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Newton's Laws
of Motion
Newton's
first law of motion
If there is a desk resting on the floor and we do not exert a force on
it, the desk will stay there forevery. it is natural to think that an
object at rest will remain at rest if there is no force acting on it.
On the other hand if we push the desk, it will slide along the floor.
If we stope pusing it, it will stop almost immediately. This phenomenon
has led to them misconception that a force must be applied to keep an
object moving.
Galileo's argument
In the sixteenth century, Galileo (1564-1642) had a new concept of motion.
he pointed out that a moving object slows down because there is a force
acting on it. This force is due to the friction between the object and
the surface on which it is moving. If friction is reduced, the object
would move a long distance before coming to rest. If there is no friction
at all, the object would move on forever.
The observations made by Galileo were summarized by Newton (1642 - 1727
) as the Newton's first law of motion An object remains in a state of
rest or moves with uniform speed along a straight lin if there is no force
acting on it
In other words, there is a tendency for a moving object to keep on moving
, just as there is a tendency for an object at rest to mremain at rest.
If an object is at rest , its velocity is zero. If the object is moving
with uniform speed along a straight line, its velocity is uniform. Newton's
first law of motion expresses the reluctance of the object to change its
state of uniform motion. The object always tends to move with uniform
velocity, including the case of zero velocity, i.e. at rest.
Inertia
The tendency of an object to maintain its state of uniform motion is called
inertia. Inertia is possessed by any object with mass. More precisely,
mass is a measure of the inertia of the object. The greater the mass of
an object, the more reluctant is the object to change its state of uniform
motion. Many tricks that we have played are based on this fact. When we
pull one book quickly out form a pile of books, the books on the top will
not go with book being pulled , but stay with the other books below .
We can break a piece of thread tied to a small mass by giving the order
end a very sudden jerk. These two tricks illustrate that a stationary
object tends to remain stationary.
A passenger tends to move backwards relative to the car when the car starts.
He tends to move forward when the moving car stops. Sea belts and head
rests are used for sately puposes. A seat belt ties the passenger to the
seat and prevents him from being thrown forrward when the car is stopped
suddenly. This happends when the car collides with an obstacle in front.
A head rest protects the neck from being injured if the head is thrown
backwards when the car is bumped from behind.
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Newton's
second law of motion
We know that a force changes the state of uniform motion of an object.
Now we want to know quantitaively how it affects motion. in particular,
how is force related to mass and acceleration?
There is an experiment in our page.
The experiment shows that a force is something which causes the acceleration
of an object. For the same mass, twice the force produces twice the acceleration.
For the same acceleration, twice the mass requires twice the force.
Such experimental results were generalized by Newton as the Newton's second
law of motion:
The acceleration of an object is directly proportional to, and in the
same direction as, the net force acting on it, and inversely proportional
to its mass.
Note that force is a vector because it has both magnitude and direction.
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Newton's
third law of motion
There is an experment in our page.
The car and the cardboard move because of the friction between the wheels
and the cardboard. The free body diagram for the car is shown in Fig.
1. The car accelerates to the right because there is a force F acting
on it. This force is exerted on the car by the cardboard.
The free body diagram for the cardboard is shown in Fig. 2. The cardboard
accelerates to the left because there is a force F' acting on it. This
force is exerted on the cardboard by the wheels. Obviously F and F' act
in opposite directions. In addition, they act on different objects. F
and F; are called an action-reaction pair. Action and reaction always
occur in pairs. In general, if there is a force F exerted by an object
A on an object B, there must be a force F' exerted by object B on object
A. The paired-forces exist no matter the two objects are stationary, moving
with uniform velocity or accelerating.
The reaction in an action-reaction pair must be distinguished from the
more general reaction discussed under Newton's first and the second laws.
Consider a block at rest on the bench. From Newton's first law we know
that there must be a reaction R' for this force. R' is exerted by the
block on the bench. In the sense of action-reaction pair, we may call
R an action and R' the reaction. We may as well call R' an action and
R the reaction.
Action-reacton pairs must also be distinguished from balanced forces.
There is a weight W of the block It is the force exerted on the block
by the earth. The block is at rest because the gravitational force W acting
on it is balanced by the rection R, also acting on it. In other words,
W and R are balanced forces acting on the same object. In contrast, paired-forces
act on different objects.
If a body A exerts a force on body B, then body B exerts an equal but
opposite force on body. The result is stated as the Newton's third law
of motion.
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