Forces and Motion
Imagine you were asked to move a heavy box. To move the box, you would most likely
apply a force. A force is a push or pull on an object. In addition to your applied force, the
box experiences a normal force from the floor, gravity from Earth, and friction. Even when
the box is not moving, it still has the normal force and gravity acting on it. How do forces
cause an object’s motion?

Newton’s First Law of Motion
Not all forces cause motion. If you push on a heavy box, it may not move. That is
because friction resists the box’s motion. However, if you increase your applied force so
that it is greater than friction, the box will begin to move. 

The effects of force on the motion of an object can be determined using Newton'’s first
law of motion. According to Newton’s first law of motion, an object at rest will stay at
rest, and an object in motion will stay in motion unless an unbalanced force acts on the
object. Unbalanced forces are forces acting on an object that combine and form a net
force that is not zero. The net force is the combination of all the forces acting on an
Object.

Balanced Forces
According to Newton’s first law of motion, an object’s motion will
not change unless acted on by an unbalanced force. That means
that the object’s motion remains the same when the forces are
balanced. Balanced forces are forces acting on an object that
combine and form a net force of zero. When the forces on an
object are balanced, they cause no change in the object’s
Velocity.

Balanced Forces and Moving Objects
According to Newton’s first law, moving objects can also experience balanced forces.
When the law states that objects will stay in motion, it means that they will move at a
constant velocity. For example, if a tennis ball rolls across an icy lake with a velocity of
2 mys, its velocity will remain 2 m/s. That is because there is very little friction acting on
the tennis ball. The forces on the tennis ball are balanced, so its motion remains the
Same.

Predicting Effects On Motion
Newton's first law of motion allows scientists and engineers to
predict an object's motion. When researching or designing
objects, scientists and engineers analyze the forces. If the forces
are balanced, then the scientists and engineers know that the
object’s motion will remain the same. If the object is at rest, it will
remain at rest. If the object is moving, it will continue to move at
the same velocity.

Unbalanced Forces
Newton'’s first law of motion only describes what happens when balanced forces act on
an object. When unbalanced forces act on an object at rest, the object starts moving.
When unbalanced forces act on an already moving object, the object's velocity changes.
That means that the object’s speed, direction of motion, or both can change. Objects that
speed up, slow down, or move in a circle all experience unbalanced forces.

Reducing Unbalanced Forces

While friction can be useful when it comes to vehicle safety, it can be problematic for

many objects and systems. When an object is moving, friction opposes its motion. If

friction causes the forces on an object to become unbalanced, the object will slow down.

That is why engineers often design objects and systems that reduce friction

Inertia

According to Newton’s first law of motion, the motion of an object

will not change if balanced forces act on it. The tendency for

objects in motion to resist changes to their direction and speed is

called Inertia. Inertia explains the motion of a crash test dummy

during an accident. Before the crash, the car and dummy are

moving at constant velocity. The impact with a barrier results in

an unbalanced force on the car, and the car stops. The dummy

continues moving forward because of its inertia.