Download Ch. 2 The Laws of Motion

Ch. 2 The Laws of Motion
Lesson 1 Gravity and Friction
Force - A push or pull
we pull on a locker handle
push a soccer ball or on the
computer keys
Contact force - push or pull on one object
by another that is touching it.
Noncontact force - a force that one object
can apply to another object without
touching it
Some contact sports?
Noncontact sports?
Forces have both strength and direction.
can be shown by arrows
The SI unit of force is the Newton (N)
Gravity - an attractive force that exists
between all objects that have mass.
Mass - the amount of matter in an object
The law of universal gravitation states that
1. All objects are attracted to each other by
a gravitational force.
2. The strength of the force depends on the
mass of each object and the distance
between them.
As mass increases, gravitational forces
increase
As distance increases, gravitational forces
decrease
Weight - the gravitational force exerted on
an object
Friction - a force that resists the motion of
two surfaces that are touching
there are several types:
Static Friction - friction between two
objects that are not moving
Sliding Friction - friction between two
objects that are sliding past each other
Fluid Friction - friction between a surface
and a fluid (liquid or gas)
in air - air resistance
Causes of friction - roughness of the surfaces
- weak electrical charges between
atoms and molecules
How can friction be reduced?
- reduce the contact area between the
microscopic bumps on the surface
- reducing the surface area
(air resistance)
Lesson 2 Newton’s First Law
Review Forces - a push or pull
Net Force - the combination of all the forces
acting on an object
need reference direction
Balanced Forces - forces acting on an object
that combine and form a net force of zero
Unbalanced Forces - forces acting on an
object that combine and form a net force that is
not zero
Newton’s First Law of Motion
Every object continues in a state of rest or of
uniform speed in a straight line unless acted on
by a nonzero net force.
If the net force on an object is zero, the
motion of the object does not change.
Inertia - the tendency of an object to resist a
change in its motion
Lesson 3
Newton’s Second Law
Forces change an object’s motion
Only unbalanced forces change an object’s
velocity
Change in velocity means a change in speed,
or direction or both.
Velocity (from last chapter) is speed and
direction of an object.
Acceleration is the change in velocity over
time.
Unbalanced forces can make an object
accelerate by changing its speed, its
direction, or both.
Newton’s Second Law
The acceleration of an object is equal to
the net force acting on the object divided by the
object’s mass.
a = F/m
Circular Motion - any motion in which an
object is moving along a curved path
Centripetal Force - in circular motion, a force
that acts perpendicular to the direction of
motion, toward the center of the curve
Lesson 4
Newton’s 3rd Law
The 3rd law states: when one object exerts a
force on a second object, the second
object exerts an equal force in the
opposite direction on the first object.
Force Pair - the forces two objects apply to
each other
Momentum is the measure of how hard it is to
stop a moving object.
The Law of Conservation of Momentum
The total momentum of a group of objects
stays the same unless outside forces act on the
objects.
Momentum Equation
momentum = mass x velocity
p=m x v
or v = p/m
or m = p/v
momentum before collision = momentum after
collision
momentum of 2 cars
before collision
=
momentum of both
cars after collision
Elastic Collision - objects bounce off each
other
Inelastic Collision - objects stick together
A ball with a momentum of 16 kg m/s strikes a
ball at rest. What is the total momentum of
both balls after the collision?
first ball p = 16 kg m/s
second ball at rest p = 0
total of both before impact p = 16 kg m/s
since momentum is conserved - same after
collision as before, after collision has to be
16 kg m/s also.
The force of a bat on a ball and the force of a
ball on a bat are _____ _____.
A 2.0 kg ball rolls to the right at 3.0 m/s.
A 4.0 kg ball rolls to the left at 2.0 m/s.
What is the momentum of the system after a
head on collision of the two balls?
Reference direction - to the right
Total momentum before collision:
(2.0 kg)(3.0 m/s) + (4.0 kg)(-2.0m/s)
6.0
+
-8.0
= -2 kg
m/s
What is Newton’s 1st Law of Motion?
An object at rest remains at rest, or an object in
motion remains in motion, in a straight line,
unless an unbalanced force acts on it.
What is inertia?
The tendency of an object to resist a change in
motion.
What force resists motion?
Friction
The gravitational force exerted on an object is
its weight or mass?
Weight.
What is mass?
The amount of matter in an object
The strength of gravitational force depends on
________ and ___________
mass of the objects and distance between
them
What is the direction of the force of air
resistance?
UP
What is Newton’s 2nd Law of Motion?
The acceleration of an object is equal to the net
force acting on the object divided by the mass
a = F/m
or
F=mxa
or
m = F/a
A cat pushes a 0.25 kg toy with a net force
of 8 N. According to Newton’s 2nd Law, what
is the acceleration of the toy?
a = F/m = 8 N / 0.25 kg
= 8/.25
= 32
What is the difference between a balanced and
unbalanced forces?
Forces acting on an object and combine and
form a net force of 0 are balanced forces.
Forces that combine and add to a value that is
not 0 are unbalanced forces.
What is Newton’s 3rd Law of Motion?
When one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object.
How was Newton’s 3rd law demonstrated in
the balloon lab?