There is real science behind my beliefs! Ever heard, "Every action has an equal reaction?" Or studied the 'butterfly effect'.? Every thing we do, actions, words, thoughts, all have an impact on EVERY OTHER part of the interconnected universe.
Newton's Third Law of Motion
Newton's Third Law |
Identifying Action and Reaction Force Pairs
Student Extras
www.flickr.com/photo
Visit The Physics Classroom's Flickr Galleries and take a visual overview of Newton's laws of motion.
PhET Simulation: Lunar Lander 
Use Newton's third law to control a lunar lander in this simulated game from PhET.
Teacher's Guide
www.physicsclassroom
Learning requires action. Give your students this sense-making activity from The Curriculum Corner.
Newton's Third Law of Motion: Actions and Reactions These ideas from NASA can be adapted to engage students in a collection of activities that explore action-reaction.
Newton's Third Law
A force is a push or a pull upon an object that results from its interaction with another object. Forces result from interactions! As discussed in
Lesson 2, some forces result from
contact interactions (normal, frictional, tensional, and applied forces are examples of contact forces) and other forces are the result of action-at-a-distance interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called
action and
reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is:
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object
equals the size of the force on the second object. The direction of the force on the first object is
opposite to the direction of the force on the second object. Forces
always come in pairs - equal and opposite action-reaction force pairs.
A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.
Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.
Consider the motion of a car on the way to school. A car is equipped with wheels that spin in a clockwise direction. As the wheels spin clockwise, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.
Check Your Understanding
1. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?
