Download Newton`s First Law

Forces and Motion
Force
FORCE
is a push or a pull
applied to an object that will
cause it to start moving, stop
moving or change its speed or
direction
Demonstration
Force
Force = Mass x Acceleration
 F = MA
 Force is measured in Newtons (N)
which is one kilogram meter per second
squared
 N = kg x m/s2

Newton’s First Law
(law of inertia)
MASS
is the measure of the
amount of matter in an object
measured
in grams (g)
or kilograms (kg)
Newton’s First Law
(law of inertia)
WEIGHT
is a measure of the
force of gravity on the mass of
an object
measured
in Newtons (N)
Force
But weight, what’s my mass?
 Please do not confuse the two.
 Weight is determined by the
acceleration due to gravity.
 If you were on another planet that had
less gravity, you would weigh less.

NET FORCE

In order for motion to occur, the
net force must be >0
10 N
10 N
10 N
20 N
m
=
m
10 N
=
m
10 N
=
m
20 N
0N
10 N
m
m
THE EQUILIBRIUM RULE
Scales pushing up
Examples of
Mechanical
Equilibrium:
Normal up
Weight down
Computer setting on a table
 Weighing yourself on a set of scales
 Hanging from a tree
Tree
 Car parked on an incline

pulling up
Friction
Weight down
Normal
Weight down
Weight down
The Equilibrium Rule
ΣF=0
SUPPORT FORCE
Normal up
Scales pushing up
Weight down
In the first example of mechanical equilibrium
the table supplied a force upward that was
called the normal force. It is a support force.
 Consider the second example of mechanical
equilibrium. The scales supply a support
force on the man.

Weight down
EQUILIBRIUM OF MOVING
THINGS
Equilibrium is a state of no change.
 If an object moves in a straight line with no
change in speed or direction, it is in equilibrium.

Examples:
Driving at constant velocity
Normal up
Air resistance
Force from road
Air
Resistance
Weight down
Terminal velocity in parachuting
Weight down
What do you weigh?

Weight on Other Planets
Force Problems
Let’s start with an easy one, your
weight.
 Remember that weight is relative – your
mass isn’t changing (the amount of
matter in you) but you weigh different
amounts because of gravity
 Gravity’s acceleration is 9.8m/s2
 On earth you take your weight to be
what it is

Force Problems
If you lived on another planet, such as
mars for example, the acceleration due
to gravity is 3.8m/s2
 In order to find out weight, we use the
following formula
w=mxg

Force Problems
Since gravity is a force (pulling you
towards the center of the planet) this is
technically a force problem
 My weight on earth is 185lbs, or 84kg
(just divide your weight by 2.2)
 That means that if we stick my weight
in and we know the acceleration due to
gravity here on earth, we can find out
my mass

Force Problems
w=mxg
 84kg x m/s2 = m x 9.8m/s2
 M = 8.57kg
 Kg x m/s2.. That’s also called a Newton!
 So my mass is 8.57kg.

Force Problems
But what if we were on another planet?
 Well, we use w = m x g
 W = 8.57kg x 3.8m/s2
 W = 32.57N
 As a reminder, weight is m x g, so it
equals a kg m/s2, or a N.
 Mass is measured in kg.

Force Problems
Ok, now you try one
 What would be your weight on Jupiter,
where gravity is 22.88m/s2?
 What would be your weight on the sun,
274.4m/s2? That’s assuming you could
stand on it

And now for something
completely different…

The Galaxy Song
Force Problems
Ok, let’s move on to earthly stuff.
 We remember that f = ma
 What would be the force exerted by a
truck with a mass of 1818kg
accelerating at 15m/s2?
 f = ma
 F = 1818kg x 15m/s2
 F = 750N

Force Problems
If you accelerate a rocket with a mass
of 300kg at Taber’s face at 500m/s2
with what amount of force will it hit
him?
 F = ma
 F = 300kg x 500m/s2
 F = 150,000

More Practice

Troy Polamalu, with a mass of 115kg,
hits Adrian Peterson with a force of
2300N. With what acceleration does
Troy hit Adrian? What force does
Adrian exert on Troy?
•F = ma
•2300N = 115kg x a
•A = 2300N / 115kg
•A = 20m/s
One More
A 20g sparrow mistakes a pane of glass
for air and slams into a window with a
force of 2N. What is the bird’s
acceleration?
 F = ma
 2N = .02kg x a
 A = 100m/s2 or 10g’s!!

