1. NEWTON’S LAWS
4(C) Investigate how an object’s motion changes only when a net force is applied, including activities and equipment such as toy cars, vehicle restraints, sports activities, and classroom objects
4(A) Describe and calculate an object’s motion in terms of position, displacement, speed, and acceleration.
4(D) Assess the relationship between force, mass, and acceleration, noting the relationship is independent of the nature of force, using equipment such as dynamic carts, moving toys, vehicles, and falling objects.
3(F) Research and describe the history of physics and chemistry and contributions of scientists.
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Newton’s Laws of Motion

2. Essential Questions
What is inertia and how is it related to Newton’s first law of motion?
How is acceleration calculated using Newton’s second law of motion?
According to Newton’s third law of motion, how are the forces between interacting objects related?
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Newton’s Laws of Motion

3. Vocabulary Review New acceleration Newton’s first law of motion
inertia
Newton’s second law of motion
Newton’s third law of motion
Newton’s Laws of Motion
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4. Newton’s First Law of Motion
Newton's first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.
If an object is at rest, it stays at rest unless an unbalanced net force acts on it.
This law is sometimes called the law of inertia.
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Newton’s Laws of Motion

5. Inertia and Mass
Inertia (ih NUR shuh) is the tendency of an object to resist any change in its motion.
If an object is moving, it will have uniform motion.
It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.
If an object is at rest, it tends to remain at rest. Its velocity is zero unless a force makes it move.
The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.
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Newton’s Laws of Motion

6. Force and Acceleration
Newton’s second law of motion describes how the forces exerted on an object, its mass, and its acceleration are related.
What’s different about throwing a ball horizontally as hard as you can and tossing it gently?
When you throw hard, you exert a much greater force on the ball.
The hard-thrown ball has a greater change in velocity, and the change occurs over a shorter period of time.
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Newton’s Laws of Motion

7. Newton’s Second Law of Motion
Newton’s second law of motion states that the acceleration of an object is in the same direction as the net force on the object, and that the acceleration can be calculated from the following equation:
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Newton’s Laws of Motion

9. Calculating Net Force with the Second Law
Newton’s second law also can be used to calculate the net force if mass and acceleration are known.
To do this, the equation for Newton’s second law must be solved for the net force, F.
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Newton’s Laws of Motion

10. Calculating Net Force with the Second Law
To solve for the net force, multiply both sides of the equation by the mass:
The mass, m, on the left side cancels, giving the equation:
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Newton’s Laws of Motion

11. SOLVE FOR ACCELERATION
Use with Example Problem 2.
Problem
You push a wagon that has a mass of 12 kg. If the net force on the wagon is 6 N south, what is the wagon’s acceleration?
SOLVE FOR THE UNKNOWN
Set Up the Problem
a = Fnet m
Solve the Problem
a = 6 N south 12 kg = 0.5 m/s2 south
Response
ANALYZE THE PROBLEM
EVALUATE THE ANSWER
The value of the net force (6) is less than the value of the wagon’s mass (12), so we would expect the acceleration’s value to be less than one. Our answer (0.5 m/s2) makes sense.
KNOWN
mass: m = 12 kg
net force: Fnet = 6 N south
UNKNOWN
acceleration: a
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Newton’s Laws of Motion

12. Newton’s Third Law
Newton’s third law of motion states that when one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.
These forces are sometimes called the action and reaction forces.
But, the action force doesn’t cause the reaction force. They occur at the same time.
When you jump on a trampoline, for example, you exert a downward force on the trampoline.
Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.
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Newton’s Laws of Motion

13. Action and Reaction Forces Don’t Cancel
According to the third law of motion, action and reaction forces act on different objects.
Thus, even though the forces are equal, they are not balanced because they act on different objects.
For example, consider a student pushing on a box.
The box pushes on the student, but the student remains in place because of the friction between her shoes and the floor.
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Newton’s Laws of Motion

14. Forces are Interactions
A force is an interaction between two objects.
The forces on the box include the student’s push to the right and sliding friction to the left.
The net force on the box is not zero and the box accelerates to the right.
For example, it does not make sense to say, “The box has a force of 20 N.”
However, it does make sense to say, “The student pushes on the box with a force of 20 N.”
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Newton’s Laws of Motion

15. Review Essential Questions Vocabulary
What is inertia and how is it related to Newton’s first law of motion?
How is acceleration calculated using Newton’s second law of motion?
According to Newton’s third law of motion, how are the forces between interacting objects related?
Vocabulary
Newton’s first law of motion
inertia
Newton’s second law of motion
Newton’s third law of motion
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Newton’s Laws of Motion