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7.3 The resultant force
Two teams compete
with each other,
pulling a rope in
opposite directions.
How do we know which
is the winning team?
textbooks nis
70
Assessing the forces
When you are studying the forces that act on different objects (or a
system) and what happens to the objects (or system) as a result of these
forces, you must take into account all the forces acting on the object.
is NOT zeroedu kz
Below is a sequence of questions we should ask when considering the
forces acting on an object.
Is the object moving or not moving?
It is moving It is not moving
Is it moving at Is it accelerating?
constant speed?
Forces acting on Forces acting on Forces acting on
the object must the object must the object must
be balanced be unbalanced be balanced
Resultant force Resultant force Resultant force
must be zero must be zero
● An object that is not moving will remain stationary when the sum of
the forces acting on it – the resultant force – is zero.
● A moving object with a resultant force equal to zero (i.e. when the
sum of all the forces acting on it is zero) will keep moving in the same
direction and at the same speed.
● When the resultant force on an object is different from zero, two things
could happen depending on the initial condition of the object:
An object that is at rest (stationary) will speed up (accelerate) in the
same direction as the resultant force.
An object that is already moving can either speed up (accelerate),
slow down (decelerate) or change direction, depending on the
direction of the resultant force.
– If the resultant force is in the same direction as the motion,
it will accelerate.
– If the resultant force is in the opposite direction to the motion,
it will decelerate.
– If the resultant force is in any other direction, it will change
direction (usually in a curved path).