Transcript 4. forces and motion

Motion and Forces
Types of Forces
1.
2.
3.
4.
Gravitational
Electromagnetic
Frictional
Strong and Weak Nuclear
Gravitational Forces
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Force of attraction between all
objects as a result of their masses
and distances between them.
The higher the masses and the
shorter the distances between two
objects, the greater the force of
gravity.
The smaller the masses and the
larger the distances between two
objects, the weaker the force of
gravity.
Gravity exists between ANY
two objects
Why things fall to the ground
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When we drop an object, the force
of gravity is strong because the
earth has a large mass and is close
by. The object gets pulled to the
ground.
Near the Earth’s surface, objects
accelerate to the ground at a rate of
9.8 m/s2, regardless of their
masses.
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The gravitational force of the Earth
decreases as you move away from
the planet.
Different planets and stars will have
different forces of gravity at their
surfaces due to their different
masses. The bigger the object the
greater the force of gravity.
Gravitational forces also explain the
Earth’s tides
Mass vs. Weight
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Mass is a measure of the quantity
of matter in an object. This does
not depend on where the object is.
Weight is a measure of the
gravitational force acting on an
object. This DOES depend on where
the object is.
Weight
However, weight and mass are different, how
come?
weight = Fg = mg which is similar to F=ma
m - is mass measured in kilograms (kg)
g - gravitational field intensity measured in
Newtons/kilogram (N/kg); this is also
known as acceleration (m/s2).
Fg - is the force of gravity measured in
Newtons (N)
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1 N = 1 kg x m
s2
We know that earths gravitational pull is
9.8 N => 9.8 kg x m = 9.8 m
kg
s2 x kg
s2
Example
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What is the
gravitational force
(weight) of a 60 kg
astronaut on the
earth’s surface?
Intensity of gravitation
on earth is 9.8 N/kg
F = mg
F = 60 kg X 9.8 N/kg
F = 588 N
Example
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What is the gravitational
force (weight) of a 60 kg
astronaut on the Moon’s
surface?
F=mg
F= 60 kg X 1.67 N/kg
(force of gravity on the
Moon)
F= 100.2 N
Gravitational Intensity on the
Planets in our Solar System
Planet
Acceleration (m/s)
Intensity (N/kg)
Mercury
3.7 m/s²
3.7 N/kg
Venus
8.87 m/s2
8.87 N/kg
Earth
9.8 m/s2
9.8 N/kg
Moon
1.62 m/s2
1.62 N/kg
Mars
3.71 m/s2
3.71 N/kg
Jupiter
24.79 m/s2
24.79 N/kg
Saturn
10.44 m/s2
10.44 N/kg
Uranus
8.69 m/s2
8.69 N/kg
Neptune
11.15 m/s2
11.15 N/kg
Pluto
.658 m/s2
.658 N/kg
How much do you really weigh???
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Take your weight in kg
(pounds/2.2) and calculate how
much weight in Newton’s you are on
all the planets in our solar system.
Electromagnetic Force
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Force of attraction or
repulsion between two
charged objects or
between the poles of two
magnets
We already discussed
this earlier in the year
Likes repel, opposites
attract
Frictional Force
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The force that prevents two objects
from slipping over each other
Air resistance is also a force of
friction
Friction depends on:
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The smoothness of the surfaces (the
rougher the surfaces the greater the
friction)
The pressure between the surfaces (the
greater the pressure the greater the
friction)
Friction
Strong and Weak Nuclear
Forces
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Act within the nucleus of an atom
Short range forces
Holds the nucleus together
Hippo & Ping Pong Ball
In a vacuum, all bodies fall at the same rate.
If a hippo and a
ping pong ball
were dropped
from a helicopter
in a vacuum
(assuming the
copter could fly
without air),
they’d land at the
same time.
When there’s no air resistance, size and shape
don’t matter!
Video of a Vacuum and two objects
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https://www.youtube.com/watch?v
=8RKAb5accC0
Question why do both objects fall at
the same acceleration?
Table of Equations from this unit
Kinetic Energy Speed of an Object
KE = ½mv2
d=vt
Acceleration
a = change velocity
change of time
a=∆v
∆t
Force
Weight
F=ma
Fg = mg
Measurement
Definition
Unit
Symbol
Kinetic Energy
Amount of energy needed to
move an object
Joules
J
Mass
How heavy an object is in
kilograms
Kilograms
kg
Velocity
How fast an object is moving in
meters/second
meters/second
m/s
Distance
The distance from one point to
another point
meters
m
Time
Amount of time measured in
seconds
seconds
s
Acceleration
The rate of change of velocity
per second
meters/second2
m/s2
Force
The amount of effort required to Newton
move an object
N
Force of Gravity
The amount of gravitation pull
on an object.
Newton
N
Gravitational
Pull
The intensity of gravity
measured in N/kg
Newton/kilogram
N/kg