• What is Newton’s first law of motion?
• How is motion related to balanced and unbalanced forces?
• What effect does inertia have on the motion of an object?
• How is motion related to balanced and unbalanced forces?
• What effect does inertia have on the motion of an object?
Newton's 1st LawThe law of inertia
An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
How do you calculate the net force on an object if two forces are acting on it in the same direction?
Hutchings Photography/Digital Light Source
Figure 2 When forces in the same direction combine, the net force is also in the same direction. The strength of the net force is the sum of the forces.
Visual Check What would the net force be if one boy pushed with 250 N and the other boy pushed in the same direction with 180 N?
Combining Forces in Opposite Directions
When forces act in opposite directions on an object, the net force is still the sum of the forces. Suppose you choose “to the right” again as the reference direction in Figure 3. A force in that direction is positive, and a force in the opposite direction is negative. The net force is the sum of the positive and negative forces. The net force on the dresser is 100 N to the right.
Hutchings Photography/Digital Light Source
Figure 3 When two forces acting on an object in opposite directions combine, the net force is in the same direction as the larger force. The strength of the net force is the sum of the positive and negative forces.
Balanced and Unbalanced Forces
When equal forces act on an object in opposite directions, as in Figure 4, the net force on the object is zero. The effect is the same as if there were no forces acting on the object. Forces acting on an object that combine and form a net force of zero are balanced forces. Balanced forces do not change the motion of an object. However, the net force on the dresser in Figure 3 is not zero. There is a net force to the right. Forces acting on an object that combine and form a net force that is not zero are unbalanced forces.
Hutchings Photography/Digital Light Source
Figure 4 When two forces acting on an object in opposite directions are the same strength, the forces are balanced.
Visual Check How are the force arrows for the balanced forces in the figure alike? How are they different?
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REVIEW VOCABULARY
reference direction
a direction that you choose from a starting point to describe an object’s position
a direction that you choose from a starting point to describe an object’s position
Newton’s First Law of Motion
Sir Isaac Newton studied how forces affect the motion of objects. He developed three rules known as Newton’s laws of motion. According to Newton’s first law of motion, if the net force on an object is zero, the motion of the object does not change. As a result, balanced forces and unbalanced forces have different results when they act on an object.
1. Key Concept Check What is Newton’s first law of motion?
Balanced Forces and Motion
According to Newton’s first law of motion, balanced forces cause no change in an object’s velocity (speed in a certain direction). This is true when an object is at rest or in motion. Look again at Figure 4. The dresser is at rest before the boys push on it. It remains at rest when they apply balanced forces. Similarly, because the forces in Figure 5—air resistance and gravity—are balanced, the parachutist moves downward at his terminal velocity. Terminal velocity is the constant velocity reached when air resistance equals the force of gravity acting on a falling object.
2. 
Reading Check What happens to the velocity of a moving car if the forces on it are balanced?
Reading Check What happens to the velocity of a moving car if the forces on it are balanced?
Tim Keatley / Alamy
Figure 5 Balanced forces acting on an object do not change the object’s speed and direction.
Unbalanced Forces and Motion
Newton’s first law of motion only applies to balanced forces acting on an object. When unbalanced forces act on an object, the object’s velocity changes. If an object is at rest, unbalanced forces cause it to start moving. If an object is already moving, unbalanced forces cause its velocity to change.
3. Key Concept Check How is motion related to balanced and unbalanced forces?
Inertia
According to Newton’s first law, the motion of an object will not change if balanced forces act on it. The tendency of an object to resist a change in its motion is called inertia (ihn UR shuh). Inertia explains the motion of the crash-test dummy in Figure 6. Before the crash, the car and dummy moved with constant velocity. If no other force had acted on them, the car and dummy would have continued moving with constant velocity because of inertia. The impact with the barrier results in an unbalanced force on the car, and the car stops. The dummy continues moving forward because of its inertia.
4. Key Concept Check What effect does inertia have on the motion of an object?
AP Photo/Insurance Institute for Highway Safety
Figure 6 Inertia causes the crash-test dummy to keep moving forward after the car stops.
5. Visual Check What effect would a shoulder belt and a lap belt have on the inertia of the crash-test dummy?
Visual Check What effect would a shoulder belt and a lap belt have on the inertia of the crash-test dummy?
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WORD ORIGIN
inertia
from Latin iners, means “without skill, inactive”
from Latin iners, means “without skill, inactive”
Why do objects stop moving?
Think about how friction and inertia together affect an object’s movement. A book sitting on a table, for example, stays in place because of inertia. When you push the book, the force you apply to the book is greater than static friction between the book and the table. The book moves in the direction of the greater force. If you stop pushing, friction stops the book.
What would happen if there were no friction between the book and the table? Inertia would keep the book moving. According to Newton’s first law, the book would continue to move at the same speed in the same direction as your push.
On Earth, friction can be reduced but not totally removed. For an object to start moving, a force greater than static friction must be applied to it. To keep the object in motion, a force at least as strong as friction must be applied continuously. Objects stop moving because friction or another force acts on them.
Lesson Review
Visual Summary
Unbalanced forces cause an object to move.
Hutchings Photography/Digital Light Source
According to Newton’s first law of motion, if the net force on an object is zero, the object’s motion does not change.
Tim Keatley / Alamy
Inertia is a property that resists a change in the motion of an object.
AP Photo/Insurance Institute for Highway Safety
Lesson Assessment
Use Vocabulary
1. Define net force in your own words.
2. Distinguish between balanced forces and unbalanced forces.
Understand Key Concepts
3. Which causes an object in motion to remain in motion?
A. friction
B. gravity
C. inertia
D. velocity
4. Apply You push a coin across a table. The coin stops. How does this motion relate to balanced and
unbalanced forces?
unbalanced forces?
5. Explain Use Newton’s first law to explain why a book on a desk does not move.
6. A train moves at a constant speed on a straight track. Which statement is true?
A. No horizontal forces act on the train as it moves.
B. The train moves only because of its inertia.
C. The forces of the train’s engine balances friction.
D. An unbalanced force keeps the train moving.
Interpret Graphics
7. Analyze What is the missing force?
8. Organize Information Copy and fill in the graphic organizer below to explain Newton’s first law
of motion in each case.
of motion in each case.
Critical Thinking
9. Extend Three people push a piano on wheels with forces of 130 N to the right, 150 N to the left,
and 165 N to the right. What are the strength and direction of the net force on the piano?
and 165 N to the right. What are the strength and direction of the net force on the piano?
10. Assess A child pushes down on a box lid with a force of 25 N. At the same time, her friend
pushes down on the lid with a force of 30 N. The spring on the box lid pushes upward with a
force of 60 N. Can the children close the box? Why or why not?
pushes down on the lid with a force of 30 N. The spring on the box lid pushes upward with a
force of 60 N. Can the children close the box? Why or why not?
11. Evaluate In terms of Newton’s first law of motion, explain why automobiles have seat belts and
amusement park rides have restraint bars to keep people safe.
amusement park rides have restraint bars to keep people safe.
12. Give an example of unbalanced forces acting on an object.
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