7/17/2019 Laws of Motion

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For every action, there is an equal and opposite reaction.

The statement means that in every interaction, there is a pair of forces acting onthe two interacting objects. The size of the forces on the rst object equals the sizeof the force on the second object. The direction of the force on the rst objectis opposite to the direction of the force on the second object. Forces always come inpairs - equal and opposite action-reaction force pairs.

The book lying on the table is exerting a downward force on the table, while the table isexerting an upward reaction force on the book. Because the forces are equal andopposite, the book remains at rest. Notice also that the table legs are in contact with thefloor and exert a force downward on it, while the floor in turn exerts an equal andopposite force upward.

The force of the nail on the hammer and the force of the hammer on the nail

The first law states that if the net force (the vector sum of all forces acting on an ob ect! is "ero, then

the velocity of the ob ect is constant. #elocity is a vector quantity which expresses both the

ob ect$s speed and the direction of its motion% therefore, the statement that the ob ect$s velocity is

constant is a statement that both its speed and the direction of its motion are constant.

The first law can be stated mathematically as

&onsequently,

• 'n ob ect that is at rest will stay at rest unless an external force acts upon it.

• 'n ob ect that is in motion will not change its velocity unless an external force acts upon it.

• The second law states that the net force on an ob ect is equal to the rate of change (that is,

the derivative ! of its linear momentum p in an inertial reference frame

• The second law can also be stated in terms of an ob ect$s acceleration. )ince Newton$s

second law is only valid for constant*mass systems, + - +/ + 0 mass can be taken outside

the differentiation operator by the constant factor rule in differentiation . Thus,

•

7/17/2019 Laws of Motion

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• where F is the net force applied, m is the mass of the body, and a is the body$s acceleration.

Thus, the net force applied to a body produces a proportional acceleration. 1n other words, if

a body is accelerating, then there is a force on it.

The third law states that all forces exist in pairs if one ob ect A exerts a force F A on a secondob ect B , then B simultaneously exerts a force F B on A , and the two forces are equal andopposite F A 2 3 F B .+45 The third law means that all forces are interactions between different bodies ,+46

+4- and thus that there is no such thing as a unidirectional force or a force that acts on only one body.This law is sometimes referred to as the action-reaction law , with F A called the 7action7 and F B the7reaction7. The action and the reaction are simultaneous, and it does not matter which is calledthe action and which is called reaction % both forces are part of a single interaction, and neither forceexists without the other. +45