Balanced and unbalanced forces:

Objectives:

1.    Distinguish between balanced and unbalanced forces.

2.    Explain how equal & opposite forces cause an object to remain at rest or move at a  constant velocity.

3.    Explain how unbalanced forces change the motion of an object.

4.    Learn to diagram forces with arrows that show the relative size & direction of ea. force

Key Terms:

equilibrium     kilogram    law of inertia    mass    net force    newton    normal force    support force    weight

Web Resources:

Notes:

Mass & Inertia:

Mass is the measure of inertia.  The more mass an object has, the more inertia it has.

• Newton's Law of Inertia:
• A body at rest will strive to stay at rest and a body in motion will strive to stay in motion.
• Newton's first law really talks about the mass of an object.

Here are a few properties of mass.

• Mass is the measure of matter in an object
• It is NOT volume (base * height * width)
• It is NOT density (mass/volume = density)
• It is NOT weight (mass * gravity = weight)
• Weight is the force of gravity acting on an object.

Force:

The SI unit of force is the N, therefore, the measure of weight is measured in Newtons

example:    If a person has a mass of 100kg, what is his weight?

If the moon has 1/6 the gravity of the earth, how much does the same person weigh on the moon?

Balanced and Unbalanced Forces:

Unequal forces can cause objects to accelerate while balanced forces can cause an object to stay at rest or at constant velocity.  In order to understand this we are reminded of the condition of weight. (mass * acceleration due to gravity)

Formulas:

F = ma (= m x v/t = kg x m/s2 or N (Newton’s))

**** Remember that forces are vectors! ****

Why is it then that when we are standing on the Earth that we are not moving?  The answer is that the Earth is pressing on us with the same force that we are exerting on it.  The net force = 0.

Balanced forces:

Balanced forces are those that are opposite in direction and equal in size.   Balanced forces are considered to be in a state of equilibrium.

•  When forces are balanced there is no change in direction.
•  Combined forces that are balanced are always equal to zero. (combining vectors)

•  Balance forces cannot change the motion or direction of an object.

•  A balanced force keeps an object moving at a constant velocity. (Newton's 1st Law of Motion)

What are some balanced forces in the room?

A book on the table is an example of a balanced force.  The force of the book's weight is counteracted by the normal force (support force) of the table.  The two forces are completely equal and opposite.

What is an example of a balanced force that keeps an object moving a constant velocity?

The cruise control on a car attempts to equal the forces of friction with a forward force.  Once constant velocity is obtained the two sets of forces are completely equal and opposite.

Unbalanced Forces:

Forces that are not opposite in direction and equal in size.

• Always cause a change in motion. (can change speed or direction of an object)

• When 2 unbalanced forces are exerted in opposite directions, the combined force is the difference between the 2 forces.

• When 2 unbalanced forces are exerted in the same direction, the combined force is the sum of the 2 forces.
• When the forces are at right angles the net force is equal to the resultant