If you are
going to remove a table, you will push or pull the table so that it moves. But if
you stop pushing or pulling, the table will also stop. Based on this fact,
former scientific philosopher stated that a force is needed to keep the body
moving. However, Galileo and Newton’s researches found that the reality is not
like that. Newton’s statements related to the movement of the body were
declared in three formulas known as Newton’s Laws.
1. 1. Newton’s First law
Newton’s first law states, “A body acted on by no net force moves with
constant velocity (which may be zero) and zero acceleration”. In other words: “When
no net force acts on a body, the body either remains at rest or moves with
constant velocity in a straight line. Once a body has been set in motion, no
net force is needed to keep it moving”. It is mathematically formulated as :
According to Newton’s first law, a body at rest condition will always
stand at its rest and the body that moves will always stands its movement. The property
of a body which always stands its first condition or its position is known as inertia.
An inertial property of a body can be felt when you are standing or sitting
in a moving city bus. When the bus braked immediately, you will be pushed
forward. Why? The bus initially moves so that it tends to keep moving and so do
you in the bus. As the result, when the bus braked immediately, you are pushed
forward because you stand the initial moving condition. On the other hand, when
the bus is initially at rest condition and then the bus moves, you will be
pushed backward because you stand the initially rest condition.
2.
2. Newton’s Second Law
Bind a toy car with a rubber bangle, and the give force by pulling it. The
toy car will move and get acceleration in the same direction with the direction
of the pulling force you gave as shown in figure (a) below. Observe the strain
of the rubber! Now, add a wooden block on the toy car as shown in figure (b),
and then give the same pulling force to it. Which figures will get bigger
acceleration, car in figure (a) or figure (b)? if the given force is the same,
the toy car in figure (a) will get bigger acceleration, because it has less
mass. In order to have a bigger acceleration, the toy car in figure (b) needs a
bigger force. This additional force will be seen when the length of the rubber increases.
Based on these phenomena, Newton formulated his second law which states, “If
a net external force act on body, the body accelerates. The direction of
acceleration is the same as the direction of net force. The net force vector is
equal to the mass of the body times the acceleration of the body.” On other
words: “Acceleration caused by forces working on a body is equal to the
magnitude of the force and unequal to the mass of the body”. Is it formulated
mathematically as follows:
Where:
a = the body acceleration (m/s2)
∑F = the total force (N)
m = the mass of the body (kg)
3. 3. Newton’s Third law
Newton’s third law of motion states that : “If body 1 exerts a force on
body 2 (an action), then the body 2 exerts a force on body 1 (a reaction). These
two forces have the same magnitude but are opposite in direction. There are two
forces act on different bodies.”
 |
| Child A exerts a force to child B, child B also exerts a force to child A with the same magnitude but from the opposite direction. As the result, A and B will move in the opposite direction |
If a body exerts a force (action) to another body, then the other body
will also exerts a force (reaction) to the first body within the same
magnitude. The pair of these two forces is often known as the pair of action-reaction
force. Mathematically, Newton’s third law can be formulated as :
F1 = - F2
Where
F1 = action force
F2 = reaction force (negative sign shows that the two forces
are in the opposite direction).