Download 2.2 Newton`s Laws of Motion I. Newton`s First Law of Motion “An

2.2 Newton’s Laws of Motion
I. Newton’s First Law of Motion
“An object at rest remains at rest and an object in motion remains in motion at a
constant speed and in a straight line unless acted on by an unbalanced force.”
A. Objects at Rest
1. an object that is not moving
2. objects at rest stay at rest until acted on by an unbalanced force
B. Objects in Motion
1. an object moving at a certain velocity will continue to move at the same speed
and in the same direction unless some unbalanced force acts on it
2. remember, velocity includes speed and direction
C. Friction and Newton’s First Law
1. friction is an unbalanced force that acts on objects to affect its motion
2. ex: rolling ball on grass, car decelerating, etc
D. Inertia is Related to Mass
1. First Law = Law of Inertia
2. inertia: tendency of all objects to resist any change in motion
E. Mass is a Measure of Inertia
1. small mass = less inertia
2. easier to start and change motion of an object with little mass than that with
more mass
II. Newton’s Second Law of Motion
“The acceleration of an object depends on the mass of the object and the amount
of force applied.”
A. Acceleration Depends on Mass
1. An object’s acceleration decreases as its mass increases
2. its acceleration increases as its mass decreases
B. Acceleration Depends on Force
1. acceleration increases as the force on it increases
2. acceleration decreases as the force decreases
3. acceleration of an object is always in the same direction as the force applied
4. to change direction, exert a force in the direction you want the object to go
C. Expressing Newton’s Second Law Mathematically
1. a = F/m (acceleration equals force divided by mass)
2. F = m * a
3. This is why objects fall to Earth with the same acceleration
4. less mass = less inertia = easier to move
III. Newton’s Third Law of Motion
“Whenever one object exerts a force on a second object, the second object exerts
and equal and opposite force on the first.”
A. Force Pairs Do Not Act on the Same Object
1. all forces act in pairs
2. action & reaction force pairs occur even when there is no motion
3. action and reaction forces do not act on the same object
4. If they did act on the same object, the net force would always be zero and
nothing would move
B. The Effect of a Reaction Can be Difficult to See
1. difficult to observe on falling objects
2. the force on an object is equal to the force of Earth, but the mass is smaller so
the acceleration of Earth is smaller than the acceleration of the object.
C. More Examples of Action & Reaction Force Pairs
1. rabbit’s legs exert force on Earth, Earth exerts an equal force on legs; rabbit
accelerates upward
2. bat exerts force on ball, ball flies outfield; ball exerts equal force on bat, but
batter exerts another force on the bat
IV. Momentum is property of Moving Objects
A. property of a moving object that depends on object’s mass and velocity
B. P = m * v
C. Momentum is Conserved
1. when a moving object hits another object, some or all of the momentum of the
first is transferred to the other
2. law of conservation of momentum: any time two objects interact, they
exchange momentum, but the total amount of momentum stays the same
D. Conservation of Momentum and Newton’s Third Law
1. when one object hits another, it is “action force”
2. reaction force is equal but opposite force exerted on the first object