Transcript Chapter 3--FORCES - Scioto Valley Home

Section 1: NEWTON’S SECOND LAW
REVIEW—NEWTON’S 1ST LAW
(LAW OF INERTIA)

If an object is moving
at constant velocity, it
keeps moving at that
velocity unless a net
force acts on it; if an
object is at rest, it
stays at rest, unless a
net force acts on it.
INERTIA
Inertia is the tendency of
an object to resist any
change in motion.
 The greater the mass of
an object, the greater the
inertia.
 ↑ MASS = ↑ INERTIA

NEWTON’S SECOND LAW

The net force acting on an object
causes the object to accelerate in the
direction of the net force. The
acceleration of an object is determined
by the size of the net force and the
mass of the object. (LONG VERSION)
OR

This law describes how force, mass, and
acceleration are connected. (SHORT
VERSION)
FORCE AND ACCELERATION
The greater the force that is
applied to an object, the
greater the acceleration will
be.
 Examples…
Ball thrown hard = greater
force and acceleration
Ball thrown gently = less force
and acceleration

FORCE AND MASS
If you throw a softball and a
baseball as hard as you can…why
don’t they have the same speed?
 The difference is due to their
masses.
 MASS of softball=
0.20 kg
 MASS of baseball=
0.14 kg

NEWTON’S 2ND LAW
The acceleration of an object depends on its
mass, as well as the force, exerted on it.
 Remember—If more than one force acts on an
object, the forces combine to form a net force.
 NEWTON’S 2ND LAW can be written as the
following equation
 Acceleration =net force

mass
OR
a =F
m
SI UNITS

The unit of MASS is the kg

The unit of ACCELERATION is m/s2

The unit of FORCE is kg x m/s2, which is
also called the Newton (N)
PROBLEM
A student pedaling a
bicycle applies a net
FORCE of 200 N.
 The MASS of the rider and
the bicycle is 50 kg.
 What is the
ACCELERATION of the
bicycle and the rider?

SOLVING FOR ACCELERATION



a = F
m
a = 200 N =
50 kg
4 m/s2
PROBLEM

MASS of a tennis ball is
0.06 kg

ACCELERATION of the
ball leaving the racket is
5,500 m/s2

What is the FORCE that is
applied to the racket?
SOLVING FOR FORCE

F = ma

F = (0.06 kg) (5,500 m/s2)

F = 330 N
PROBLEM
A student riding a
skateboard applies a net
FORCE of 180 N.
 The ACCELERATION of
the skateboard and the
rider is 3 m/s2.
 What is the MASS of the
skateboard and the rider?

SOLVING FOR MASS

m =F
a

m = 180 N (kg x m/s2)
3 m/s2

m = 60 kg
FRICTION
The force that opposes motion
between 2 surfaces that are touching
each other
 The amount of friction depends on 2
factors
1. the types of surfaces
2. the force pressing the surfaces
together

FRICTION

What causes friction?
If 2 surfaces are pressed tightly together,
welding or sticking occurs in those areas
where the highest bumps come into
contact with each other. These areas
where the bumps stick together are
called microwelds and are the source of
friction.
MICROWELDS
STATIC FRICTION
Friction between 2 surfaces that are
NOT moving past each other.
 MICROWELDS have formed between
the bottom of the box and the floor.

SLIDING FRICTION
Force that opposes the motion of 2
surfaces sliding past each other.
 Caused by MICROWELDS constantly
breaking and then forming again as
the box slides across the floor.

ROLLING FRICTION
Friction between a rolling object and
the surface it ROLLS on.
 Because of rolling friction, the
wheels of a train rotate when they
come into contact with the track,
rather than sliding over it.

ROLLING FRICTION

Rolling friction is usually much less
than static or sliding friction. That is
why it is easier to pull a load in a
wagon, rather than pushing it on the
ground.
AIR RESISTANCE

When an object falls
toward Earth, it is
pulled downward by
the force of gravity;
however, another
force called air
resistance acts on
objects that fall
through the air.
FORCES THAT OPPOSE MOTION

Friction, as well as air resistance,
acts in the direction opposite to that
of the object’s motion.
AIR RESISTANCE

The amount of air
resistance on an
object depends on
the speed, size,
and shape of the
object (NOT
MASS).
Section 2--GRAVITY
GRAVITY

Anything that has mass is attracted by
the force of gravity.

