Download long range force

Chapter 4: Forces
Force and Motion
Objectives
Define a force and differentiate between
contact force and long range force.
Explain the meaning of Newton’s First Law of
Motion and describe an object in equilibrium.
Recognize the significance of Newton’s
Second Law of Motion and use it to solve
motion problems.
Activator:
How do Skydivers control their
velocity so they can perform
team maneuvers?
What is the FORCE pulling down?
What is the FORCE pushing up?
Force:
An object that experiences
a push or pull has a force
exerted on it.
Latin: Fortis (strong)
Contact versus Long-range Forces

Contact Force - A force that acts on the
object by touching it. It “makes contact”
with it. Con - with; tact (tactile) - touch.
Examples: desk on a book, floor on a
person.
Long-range Force - A force that
acts on the object without touching
it.
Examples: magnets, force of gravity.
Types of Forces
Force
Symbol Definition
Direction
Friction
Ff
Contact force that acts to oppose
sliding motion between surfaces.
Parallel to surface; opposite
to sliding direction
Normal
FN
Contact force exerted by surface
on object.
Perpendicular to and away
from surface.
Spring
Fsp
A restoring force, push or pull
spring exerts on object.
Opposite the displacement of
object at end of spring.
Tension
FT
Pull exerted by string, rope, cable
when attached and taut.
Away from object, parallel to
rope at point of attachment
Forces that move rockets,
planes, cars…
Same direction as
acceleration, travel
Long-range gravitational force.
Down towards center of
earth.
Thrust
Fthrust
Weight
Fg
Free-Body Diagram:
A Free-Body Diagram shows all the
forces, in vector form, acting on an
object.
Physical Diagram
Free-Body Diagram - A
Free-Body Diagram - B
Newton’s First Law of
Motion

An object that is at rest will remain at rest or an
object that is moving will continue to move in a
straight line with constant speed, if and only if
the net force acting on that object is zero.

A.K.A. Law of Inertia. Inertia is the tendency of
an object to resist change in its current motion.
(Example: Car without gas moving or at rest)

Equilibrium: An object is said to be in equilibrium
when the net force is zero. (net = sum of all
vector forces)
Newton’s First Law of
Motion
Newton’s Law of Inertia
Paul G. Hewitt
Disc One - Newton’s Law of Inertia
Law of Inertia Demo
Paul G. Hewitt
Disc One - The Old Table Cloth Trick
Cylinder Inertia Demo
Paul G. Hewitt
Disc One - Inertia of Cylinder
Doing the T.P. Roll
Paul G. Hewitt
Why you don’t have to hold the T.P. roll
Weight Mass Distinction
Paul G. Hewitt
Disc One - Weight-Mass
Newton’s Second Law of Motion
The acceleration of an object as produced by a net force
is directly proportional to the magnitude of the net force,
in the same direction as the net force, and inversely
proportional to the mass of the object.



a  F net ; a 1 / m
; a = F net / m
Most Common Form:
F net = m a

Force = mass x acceleration
Newton’s 2nd Law of Motion
Paul G. Hewitt
Disc One - Newton’s 2nd Law
All Together Now….
v = 0 m/s
v  0 m/s
1)
2)
Depends directly on Net Force
Depends inversely on Mass
Using Newton’s Laws
Sir Isaac Newton used prisms to show
that sunlight was made up of all the
colors of the rainbow. This proved
that the ancient Greeks ideas about
light were wrong.
Friction: What is it?


Frictional resistance to the relative motion of two solid
objects is usually proportional to the force which presses
the surfaces together as well as the roughness of the
surfaces.
Rub your hands together. Feel the heat that is generated. That is
friction. Why do we put oil in our car engines? To overcome friction
in the pistions. Why can ice skaters glide so easily? How do car
brakes work?
Friction: Static and Kinetic
Friction: Static and Kinetic
Friction
Gravity: Feather and Coin
Why do all objects fall at the same rate?
Paul G. Hewitt Free-fall Acceleration Explained
Interaction Forces
Objectives

Define Newton’s Third Law of motion.

Explain the tensions in ropes and
strings in terms of Newton’s third law.

Define the Normal force.

Determine the value of the normal force
by applying Newton’s Second Law.
Newton’s Third Law of Motion


For every action, there is an equal and
opposite reaction.
All forces come in pairs. These interaction
pairs are equal in magnitude and opposite
in direction.
Examples
Interaction Forces
Rifle Recoil
Tensions on Ropes
In this simple
rope tension
diagram, how
can you find the
tension force in
the rope?
How does this
relate to
Newton’s Third
Law?
Tensions on Ropes
In this simple
rope tension
diagram, how
can you find the
tension force in
the rope?
How does this
relate to
Newton’s Third
Law?
Normal Force
The Normal
Force is the
perpendicular
contact force that
a supporting
surface exerts on
another object.
Normal Force
The Normal
Force is equal to
the weight of the
object if the
supporting
surface is
horizontal.
i.e. FN = Fg = mg
Pressure: The Bed of Nails
Paul G. Hewitt Disc One - Pressure: The Bed of Nails
Pressure: The Bed of Nails
Why doesn’t the assistant get hurt?
Einstein’s Angels
Einstein's Angels
Summary

How would you summarize today’s
lesson?

Forces come in interaction pairs that are
equal and opposite.

Although the forces are the same in
magnitude, the accelerations can vary
greatly.
Assignments

Chapter 4 Review Questions 2-30
Evens only, p. 76-77 due Tuesday 11/2.

Study Guide due Thursday.

Quiz next Monday.