Based on the previous two laws, motion still
can not be completely explained. Sometime, over objects, there is not a clear
external force acting on the object; like for example, a person standing on the
middle of a room, alone. If the person choose to walk out of the room. Where is
the net external force need for accelerating the person coming from? Originally,
the person is at rest (zero velocity) and later the person is moving with a
given velocity; thus, the acceleration is not zero. What does apply the net
external force to the person? The answer to this question is provided by
Newton's third law, Action and Reaction.
If body A applies a net external force on
body B (action), FBA, body
B reacts by applying a force of the same magnitude but opposite
direction on body A (reaction), FAB.
These pair of forces is called an action and reaction pair.
Characteristics of action and reaction pairs:
action and reaction forces have the same magnitude:
action and reaction forces are opposite to each other:
two forces, action and reaction, act on different objects;
therefore, the two forces do not cancel each other. In fact on body A,
there is only one force acting on it:
As an example of action and reaction, consider a
person pushing a block as illustrated in the figure. The action is the
force applied by the woman on the block. To this action, there is a
reaction force exerted by the block and acting over the arms of the
woman. The body of the person transmit this force all the way to the
feet, blue vector. In the absence of friction (see
below), the woman will slide back because of the reaction force
acting on her. Static friction prevents sliding of the feet. On the
figure, the friction between the floor and the block is not represented
even when it is present. In most cases, the force that the woman exerts
over the block is such that the block can be pushed at constant velocity
(excepts when it just starts the motion where the push must also
accelerate the block from rest).
Depending on the friction between the different
surfaces, the result of this action and reaction pair can be (starting
Static friction between floor and shoes and
between block and floor are greater than the force applied by the
woman. Neither the block nor the woman move.
Static friction between floor and shoes greater
than force applied by the woman. However, static friction between
floor and block is smaller than the force applied by woman. Block
starts sliding forward. Woman must maintain an applied force in
order to over come the kinetic friction (see
below) between the block and the floor.
Static friction between block and floor greater
than applied force by the woman. In this case, the static friction
between the floor and the shoes is smaller than the reaction force.
Woman will slide back without being able to push the block.
Static friction between the two pairs of surfaces
is smaller than the force applied by the woman (imagine the floor is
iced) both the woman and the block will slide in opposite
directions. Woman to the left, and block to the right.
Using Newton's third law, the change in the state of motion for a person can
Over the player
there are only two forces acting, the weight and the reaction of the
floor to the pushing of the player against the ground. The reaction
force of the floor, acting on the player, can be decomposed into
vertical and horizontal components, V and H. Between this
vertical component of the reaction force and the weight of the player,
the resultant force in the vertical direction is zero (in average when
the player is allowed to jump while running). However, the
horizontal component of the reaction force is unbalance.
Therefore, there is
a net external force acting over the player. This force produces the
acceleration of the player.