Newton’s Laws of Motion

1. Historical Perspective
1. Aristotle
• Observer not an experimenter
• A force has to act on an object for it to move
• Velocity was proportional to that force
1. Galileo
• supported heliocentric model of the earth
• no force is required to maintain motion once it is initiated
• included acceleration in his theories
1. Isaac Newton
• Proposed a theiry of the causes of motion that could explain the motion of ANY object
• Three laws of motion and the law of universal gravitation
• Could confirm predictions using these laws
1. Newton’s Laws of Motion
1. Newton’s first law of motion
• explains what happens in the absence of force
• An object remains at rest, or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force.
• The velocity will not change unless a force acts upon the object (constant velocity)

If F = 0

v = 0 (at rest) remain at rest

v v v v

• Why do things that are moving stop?

1. Newton’s second law of motion
• Explains the effects of applying a force to an object
• The central idea of his theory of motion
• The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object.
• The acceleration is in the same direction as that of the imposed force.

or

Acceleration is proportional to force

Acceleration is inversely proportional to mass

• Second Law is the central idea of Newton's theory of motion
• Total force => the imposed force in the NET force.

( 1N = 1 kgm/s2)

5 kg

F1 = 10 N

F2 = 2N

• Total force acting on the block?

FT = F1 + (- F2)

FT = 10N + (- 2N) = 8N

• Acceleration of the block?

F = ma

a = F/m

a = 8N/5kg

a = 1.6 m/s2

1. Mass and Weight
• Mass (kg) => The property of matter that determines its resistance to a change in motion. The measure of an objects inertia.
• Weight (N) => A measurement of the gravitational force acting on an object.
1. How are mass and weight related?
2.

3. Why is gravitational acceleration independent of mass?

• Therefore, gravitational force is proportional to mass but acceleration is inversely proportional to mass.
1. Newton’s third law of motion
• action/reaction principle
• For every action there is an equal and opposite reaction
• Force comes from interaction with other objects
1. How to identify the different forces
2. The forces that cause a car to accelerate
1. Applications of Newtons Laws
2. Newton’s law of universal gravitation
• The gravitational force between two objects is proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the masses.

G m1m2

F =
r2

F1 F2

Mass 1 Mass 2

r