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Newton’s First & Second Laws of Motion

Chp 12.2 Pg 363-369

Newton’s First Law of Motion

 According to the 1st Law of motion, the state of motion of an object does not change as long as the net force on the object is zero  Unless an unbalanced force acts,

an object at rest will remain at rest and an object in motion will remain in motion at constant velocity

Inertia  Newton’s 1st Law is known as the Law of

Inertia

 Inertia is the tendency of an object to resist change in its motion.  An object at rest stays at rest and an object

in motion stays in motion at constant velocity

The coin stays in place above the cup, then falls

The coin wants to stay at rest

Inertia continued

The man wants to stay in motion. So he continues moving forward at constant velocity. However, gravity begins to act on him while he is airborne and eventually he will fall to the ground

Inertia and Car Accidents

Wear your seatbelt!!!!!

Head keeps moving

Newton’s Second Law

 Mass is a measure of the inertia of an object that depends on the amount of matter the object contains. (an amount of matter)

 Acceleration of an object is equal to the net force acting on it divided by the object’s mass

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2nd Law Formula variations and units  Force = mass x acceleration   F = m x a units = Kg x m/s2 = Kgm/s2 = N

 Acceleration = Net force Mass

  a = _F_ units = N or Kgm/s2 = m/s2

m Kg Kg

 Mass = force

acceleration  M = F units = N or Kg m/s2 = Kg a m/s2 m/s2

A car with a mass of 1200 kg accelerates at a rate of 2 m/s2 in the forward direction. What

is the net force acting on the car? Given info: m = 1200 kg

a = 2 m/s2

Step 1 Step 2 Step 3

F = m x a F = 1200 kg x 2 m/s2 F = 2400 kgm/s2

or 2400 N

A car with a mass of 1000 kg accelerates when the light turns green. If the net force on

the car is 2000 N what is the car’s acceleration?

  Given info: m = 1000 kg

F = 2000N

Step 1 Step 2 Step 3

a = F a = 2000 N kgm/s2 a = 2 m/s2

m 1000 kg

A 30 N force accelerates a boy on a skateboard at 0.5 m/s2. What is the mass of

the boy and skateboard?

  Given info: a = 0.5 m/s2

F = 30 N

Step 1 Step 2 Step 3

m = F m = 30 N kgm/s2 m = 60 kg

a 0.5 m/ss

Weight and Mass  Weight is the force of gravity acting on

an object. It is the product of the object’s mass and acceleration due to gravity acting on it.  Mass and weight are not the same. The

unit for mass is kg and unit for weight is N

 Mass is the measure of inertia on an object (an amount of matter); weight is a measure of the force of gravity acting on an object

Weight problems  Weight = mass x acceleration due to

gravity

 w = m x g  Units = kg x m/s2 = kgm/s2 = N

 What is the weight of a man if his mass is 112 kg and gravity is 9.8m/s2?

w = m x g

w = 112 kg x 9.8 m/s2

w = 1100 kgm/s2 or 1100N

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Weight changes if gravity changes

  If gravity the moon is 1/6 that of Earth’s gravity the weight of a 60 N rock on Earth will only be 10 N on the moon.

  What would the weight of a dog be on the moon if its weight on earth is 54 N?   54 N x 1 = 9 N 6

  No matter the gravity,

mass is always the same