Oh yeah, one more.
 Suppose
your car is parked on an
incline of 10 degrees. If the
parking brake lets go and your car
starts rolling, with what force are
you going down the hill? What is
your force on the ground? Assume
the car weighs 1500kg.
Friction
FRICTION
is the force
that acts in the opposite
direction of the motion
of the object
Types of Friction

Static Friction – Friction due to gravity when an
object is at rest.
– Demonstration

Sliding Friction – Friction while an object is at
motion.
– Example
Rolling Friction – Similar to sliding friction, but
the object is on wheels or castors to reduce the
sliding friction.
 Fluid Friction – Friction through water or air

– Terminal Velocity
Types of Friction
Sliding Friction
Ffriction = µFnormal
µ = the coefficient of sliding friction
(has no units)
product of the friction b/w materials and amount of force
1. Ben is walking through the school cafeteria
but does not realize that the person in front
of him has just spilled his glass of chocolate
milk. As Ben, who weighs 420 N, steps in the
milk, the coefficient of sliding friction between
Ben and the floor is suddenly reduced to
0.040. What is the sliding force of friction
between Ben and the slippery floor?
Friction
2.
While redecorating her apartment, Kelly slowly
pushes an 82 kg china cabinet across the wooden
dining room floor, which resists motion with a force
of 320 N. What is the coefficient of sliding friction
between the china cabinet and the floor?
3.
A rightward force is applied to a 10-kg object to
move it across a rough surface at constant velocity.
The coefficient of friction, µ, between the object
and the surface is 0.2. Use the diagram to
determine the gravitational force, normal force,
applied force, frictional force, and net force.
(Neglect air resistance.)
Terminal Velocity
Projectile Motion






What is a projectile? – Throw ball
Projectiles near the surface of Earth follow a
curved path
This path is relatively simple when viewed
from its horizontal and vertical component
separately
The vertical component is like the free fall
motion we already covered
The horizontal component is completely
independent of the vertical component (roll
ball)
These two independent variables combined
make a curved path!
Projectile Motion
Projectile Motion
No Gravity
With Gravity
Projectile Motion
Horizontally Launched Projectile
(initial speed (vx) = 25 m//s)


Time
(x)
0s
 1s
 2s
 3s
 4s
 5s
 Ts
Horizontal Displacement

0m
 25m
 50m
 75m
 100m
 125m
 ½ gt2

0m
25m
50m
75m
100m
125m
vxt
Vertical Displacement (y)
Horizontally Launched
Projectiles

What will hit the ground first, a projectile
launched horizontally, a projectile dropped
straight down, or a project fired up?
The Plane and the Package
Projectile Motion
Remember that nothing is accelerating the
projectile after it leaves
 The only thing accelerating the projectile
after launch is gravity
 The two vectors can be separated into the
velocity at launch and the acceleration of
gravity

Truck and Ball

Imagine a pickup truck moving
with a constant speed along a
city street. In the course of its
motion, a ball is projected
straight upwards by a launcher
located in the bed of the truck.
Imagine as well that the ball does
not encounter a significant
amount of air resistance. What
will be the path of the ball and
where will it be located with
respect to the pickup truck?
Fast-Moving Projectiles—
Satellites
What if a ball were thrown so fast that the
curvature of Earth came into play?
 If the ball was thrown fast enough to exactly
match the curvature of Earth, it would go into
orbit
 Satellite – a projectile moving fast enough to
fall around Earth rather than into it (v = 8
km/s, or 18,000 mi/h)
 Due to air resistance, we launch our satellites
into higher orbits so they will not burn up

Satellites
Launch Speed less than 8000 m/s
Projectile falls to Earth
Launch Speed less than 8000 m/s
Projectile falls to Earth
Launch Speed equal to 8000 m/s
Launch Speed greater than 8000 m/s
Projectile orbits Earth - Circular Path Projectile orbits Earth - Elliptical Path
ARISTOTLE ON MOTION
Aristotle
attempted to
understand motion by
classification
Two
Classes:
– Natural and Violent
Natural Motion
Natural motion depended on nature of
the object.
 Examples:
 A rocks falls because it is heavy, a
cloud floats because it’s light
 The falling speed of an object was
supposed to be proportional to its
weight.

Natural Motion

Natural motion could be circular
(perfect objects in perfect motion
with no end).
Violent Motion
Pushing or pulling forces imposed
motion.
 Some motions were difficult to
understand.
 Example: the flight of an arrow


There was a normal state of rest
except for celestial bodies.
Aristotle
Aristotle was
unquestioned for
2000 years.
 Most thought that
the Earth was the
center of everything
for it was in its
normal state.
 No one could imagine
a force that could
move it.

