Newton’s Laws of Motion
Newton’s First Law of Motion:
An object at rest remains at rest and an object in motion
remains in motion at constant speed and in a straight line
unless acted on by an unbalanced force.
Part I: Objects at Rest
-Objects at rest mean they are not moving
Ex: chair on the floor, a plane on a runway
-Objects will not start moving until a push or a pull is
exerted on them.
Ex: You tend to fall backward when your school bus
starts to move.
Part II: Objects in Motion
-An object moving at a certain velocity will continue to
move forever at the same speed and in the same direction
unless some unbalanced force acts on it.
Ex: Riding in a bumper car…..car stops, but you
continue to move forward until your seat belt
stops you.
www.physicsclassroom.com/mmedia/newtlaws/il.html
Friction and Newton’s First Law
Friction often prevents us from observing Newton’s First Law on
everyday objects. (meaning: we don’t see things moving forever)
Ex: You want to give your desk a push so you can see it move
forever across the floor. But when you do, it quickly comes to a
stop.
WHY?
Because an unbalanced force acted on the desk to stop its
motion. That unbalanced force is FRICTION!!!
Inertia is Related to Mass
Newton’s first law is sometimes called the law of inertia
Inertia: the tendency of all objects to resist any change in motion
Inertia is the reason objects at rest remain at rest and objects in motion
remain in motion.
Inertia is the reason you slide toward the side of a car when the
driver makes a sharp turn.
Mass Is a Measure of Inertia
An object with small mass has less inertia
An object with large mass has greater inertia
This is why it is easier to start and to change the motion of an object
with a small mass.
Which of these two cars would be easier to push and stop once it
was moving?
Matchbox car
Elvis’s pink cadillac
Newton’s Second Law of Motion
The acceleration of an object depends on the
mass of the object and the amount of force
applied.
Part I: Acceleration Depends on Mass
Acceleration decreases as its mass increases
Acceleration increases as mass decreases
Example: You are pushing a shopping cart at the
grocery store. At the beginning of your shopping
trip, you exert a small force on the cart to accelerate
it. (smaller mass = greater acceleration)
Exert the same amount of force when the cart is full
and the cart will not accelerate as much. (greater mass
= smaller acceleration)
http://www2.hawaii.edu/~kobatake/secondlaw4.html
Part II: Acceleration Depends on Force
Acceleration increases as the force on it increases
Acceleration decreases as the force on it decreases
Example: When pushing the full shopping cart,
if you push harder (greater force), the cart will
move faster.
If you push the full shopping cart with less
force, the cart will move slower.
**The acceleration is always in the same direction as the force
applied. The shopping cart moved forward because the push was in
a forward direction
Expressing Newton’s Second Law Mathematically
Newton’s second law shows the relationship between:
1. Acceleration = (a)
2. Mass = (m)
3. Force = (F)
Formula to find acceleration: (Force divided by mass)
a = F /m
Formula to find Force: (mass times acceleration)
F =m*a
Formula to find mass: (Force divided by acceleration)
m = F/a
http://www2.hawaii.edu/~kobatake/chpt3review.html
Newton’s second law explains why objects fall to Earth
with the same acceleration (9.8 m/s)
Less mass
More mass
Less Gravitational force
More Gravitational force
Less inertia = easier to
move
More inertia = harder to
move
Second-Law Problems
a=F/m
F=m*a
m=F/a
1. What is the acceleration of a 7kg mass if a
force of 68.6 N is used to move it toward
Earth?
A=F/m
A= 68.6 / 7kg
A=9.8 m/s/s
a=F/m
F=m*a
m=F/a
2. What force is necessary to accelerate a
1,250 kg car at a rate of 40 m/s/s?
F = m*a
F = 1,250 kg * 40 m/s/s
F = 50,000 N
a=F/m
F=m*a
m=F/a
3. What is the mass of an object if a force of
34 N produces an acceleration of 4 m/s/s?
m = F/a
m = 34 N / 4
m = 8.5 kg
Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the second
object exerts an equal and opposite force on the first.
Simply stated:
For every action, there is an equal and opposite reaction
*All forces act in pairs (action and reaction)
*If a force is exerted, another force occurs (equal in size and opposite
in direction).
Action and reaction force pairs occur even if there is no motion.
Your weight pushing down on
the chair is the action force
The force exerted by the
chair that pushed up on
your body and is equal to
your weight is the reaction
force.
Force Pairs Do Not Act on the Same Object
Remember:
A force is always exerted by one object on another object.
If action and reaction forces acted on the same object, the net force
would always be zero and nothing would ever move.
Example:
Action force was exerted on the water by the swimmer’s hands and feet
Reaction force was exerted on the swimmer’s hands and feet by the
water
More Examples of Action and Reaction Force Pairs
Momentum Is a Property of Moving Objects
Momentum: is a property of a moving object that depends on the
object’s mass and velocity.
The more momentum an object has, the harder it is to stop the object or
change its direction.
If an object has more mass, it has more momentum.
If an object has a greater velocity, it has more momentum.
http://www2.hawaii.edu/~kobatake/thirdlaw2.html
http://www.physicsclassroom.com/class/newtlaws/U2L4b.html
Momentum is Conserved
When a moving object hits another object, some or all of the momentum
is transferred to the other object.
Example: You are playing pool. You hit the cue ball. It travels across
the pool table and hits another ball. That ball starts moving, but the cue
ball stops.
All of the cue ball’s momentum was transferred to the other pool ball.
That ball now moved away with the same amount of momentum the cue
ball had.
Law of conservation of momentum: any time two or more objects
interact, they may exchange momentum, but the total amount of
momentum stays the same.
Explain the conservation of momentum in this photo
The bowling ball rolls down the lane with a certain amount of
momentum
When the ball hits the pins, some of the ball’s momentum is transferred
to the pins and the pins move off in different directions.
Some of those pins that were hit with the ball will go on to hit other pins,
transferring the momentum again.
A Review of Newton’s Laws of Motion
Newton’s First Law:
An object at rest remains at rest and an object in motion
remains in motion at constant speed and in a straight
line unless acted on by an unbalanced force.
Newton’s Second Law:
The acceleration of an object depends on the mass of
the object and the amount of force applied.
Newton’s Third Law:
For every action, there is an equal and opposite
reaction
A Check-Up on Newton’s Laws
1. How does Newton’s second law explain why it is easier to
push a bicycle than to push a car with the same acceleration?
The bicycle has a smaller mass, so a smaller force is required to give
it the same acceleration as the car.
2. What are two ways that you can increase the acceleration
of a loaded grocery cart?
You can increase the force applied to the cart, or you can decrease
the mass of the cart by removing some of the objects from it.
3. How does Newton’s third law explain how a rocket takes
off?
The hot gases expelled from the back of the rocket produce a
reaction force on the rocket that lifts and accelerates the rocket.