton's Third Law of Motion explains that forces always come in action-reaction pairs. The Third Law states that for every action force, there is an equal and opposite reaction force. Imagine hitting a baseball. The bat exerts a force on the ball. This is the action force. The ball exerts an equal and opposite force on the bat. This is the reaction force. Such an interaction pair is another example of Newton's Third Law. The baseball forces the bat in one direction and the bat forces the ball in the opposite direction. The two forces create an interaction pair on different objects and are equal in strength and opposite in direction. The force(F) of A(the bat) on B(the ball) is equal in magnitude and opposite in direction of the force of B on A: F(A on B) = - F(B on A).
Newton realized that if one object pulls on another, the second object also pulls back on the first object. If one object pushes on another, the second pushes back on the first object. In other words, for every action by a force there is a reaction by another force.
When sorting out action and reaction forces, it is helpful to draw diagrams. Draw each object apart from the other. Represent each force as an arrow in the appropriate direction. The guidelines on this chart can help you sort out action and reaction forces.
Consider the situation of holding a book in your hand. You can draw one diagram for you and one for the book. Are there any interaction pairs? You can use arrows to represent force and the direction of the force. In this case, the interaction pair is the force of the book on the hand and the force of the hand on the book.
We've gone through some examples already. But what are some other action-reaction examples? Let's look at a rocket engine. Newton's Third Law explains how rocket engines work. Hot gases are forced out of the back of the rocket. This is the action force. The gases exert an equal and opposite force on the rocket. This is the reaction force. The reaction pushes the rocket upward and off the ground.
Also consider what happens when a diver jumps on a diving board? The board springs back and forces the diver into the air. The action force exerted on the board by the diver causes a reaction force by the board on the diver. The force of the diver on the board is equal and opposite to the force exerted by the diving board. Think about the way the force of the diving board affects the diver's performance. The greater the force exerted upon the diving board, the higher the dive will be.
Finally, think about how a crew team uses Newton's Third Law of Motion to move a boat. When an oar is put into the water, the water exerts an equal force on both sides of the oar. However, when the members pull on their oars, the surface of the flat side of the oars pushes against the water. The water pushes back on the oars with an equal and opposite force. The boat moves in the opposite direction of the oars with a force that is equal to that of the oars as they push against the water. The boat moves because the forces against it are unbalanced. Why do you think it is important for all the crew members to pull on their oars at the same time? Well if the crew members do not work together, their own forces will balance each other, decreasing the overall unbalanced force they are trying to achieve.
A rotating water sprinkler is another example of action and reaction forces. Water is forced from the sprinkler. This is the action. The reaction is the movement of the sprinkler arms away from the water. You feel the same kind of reaction when you hold a water hose and turn the water on quickly. You may have seen firefighters struggling to control a fire hose. The hose is forced backward when the water leaves it. This reaction makes the hose hard to handle.
Octopus and squid make use of Newton's Third Law of Motion as well. An octopus or squid moves by first drawing water into its body. Then the animal forcefully squeezes water out of its body through an opening behind its head. The force of the expelled water moves the animal in the opposite direction.
Here's a final example when thinking about Newton's Third Law and the notion that forces react in opposite directions; you have just blown up a balloon. First, hold it with the opening downward and let go. In what direction does the balloon move? With the opening downward, the balloon moves upward. Blow up the balloon again, hold it horizontally and let it go. In what direction does the balloon move? The balloon will move horizontally away from the end from which the air is escaping. How would you explain why both balloons don't move in the same direction? The direction of motion is opposite the direction of the escaping air.
Forces always act in pairs. Newton's Third Law of Motion states that for every action force there is an equal and opposite reaction force. Action-reaction pairs can be seen in all parts of life, from baseball and skateboarding to sea animals and rocket ships. When discerning an action and reaction force you may want to consider drawing a diagram that illustrates the objects the force is acting on and the direction of those forces.
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Back To CoursePhysics 101: Intro to Physics
17 chapters | 188 lessons