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Third Law

When an object exerts a force on a 2nd object,
the 2nd object exerts an equal force,
in the opposite direction, on the 1st object.


Action and reaction pairs

Action: Object A exerts a force on object B

Reaction: Object B exerts a force on object A

Note that the force and reaction force are on different objects

forces in pairs

Another example

third law person walking on ground subscripts

Lift: How can airplanes fly?

3rd law – Jets push airplane forward – but air does not go over the wings symmetrically.

For every force there is an equal opposite force – so as the air is forced down by the wing, the wing is forced up by the air.

The aerodynamic lift produced by the motion of a pitched, or tilted, wing moving through air can be understood directly through Newton’s Laws of Motion.

Consider a pitched wing moving through stationary air at a constant speed, v. From the point of view of the wing, it is not moving. Rather, the air is rushing by it with net speed v in the opposite direction.

Air is made up of many molecules and atoms that follow Newton’s Laws of Motion. Newton’s second law says that each molecule hitting the wing experiences an acceleration proportional to the force of contact, and inversely proportional to its own mass.*

Newton’s third law tells us that the wing experiences a force that is equal in size but opposite in direction to that felt by the molecule.** While an individual collision of a molecule has little effect, putting billions of billions of impacts together each second does have a real effect – known as lift.

  • from http://www.4physics.com/phy_demo/tilted-wing.htm


Reaction force on airplane wing

This is a (large) model airplane going through fog in a wind tunnel. The fog is lit by lasers to help us visualize.

newtons 3rd law airplane reaction

Nellie Newton holds an apple weighing one newton at rest on the palm of her hand. The force vectors shown are the forces that act on the apple.

Hewitt Conceptual Physics worksheet

Hewitt Conceptual Physics worksheet

Newton’s Third law

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

A FRAMEWORK FOR K-12 SCIENCE EDUCATION: Practices, Crosscutting Concepts, and Core Ideas
How can one predict an object’s continued motion, changes in motion, or stability?

Interactions of an object with another object can be explained and predicted using the concept of forces, which can cause a change in motion of one or both of the interacting objects… At the macroscale, the motion of an object subject to forces is governed by Newton’s second law of motion… An understanding of the forces between objects is important for describing how their motions change, as well as for predicting stability or instability in systems at any scale.

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.

1.4 Interpret and apply Newton’s three laws of motion.
1.5 Use a free-body force diagram to show forces acting on a system consisting of a pair of
interacting objects. For a diagram with only co-linear forces, determine the net force acting on a system and between the objects.
1.6 Distinguish qualitatively between static and kinetic friction, and describe their effects on the motion of objects.
1.7 Describe Newton’s law of universal gravitation in terms of the attraction between two objects, their masses, and the distance between them.
1.8 Describe conceptually the forces involved in circular motion.

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