Force and Motion
A force is a push or a pull. Forces
are described not only by how strong they
are, but also by the direction in which
they act. The strength of a force is
measured using a unit called the Newton
(N), named for Isaac Newton. Picking up a
small lemon requires you to exert a force of
about one Newton. Forces can be shown
using arrows. The length of the arrow
represents the size of the force, and the
direction of the arrow shows the direction of
the force. The overall force on an object,
called the net force, is found by combining all of the forces acting on the object. The size of the
net force determines whether the object’s motion changes. The direction of the net force
determines the direction of the object’s motion. When two forces act in the same direction, they
add together. When forces act in opposite directions, they are
combined by subtracting the smaller force from the larger force.
The direction of the resulting force (resultant) is the direction of
the larger original force. When there is a net force acting on an
object, the forces are said to be unbalanced.
Unbalanced forces cause an object to start moving, stop moving,
or change direction. Unbalanced forces acting on an object will
change the object’s motion. Unbalanced forces can also stop a
moving object. The three main forces that stop moving objects
are friction, gravity and wind resistance.
Equal forces acting in opposite directions are called balanced
forces. Balanced forces acting on an object will not change the
object’s motion. When you add equal forces in opposite
direction, the net force is zero.
A force can be shown with a
vector. A vector is a line with an arrow. It begins with a dot
showing where the force begins. The length of the arrow shows
the amount of force. The arrows show the direction of the force.
Vectors are often used to show the net force acting on an object.
By combining two vectors you can determine the resulting force
(resultant) (Meter stick example)
