Download force

Modified Fall 2012
Chapter 3-4
Force, Mass and Acceleration
•
•
•
•
Force
Defined as a push or a pull.
Net forces cause acceleration (change in motion)
Unit of force is the Newton (N)
Net force is the total amount of force acting on
an object
1) Can be balanced or unbalanced
a) balanced forces occur when the net force
is equal to zero and do no change the
motion of the object
b) unbalanced forces when the net force is
NOT equal to zero and do change the motion
of the object.
Calculating the Net Force
• When the forces are acting in the same
direction, add to get the net force.
2N + 2N
=
4N
The above object will accelerate
because there is a net force acting on
it.
• When the forces are acting in opposite
directions, subtract to get the net force.
2N -
2N
=
0N
This object will not accelerate because
the net force is equal to zero.
Equilibrium  occurs when the net force
is zero, the object will not change its
motion
Calculate the net force
1) 3N
4N
Calculate the net force
1) 3N
4N
2)
2N
2N
7N to the left
Calculate the net force
1) 3N
4N
7N to the left
2)
2N
4N up
2N
3)
3N
4N
3N
4N
3N 4N
7N
1N to the left
3N
4N
1N to the left
3N 4N
Force UP  3 + 4 = 7
Force Down = 7
Net Force = Force Up – Force Down
7 – 7 = 0N
7N
5) Which direction will the block move?
5N
A
5N
C
B
5) Which direction will the block move?
5N
A
5N
C
B
6) What is the net force on the block?
6N
9N
a)
b)
c)
d)
Between 6N and 9N
Less than 6N
Exactly 9N
Greater than 9N
6) What is the net force on the block?
6N
To Solve: Use the
Pythagorean theorem.
9N
6N
a)
b)
c)
d)
Between 6N and 9N
9N
Less than 6N
a2 + b2= c2
Exactly 9N
9 2 + 62 = c 2
Greater than 9
81 + 36 = 122
122 = 11 N
Friction – A Force
• Defined as a force that results from
the movement of 2 objects in contact.
• Caused by the surface irregularities
between the 2 objects.
rough surface
smooth surface
has high friction
(carpet)
has low friction
(ice)
• Friction is what causes things to wear
out
Examples: break pads, erosion
• Friction resists motion
motion of block is to the right
The force of friction acts to the left
• Friction is what enables moving objects
to come to rest
Types of Friction
1) Static Friction  occurs between
surfaces that are not moving. This is
the friction you must overcome in order
to start motion.
a) static friction is the most difficult
friction to overcome.
2) Sliding (Rolling) Friction  occurs
between surfaces that are moving
3) Air Resistance
a) friction due to air pushing up on a
falling object
b) depends on size, shape, and speed of
the object
air resistance
gravity
c) Terminal velocity  occurs when the
force of gravity down equals the air
resistance up.
1) Highest velocity a falling object will reach
The Absence of Air Resistance
• In the absence of air resistance all objects
will fall at the same rate.
• If a hammer and a feather are dropped at
the same time on the moon, both will land
at the same time because there is no air
resistance.
Gravity – Another Force
• Is a force that every object exerts on every
other force.
• Is the weakest force in nature
• The Law of Universal Gravitation The
force of gravity depends on the directly on
the masses of the two objects and
inversely on the square of the distance
between the objects
*Increase mass = increase gravity
*Increase distance = decrease gravity
Law of Universal Gravitation
Formula
Force of
gravity
mass 2
mass 1
F = G (m1m2)
d2
Universal
constant
distance squared
Gravity & Weight
• Gravity is the force the earth exerts on you
• The force of gravity changes with location
• Weight is a measure of the force of gravity
acting on a mass
• Formula: Weight = mass X gravity
W=mXg
• Units:
Weight = Newtons (N)
Mass = kilogram (kg)
Gravity = m/s2
on earth = 9.8m/s2
W
m g
Definition
Property
Unit
Mass
Weight
The amount
of matter in
an object
Measure of the
force of gravity
on an object
Mass
Weight
Definition
The amount
of matter in
an object
Measure of the
force of gravity
on an object
Property
Does not
change based
on location
Changes based
on location
Unit
Mass
Weight
Definition
The amount
of matter in
an object
Measure of the
force of gravity
on an object
Property
Does not
change based
on location
Changes based
on location
Unit
Kilogram
Newton (SI)
or
Pound
Newton’s First Law
• Two Parts:
1) An object at rest will stay at rest unless
a net force acts on it.
2) An object in motion will continue in
motion in a straight line at the same
speed unless a net force acts on it.
• In other words – objects resist changes in
motion.
• Inertia is the ability of an object to
resist changes in its motion.