GALILEO AND THE
LEANING TOWER
 17th
Century scientist who
supported Copernicus.
 He refuted many of Aristotle's
ideas.
 Worked
on falling object problem used experiment.
GALILEO'S INCLINED PLANES

Knocked down Aristotle's push or pull ideas.

Rest was not a natural state.

The concept of inertia was introduced.

Galileo is sometimes referred to as the

“Father of Experimentation.”
NEWTON’S FIRST LAW OF
MOTION
Newton finished the overthrow of
Aristotelian ideas.
 Law 1 (Law of Inertia)
 An object at rest will stay at rest and an
object in motion will stay in motion
unless acted upon by an outside force.

Newton’s First Law
(law of inertia)
INERTIA
is a property of an
object that describes how hard
it is to change the motion of
the object
More mass = more inertia
F=MA
Newton’s Second Law
Force Causes Acceleration

In order to make an object at rest move, it
must accelerate
– Suppose you hit a hockey puck
– as it is struck it experiences acceleration, but as it travels
off at constant velocity (assuming no friction) the puck is
not accelerating
– if the puck is struck again, then it accelerates again; the
force the puck is hit with causes the acceleration

Acceleration depends on net force
 to increase acceleration—increase net force
 double acceleration—double the force

Force ~ Acceleration
 directly proportional
Newton’s Second Law

Acceleration depends on mass
– to decrease acceleration—increase mass
– to increase acceleration—decrease mass
– double the mass = ½ the acceleration

Acceleration ~ 1/mass
 inversely proportional
Newton’s Second Law

Newton was the first to realize that
acceleration produced when something is
moved is determined by two things
 how hard or fast the object is pushed
 the mass of the object

Newton’s 2nd Law
– The acceleration of an object is directly
proportional to the net force acting on the object
and is inversely proportional to the object’s mass
Newton’s Second Law

Second Law Video
Newton’s Second Law
a = F/m

–
–
or
F = ma
Robert and Laura are studying across from each other at a
wide table. Laura slides a 2.2 kg book toward Robert. If
the net force acting on the book is 1.6 N to the right, what
is the book’s acceleration?
A = F/m
= 1.6N / 2.2kg
= .73m/s2
Newton’s Second Law
2. An applied force of 50 N is used to accelerate an object to the
right across a frictional surface. The object encounters 10 N of
friction. Use the diagram to determine the normal force, the net
force, the mass, and the acceleration of the object. (Neglect air
resistance.)
3. Rose is sledding down an ice-covered hill inclined at an angle of
15.0° with the horizontal. If Rose and the sled have a combined
mass of 54.0 kg, what is the force pulling them down the hill?
Newton’s
1.
nd
2
A 4.60 kg sled is pulled across a smooth ice
surface. The force acting on the sled is of
magnitude 6.20 N and points in a direction
35.0° above the horizontal. If the sled starts
at rest, what is its velocity after being pulled
for 1.15 s?
1. V = 1.265 m/s
2.
Law & Kinematics
The fire alarm goes off, and a 97 kg fireman
slides 3.0 m down a pole to the ground floor.
Suppose the fireman starts from rest, slides
with a constant acceleration, and reaches the
ground floor in 1.2 s. What was the force
exerted by the pole on the fireman?
1. F = 201.76N
Sample Problems
Which exerts a greater force on a table: a
1.7kg physics book lying flat or a 1.7kg
physics book standing on end? Which
applies a greater pressure? If each book
measures .26m x .210m x .04m, calculate
the pressure for both.
 Same
 Standing
 Flat = 311N/m2; Standing = 2.0x103 N/m2

Sample Problems
A 1250kg slippery hippo slides down a
mud-covered hill inclined at an angle of 18
degrees to the horizontal. If the
coefficient of sliding friction between the
hippo and the mud is .09 what force of
friction impedes the hippo? If the hill were
steeper, how would this affect the
coefficient of friction?
 1068.75N

Sample Problems
Mr. Micek loves to ride his motorcycle. Mr.
Micek and his motorcycle have a combined
mass of 518kg. With what force must each tire
push down on the ground to hold the bike up?
If the contact pattern of the tire is 15cm x
35cm (for each wheel), what pressure do they
exert? If Mr. Micek accelerates at 8.8m/s2 what
force will he exert? If he parks his bike on a hill
at an angle of 25 degrees what must the force
due to friction be to keep it there?
 2590N; 49.3kPa; 4558.4N; 2175.6N
