Transcript Force

Motion & Forces
Defining Force
Force
 Newton’s First Law
 Friction

Use your paper from yesterday
and create a 4 Square!
NEWTON’S FIRST LAW
FORCES
BALANCED FORCES
INERTIA AND MASS
CAR CRASH
UNBALANCED FORCES
Laws of Motion
Copyright © 2010 Ryan P. Murphy

Newton’s 1st Law
An object at rest tends to stay at rest and
an object in motion tends to stay in motion
with the same speed and in the same
direction unless acted upon by an
unbalanced force.
 Called Law of Inertia

Copyright © 2010 Ryan P. Murphy

Newton’s 1st Law
An object at rest tends to stay at rest and
an object in motion tends to stay in motion
with the same speed and in the same
direction unless acted upon by an
unbalanced force.
 Called Law of Inertia

Copyright © 2010 Ryan P. Murphy

Newton’s 1st Law
An object at rest tends to stay at rest and
an object in motion tends to stay in motion
with the same speed and in the same
direction unless acted upon by an
unbalanced force.
 Called Law of Inertia

Learn about the 1st Law of Motion at…
http://www.physicsclassroom.com/class/newtlaws/u2l1a.cfm
Copyright © 2010 Ryan P. Murphy

Newton’s 1st Law

An object at rest tends to stay at rest and
an object in motion tends to stay in motion
with the same speed and in the same
direction unless acted upon by an
unbalanced force.

Called Law of Inertia
Learn about the 1st Law of Motion at…
http://www.physicsclassroom.com/class/newtlaws/u2l1a.cfm
Copyright © 2010 Ryan P. Murphy
An object in motion tends to stay in motion.
Forces


A force is a push or a pull on an object
All changing motion is caused by a force
or more than one force
Force

Force
 What forces are being
exerted on the football?
Fkick
Fgrav
Force


The force exerted by the players foot
is greater than the forces keeping the
ball at rest.
Therefore, the resultant force is in the
direction of the greater force,
Fkick
the kick.
Fgrav
Balanced Forces
Force does not always change
velocity.
Balanced forces are equal in size
and opposite in direction.
Force

Balanced Forces
 forces
acting on
an object that
are opposite in
direction and
equal in size
 no
change in
velocity
Force



Sometimes it is obvious that a force has
been applied.
But other forces aren't as noticeable.
A force can cause the motion of an
object to change.
Unbalanced Forces
If an object is
being pushed or
pulled, the
changing motion is
caused by forces
1. Pushing unequal forces in opposite
directions
2. Pushing in the same direction and
combining forces

Force

Net Force
 unbalanced forces that are not
opposite and equal
 velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
W
N
Some Examples from Real Life
A soccer ball is sitting at rest. It takes an
unbalanced force of a kick to change its
motion.
Two teams are playing tug of war. They are both
exerting equal force on the rope in opposite directions.
This balanced force results in no change of motion.
Balanced Forces



When two or more forces act on an
object at the same time, the forces
combine to form the net force
The net force on the box is zero
because the two forces cancel each
other.
Forces on an object that are equal in
size and opposite in direction are called
balanced forces.
Unbalanced Forces



When two students are pushing with
unequal forces in opposite directions, a
net force occurs in the direction of the
larger force.
The net force that moves the box will
be the difference between the two
forces because they are in opposite
directions.
They are considered to be unbalanced
forces.
Unbalanced Forces

The net force
that acts on
this box is
found by adding
the two forces
together.
Newton’s 1st Law

Newton’s 1st Law of Motion


An object at rest will stay at rest, and
an object in motion will stay in motion
UNLESS acted on by an unbalanced force
In other words: an object will keep
doing what it’s doing until a force
changes its motion
Newton’s First Law


Newton’s First Law of Motion
 “Law of Inertia”
Inertia
 tendency of an object to resist any
change in its motion
 increases as mass increases
Inertia and Mass



Inertia is the tendency of an object to
resist any change in its motion
If an object is moving, it will have
uniform motion.
It will keep moving at the same speed
and in the same direction unless an
unbalanced force acts on it

Two platoons line up for a tug of war. If
both platoons have the same number of
participants on a team, what other
factor is critical to either platoon's
win?
A) age of men
B) height of men
C) total mass of the team
What happens in a crash?


The law of inertia can
explain what happens
in a car crash.
When a car traveling
about 50 km/h
collides head-on with
something solid, the
car crumples, slows
down, and stops
within approximately
0.1 s.
What happens in a crash?



Any passenger not wearing a safety belt
continues to move forward at the same speed
the car was traveling.
Within about 0.02 s (1/50 of a second) after
the car stops, unbelted passengers slam into
the dashboard, steering wheel, windshield, or
the backs of the front seats.
The force needed to slow a person from 50
km/h to zero in 0.1 s is equal to 14 times the
force that gravity exerts on the person.
Car-Person Inertia

http://www.physicsclassroom.com/mmed
ia/newtlaws/cci.cfm
What happens in a crash?



The belt loosens a little as it restrains
the person, increasing the time it takes
to slow the person down.
This reduces the force exerted on the
person.
The safety belt also prevents the
person from being thrown out of the
car.
Safety Belts



Air bags also reduce injuries in car
crashes by providing a cushion that
reduces the force on the car's
occupants.
When impact occurs, a chemical
reaction occurs in the air bag that
produces nitrogen gas.
The air bag expands rapidly and then
deflates just as quickly as the nitrogen
gas escapes out of tiny holes in the bag.
Force Diagrams


Force diagrams use
arrows to represent the
sizes, directions, and
types of forces on an
object
The object is usually
represented by a box
Concept Test 1
TRUE or FALSE?
The object shown in the diagram must be at
rest since there is no net force acting on it.
FALSE! A net force does not
cause motion. A net force
causes a change in motion, or
acceleration.
Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.
Concept Test 2
You are a passenger in a car and not
wearing your seat belt.
Without increasing or decreasing its
speed, the car makes a sharp left turn,
and you find yourself colliding with the
right-hand door.
Which is the correct analysis of the
situation? ...
ConcepTest 2
1. Before and after the collision, there
is a rightward force pushing you
into the door.
2.
Starting
at
the
time
of
collision,
2. Starting at the time of collision, the
the
door
exerts
a
leftward
force
door exerts a leftward force on
on
you.
you.
3. both of the above
4. neither of the above