Download Causes of Motion Forces - Red Hook Central Schools

Motion & Force
Aristotle ~375 BC
All motion on the Earth is linear.
All motion in the heavens is perfectly circular .
Aristotle’s falling bodies – heavy fall faster.
Motion continues so long as there is an applied
force (push or pull) to an object. Removing the
force stops the object.
Everyone accepted Aristotle’s
view until Galileo.
That's about 2000 years of false
knowledge.
Galileo ~ 1600
Began Experimentation and observation.
Collected empirical data.
Galileo - all objects fall to the ground at
the same speed.
Galileo’s Experiment – objects attain same height.
What is right side made flat?
Objects will move in straight line at
constant velocity forever with no
friction (or other force) to stop them!
Sir Isaac Newton ~1660
Quantified Galileo’s Ideas
Developed Laws of Motion
Newton’s 3 Laws of Motion
• 1. Law of inertia – deals with objects with
zero net force.
• 2. Law of acceleration – deals with
accelerating objects which have a net force.
• 3. Action/Reaction or Force pairs. Deals with
equal & opposite forces exerted on & by pairs
of objects.
What is a force?
Push or pull on object
Force unit = Newton N.
1 N = kg
2
m/s .
• The amount of force needed to accelerate a
1 kg object 1 m/s2.
Newton’s 1st Law: Inertia
An object with no net (extra) force continues at
rest or in straight line motion with the constant
speed unless acted upon by an unbalanced force.
Unbalanced force = Fnet. More F in 1 direction.
If forces are balanced, they are equal & opposite.
If all forces balanced, Fnet = 0,
acceleration =?
a=0
Object at rest or moving at constant velocity!
Inertia
• Inertia = the resistance an object has to a
change in its state of motion –
• acceleration .
• If it’s moving, inertia makes it want to
keep moving.
• If it’s at rest, inertia makes it want to stay
at rest.
The truck stops from the force applied by the car.
The ladder does not have the same applied force.
It keeps going. Why?
Inertia!
The inertia of the ladder makes it keep its
state of motion.
Inertia depends on:
Mass only
High Inertia
Low Inertia
1. You are in floating space with no gravity or
friction of any kind. Your (ex) best friend
throws a 10-kg bowling ball at 1 m/s and a 4-g
penny moving at 500 m/s at you. Which one
has more inertia?
• 1. The bowling ball.
• 2. The penny.
• 3. Neither – since there is no gravity or
friction.
• 4. Impossible to tell from information given.
Force & Motion
• Objects can have force on them but
won’t necessarily move.
Net Force (unbalanced) = more
force in one direction.
Acceleration to right.
An unbalanced/net force causes the object
to accelerate in the direction of net force.
Observe the cartoon.
• What is the Fnet?
• What is the
Acceleration?
• What is the
Direction?
Inertia
• Resists acceleration or change.
• Stopping, starting, and changing direction.
• The more mass an object has, the harder it is to
change its motion. It is directly proportional to mass.
• Two times the mass = 2x the effort for Dv.
• Inertia is NOT a force!!
• It is a property of mass.
Net Force
• Fnet - Unbalanced forces cause acceleration in
the same direction as the unbalanced force.
• Equilibrium – The sum (SF) of all forces =
0. They cancel in all directions.
• Left F = Right F, Up F = down F.
• West F = East F.
• Acceleration = 0.
2. A car is speeding on a straight flat highway at 100
miles/hour. The engine is pushing the car forward
with a force of 25,000 N. What can you say about the
sum of the resistive (backward) forces on the car?
•
•
•
•
1. The resistive forces = 0
2. The resistive forces = 25,000 N
3. The resistive forces < 25,000 N.
4. Cannot tell from the information supplied.
Acceleration
• To change an object’s velocity, a, there must
be an unbalanced or net force (Fnet) acting
on the object.
• This is true for a change in speed or a
change in direction.
• The more inertia, the more Fnet needed for
acceleration.
3. This car is moving at a constant 30
mi/hr. What can you say with certainty?
•
•
•
•
1. It has no forces acting on it.
2. The forces on it are balanced.
3. It is accelerating forward.
4. Not enough information to tell.
Demo Accelerometer
2 outcomes of Balanced Forces
Objects stay
at rest
Objects at constant
velocity = no
acceleration
Balanced Forces = Equilibrium
Forces of equal magnitude
and in opposite directions,
balance each other. They
are said to be at equilibrium.
SF = 0
The forces
add to zero
there is no
net force on
the person.
In the absence of an unbalanced force to stop it,
inertia will keep an object moving in a straight
line forever! The block is in equilibrium.
Force units:
Unit of Force = Newton (N)
1 N = 1 kg m
2
s.
Force is a vector quantity
same direction as acceleration
Hwk read Holt text pg 124 – 131
do 151 #1 – 6.
Write full sentences on separate
sheet.
Film Clips
Newton’s 3 laws with hockey 6 minutes.
http://science360.gov/obj/video/642db496-d506-432e-85b44e38f75d9142/newtons-three-laws-motion
• Newton’s 1st Law space station 6 min
• http://www.youtube.com/watch?v=Q0Wz5P0JdeU