newton’s laws: force and motion
TRANSCRIPT
Newton’s Laws:
Force and Motion
The First Law: Force and Inertia
The Second Law: Force, Mass and
Acceleration
The Third Law: Action and Reaction
The First Law: Force and Inertia
Investigation Key Question:
How does the first law apply to objects at rest
and in motion?
Force changes motion
A force is a push or pull, or any action that is able
to change motion.
The definition of force
The simplest concept of force is a push or a
pull.
On a deeper level, force is the action that has
the ability to create or change motion.
5.1 Force Force is an action that can change motion.
— A force is what we call a push or a pull, or any
action that has the ability to change an object’s
motion.
— Forces can be used to increase the speed of an
object, decrease the speed of an object, or
change the direction in which an object is
moving.
Newton’s first law
Newton’s first law says that objects continue the
motion they already have unless they are acted
on by a net force.
If the net force is zero, an object at rest will stay
at rest.
If an object is acted upon by unbalanced forces,
its motion will change.
Remember…Newton’s first law tells us that
motion cannot change without a net force.
Net force
Newton’s first law is often written in terms of the net force:
“An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.” According to these vectors, in what
direction is the net force?
Force changes motion
Forces can be used to increase or decrease the
speed of an object, or to change the direction an
object is moving.
Key Question:
Why are heavier objects harder to start or stop moving?
Law of inertia
Force, mass, and inertia
Some objects resist changes in motion more than others.
Inertia is the property of an object that resists changes in its motion.
The greater an object’s inertia, the greater the force needed to change its motion.
A bowling ball has more inertia than a golf ball.
Force, mass, and inertia
Inertia comes from mass.
Objects with more mass have more inertia and are
more resistant to changes in their motion.
A 5-kilogram bowling ball is 100 times as massive as
a 50 gram golf ball, so it has 100 times the inertia.
Inertia
Inertia is a term used to measure the ability of an
object to resist a change in its state of motion.
An object with a lot of inertia takes a lot of force to
start or stop; an object with a small amount of inertia
requires a small amount of force to start or stop.
The word “inertia” comes from the Latin word
inertus, which can be translated to mean “lazy.”
The net force
The motion of objects changes in response to the
total force acting on the object, including gravity and
any other forces that are present.
The net force in the horizontal direction
The term net force is used to describe the total of
all forces acting on an object.
When used this way, the word net means “total.”
The net force in vertical direction
Gravity exerts a force downward on the box.
The floor exerts an equal and opposite force upward
on the box.
The net force in vertical direction
The net force on the box in
the “up-down” direction is
zero.
When equal forces applied
to the same object are in
opposite directions they
cancel.
Applications of Newton’s First Law
Two very important safety
features of automobiles are
designed with Newton’s first
law in mind: seat belts and air
bags.
Both supply a restraining force
to counteract your inertia and
to slow your body down.
Applications of Newton’s First Law
Prior to the invention of cup
holders, drink containers left
on the dash obeyed the first
law of motion and made quite
a mess.
Can you think of other
applications of Newton’s first
law?
The Second Law:
Force, Mass, and Acceleration
Key Question:
What is the relationship between
force, mass, and acceleration?
Newton’s second law
Newton’s first law tells us that motion cannot
change without a net force.
According to Newton’s second law, the
amount of acceleration depends on both the
force and the mass.
Newton's Second Law
If you apply more force to an object, it accelerates at a higher rate.
Newton's Second Law
If the same force
is applied to an
object with greater
mass, the object
accelerates at a
slower rate
because mass
adds inertia.
Acceleration and force
The second law says that
acceleration is
proportional to force.
If force is increased or
decreased, acceleration
will be increased or
decreased by the same
factor.
Applying the second law Keep the following important
ideas in mind:
1. The net force is what causes
acceleration.
2. If there is no acceleration, the
net force must be zero.
3. If there is acceleration, there
must also be a net force.
4. The force unit of newtons is
based on kilograms, meters,
and seconds.
Newton's Second Law
a = F
m
Force (newtons, N)
Mass (kg)
Acceleration (m/sec2)
Three forms of the second law
When using the second law, the force that appears is the net force.
Consider all the forces that are acting and add them up to find the net force before calculating any accelerations.
Using units in calculations
In terms of solving physics problems, use the
following units when using force in newtons:
— mass in kilograms (kg)
— distance or position in meters (m)
— time in seconds (s)
— velocity in meters per second (m/s)
— acceleration in meters per second per second
(m/s2)
A car has a mass of 1,000 kg. If a net
force of 2,000 N is exerted on the
car, what is its acceleration?
Using Newton’s second law
1. Looking for: … the car’s acceleration.
2. Given: …car’s mass (m= 1,000 kg) and the net force (Fnet = 2,000N).
3. Relationship: Use: a = F
m
4. Solution: a = 2,000N = 2 kg• m/s2 = 2 m/s2
1,000 kg kg
5.2 Finding force from acceleration
Wherever there is acceleration there must also be
force.
Any change in the motion of an object results
from acceleration.
Therefore, any change in motion must be caused
by force.
1. You asked for the force (F).
2. You are given the mass (m) and acceleration (a).
3. The second law applies: a = F ÷ m
4. Plug in the numbers. Remember: 1 N = 1 kg·m/s2.
Calculating force
An airplane needs to accelerate at 5 m/sec2 to reach take-off speed before reaching the end of the runway. The
mass of the airplane is 5,000 kilograms. How much force is needed
from the engine?
Newton's Third Law
Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction.
Action and reaction forces act on different objects, not on the same object.
Newton's Third Law
Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction.
Action and reaction forces act on different objects, not on the same object.
The forces cannot cancel because they act on different objects.
The Third Law: Action and Reaction
“For every action there is an
equal and opposite reaction.”
This statement is known as
Newton’s third law of motion.
Newton’s third law discusses
pairs of objects and the
interactions between them.
Forces occur in pairs
The two forces in a pair are called action and reaction.
Anytime you have one, you also have the other.
If you know the strength of one you also know the strength of the other since both forces are always equal.
Forces occur in pairs
The astronauts working on the space station have a
serious problem when they need to move around in
space: There is nothing to push on.
One solution is to throw something opposite the
direction you want to move.