According to the law of gravitation, any
2 masses exert an attractive force on
each other
GRAVITY
The attractive force depends on the
mass of the 2 objects and the distance
between them.
 Gravity is 1 of the 4 basic forces. (The
other basic forces are the
electromagnetic force, the strong
nuclear force, and the weak nuclear
force.

MASS(M) VS. DISTANCE(D)
If the mass of either of the objects
increases, the gravitational force between
them increases. (Left pic) ↑M = ↑GF
 If the objects are closer together, the
gravitational force between them
increases. (Right pic)↓ D=↑GF

GRAVITATIONAL ACCELERATION
Near Earth’s surface, the gravitational
attraction of Earth causes all falling
objects to have an acceleration of 9.8
m/s2.
 As a result, all objects fall with the same
acceleration rate, regardless of their
mass.

GRAVITATIONAL ACCELERATION
According to the second law, the force
on an object that has an acceleration of
9.8 m/s2 is as follows:
F = m x 9.8 m/s2

A force has a direction.
 The force of gravity is always directed
downward.

WEIGHT (W)

The gravitational force exerted on an
object is called the object’s weight.

You can find the gravitational force or
weight, by using Newton’s 2nd Law…
EQUATIONS
Gravitational Force(F)=mass(m) x acceleration
due to gravity(a=9.8
2
m/s )
OR
 Weight(W)=mass(m) x acceleration
due to gravity(a=9.8 m/s2)

***Because gravitational force = weight;
mass ≠ weight
***Massstays the same; WEIGHTchanges due
to gravity
WEIGHT AND MASS ARE NOT THE
SAME.

WEIGHT is a force due to gravity.

WEIGHT changes when gravity
changes.

MASS is a measure of the amount of
matter in an object.

MASS always stays the same.
PROBLEM
Mass of a person = 50 kg
What is the weight of this person?

Weight (W)=mass(m) x 9.8 m/s2
 W = 50 kg x 9.8 m/s2 =
 490 kg x m/s2 or 490 N

PROBLEM
What is my MASS?
 Mass = 150 poundsConvert to
kilograms
 1 kg = 2.2 lbs (CONVERSION
FACTOR)
 150 lbs x 1 kg
2.2 lbs
 = 68.2 kg

PROBLEM
What is my WEIGHT?
 W = mass x 9.8 m/s2 ( accel. due to
gravity)


W = 68.2 kg x 9.8 m/s2
W = 668.4 kg x m/s2
OR
668.4 N

PROJECTILE MOTION
Anything that is thrown or shot through
the air is called a projectile.
 Projectiles follow a curved path,
because they have horizontal and
vertical velocities.
 Example—
Force from a thrown ball—horizontal
velocity
Force from gravity—vertical velocity
RESULT: curved path

CENTRIPETAL FORCE
Centipetal force is a force directed
toward the center of the circle for an
object moving in a circular motion.
 The word “centripetal” means to
move toward the __________.
 Examples—amusement park rides,
bucket/water

Section 3—Newton’s Third Law of Motion
NEWTON’S THIRD LAW OF
MOTION
When one object exerts a force on a
second object, the second one exerts
a force on the first that is equal in
size and opposite in direction.
OR
 Another way to say this is “to every
action force there is an equal and
opposite reaction force.”

EXAMPLES
Trampoline—you exert a force
downward and the trampoline exerts
an equal force upward; Rocket
propulsion
 ***Even though the forces are equal,
they are not balanced, because they
act on different objects

MOMENTUM
Momentum is the property of a moving
object.
 The momentum of an object is the
product of its mass and velocity.
 Momentum is represented by the
symbol (p) and can be calculated as
follows:
Momentum (p) = mass (m) x velocity
(v)
 The SI unit for momentum is kg x m/s

THE LAW OF CONSERVATION OF
MOMENTUM

The law of conservation of
momentum states that the total
momentum stays the same unless an
outside force acts on the objects.
Momentum, however, can be
transferred from one object to the
other.
(P) BEFORE VS. (P) AFTER