1) depends on mass
a) the more mass = the more
inertia
Tennis Ball
Bowling Ball
• Newton’s 1st Law is known as the Law
of Inertia.
Newton’s
nd
2
Law
• Relates force and mass to acceleration.
• Formula:
acceleration = force
a=F
mass
m
• Units: Force (N)
mass (kg)
acceleration (m/s2)
F
(We will solve some sample problems later )
a m
• Formula tells us:
1) Net forces cause acceleration.
a) force and acceleration are
directly proportional.
1) F = A (Big Force = Big Acceleration)
2) f = a (Little Force = little Acc)
2) mass resists acceleration.
a) mass and acceleration are
inversely proportional
1) M = a (Big mass = little acceleration)
2) m = A (small mass = big acc)
Newton’s 3rd Law
• States for every action there is an equal
and opposite reaction.
• Relates forces as pairs. Each force pair is
made up of an action force and a
reaction force.
Example: Student sitting in chair
Action force: Student pushes down on
chair
Reaction force: Chair pushes up on
student
State the reaction force
• Action force: Bat hits ball
Reaction force:
State the reaction force
• Action force: Bat hits ball
Reaction force: Ball hits bat
• Action force: You push a box to the
right
Reaction force:
State the reaction force
• Action force: Bat hits ball
Reaction force: Ball hits bat
• Action force: You push a box to the
right
Reaction force: Box pushes you to
the left
• Action force: Book pushes down on
table
Reaction force:
State the reaction force
• Action force: Bat hits ball
Reaction force: Ball hits bat
• Action force: You push a box to the
right
Reaction force: Box pushes you to the
left
• Action force: Book pushes down on
table
Reaction force: Table pushes up on
book
• Remember, Newton’s 3rd Law states
that forces are EQUAL and opposite.
• Since forces are equal, that means that
you can only apply a force to an object
that an object can apply to you.
Check Understanding - Newton’s
Three Laws
1) Imagine a place in the cosmos far
from all gravitational and frictional
forces. Suppose an astronaut in that
place throws a rock. The rock will:
a) gradually stop
b) continue in the same direction and
the same speed
2) A 2-kg object is moving horizontally with a
speed of 4 m/s. How much force is required
to keep the object moving with the same
speed and in the same direction?
Zero. Think about if friction could be
eliminated~once an object is in motion, it will
continue in motion at a constant velocity (same
speed and straight line). Newton’s 1st Law INERTIA
Remember Galileo postulated that if friction could be
entirely eliminated an object in motion would continue to
move
3) Mac and Tosh are arguing in the
cafeteria. Mac says that if he throws his
jello with a greater speed, it will have
greater inertia. Tosh argues that inertia
does not depend on speed, but rather on
mass. With whom do you agree?
Tosh
4) If you were in a weightless environment
in space, would it require a force to set
an object in motion?
Yes. Newton’s 1st law states: an object
at rest will remain at rest unless a net
force acts on it.
5) Mr. Wegley spends most Sunday
afternoons at rest on the sofa watching
football games and consuming large
quantities of food. What effect (if any)
does this situation have upon his
inertia?
His inertia will increase. As he eats, he
will increase his mass. As his mass
increases so does his inertia.
6) When a bug and a car have a collision,
both experience the same force. Which
object, the bug or car, will have little
change in its motion? Why?
The car will have very little change in
its motion because it has more inertia.
7) While driving Anna Litical observed a
bug striking the windshield of a car.
The bug hits the windshield and the
windshield hits the bug. Which of the
two forces is greater:
A) The force on the bug
B) The force on the car
C) Neither
C. Newton’s 3rd Law – equal and
opposite forces
Momentum
• Inertia in motion
• Formula:
momentum = mass X velocity
p=mXv
(momentum)
(velocity)
p
(mass)
Units
momentum = kg*m/s
mass = kg
velocity = m/s
m
v
• Mass and momentum are directly
proportional.
1) increase mass = increase momentum
Explains why a bike is easier to stop than
a car traveling at the same speed.
Bike
p = mv
Car
p = mv
• Velocity and momentum are directly
proportional
1) increase velocity = increase
momentum
Explains why a slow moving bike is
easier to stop than a fast moving bike
Slow Bike
Fast Bike
p = mv
p = mv
The Law of Conservation of
Momentum
• States: As long as interacting objects
are not influenced by outside forces
(like friction), the total momentum
does not change.
Momentum is transferred between
objects
Collisions
• Examples of the Law of Conservation
of Momentum
• Two types:
1) Elastic  objects bounce off one
another
ex: billiard balls
2) Inelastic collisions  objects stick
together
ex: car crashes