Download Introduction to Forces- Reading 3: Balanced

Name: ______________________________ Date: ________________ Period: ______
Introduction to Forces- Reading 3: Balanced/Unbalanced forces and Net Forces
Directions: Write a total of at least 4 comments AND 4 highlight/underlines as you read. Then answer the
questions at the bottom.
Net Forces
Once we understand the notation we will use for describing forces, we are ready to deal with a much more
important idea than forces alone. Where motion is concerned, any single force acting on an object typically
tells you very little, unless there is only one force acting on the object. Most of the time objects experience
more than one force. In such cases it is the net force that is important to understand if one is to relate to an
object’s motion. The term “net force” refers to the overall effect of all of the forces that act on an object.
The unbalanced force has the same effect on an object as if there were only one force on the object of that
size and direction. You determine the net force by adding together each of the forces acting on an object,
taking into consideration both the size of each force, and the direction of each force.
• If an object experiences no forces, the net force is zero.
• If an object experiences only one force, the net force is that single force.
• If more than one force acts on the object, you must combine all of the forces to find the net force.
o Whenever two or more forces act in the same direction, the net force is the sum of the forces
added together, and points in the direction of each of the forces.
o If two forces act in exactly opposite directions, the net force is the difference in the
magnitudes of the two forces (subtraction), and points in the direction of the larger force.
o If an object experiences two forces of the same size in opposite directions, the net force is
zero.
In order to determine the size and direction of the net force on an object, particularly when two or more, we
will use a force diagram.
Newton’s Laws
The net force is important because it tells you how the object is accelerating! Isaac Newton was a scientist
who lived over 300 years ago. Newton published some of the most important ideas in the entire history of
science, including 3 laws about physics:
1. An object with balanced forces has a constant velocity
2. An object with unbalanced forces accelerates.
3. A force is an interaction between two objects.
We say that an object has “balanced forces” when the net force acting on the object is zero. For
example, a book sitting on a table has 2 forces acting on it: the normal force points up and the gravitational
force points down. The forces are balanced because the forces pointing up are equal to the forces pointing
down. The normal force and the gravitational force cancel each other out to create zero net force.
An object experiences “unbalanced forces” when the net force acting on the object is NOT zero.
Maybe there is more force down than up. Maybe there is more force left than right. In a situation with
unbalanced forces, the forces are not acting equally in all directions.
Check for Understanding Questions:
1. If an object is accelerating are the forces on it balanced or unbalanced? ________________
2. What is the net force on an object experiencing NO forces? ________________
3. If an object has unbalanced forces, which of the following types of motion will NOT be
possible?
a. speeding up
b. constant velocity
c. slowing down
d. turning
©Modeling Workshop Project 2012/STL Group-R. Rice
1
Unit 3, Reading 1, Introduction to Forces, v1.12