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Motion and Forces
Welcome to the Physics part of
Physical Science
• Have you ever watched a rocket launch –
like the space shuttle? Did you ever
wonder what powered the rocket or how
fast it went?
• Both of those questions can be related to
motion, forces and mechanics.
Speed
• The speed of an object is the distance the
object travels per unit of time. Speed is a
rate which tells you the amount of
something that occurs or changes in one
unit of time.
• Speed=distance over time
• Speed can be divided into two subtitles
constant speed & average speed.
Constant & Average Speed
Constant speed is the speed that does not change.
Average speed is the total distance divided by time.
Speed is not the same as Velocity!
Speed is how fast something is going but no given or
known direction – although often we can assume we
know from the information given.
Velocity
• Velocity is Speed in a given direction.
• If you know the velocity of an object you
know its speed and its direction.
• Velocity is a vector quantity. It’s also
“direction-aware”. The direction of the
velocity vector is simply the same as the
direction which an object is moving. If
going downward the velocity is downward.
If going upward the velocity is upward.
Velocity
• V1 represents the initial or starting velocity
– If the object starts from a rest, V1 will = 0
• V2 represents the final velocity of an object
– If the object ends with a stop, then V2 right at
the end will be a zero, but not just a
millisecond before that!
–V = d/t
– And this means d = vt;
and t = d / v
Acceleration
• 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.
• Acceleration (a) = ΔV / Δt
-or• Acceleration = force over mass
Let’s do an example. . .
Superman Leaps To the Top of a Building. . . . . . .
• Let’s say the building has a height of 660 feet
• His final velocity (V2) at the top is equal to 0 (cause he
stopped to admire the view)
• We don’t know his starting velocity (V1) or how long it
took to get to the top. . . .
• So – modifying the equations you are going to learn – we
can get:
– V2 = 2 g h where g is the acceleration due to gravity
– This means V2 = 2 (32 ft/s2) (660 ft) = 205 ft/s = 140
mph!
• Superman’s acceleration is definitely unearthly!
In addition to speed and velocity,
there is a difference between
Weight & Mass
Mass of an object
will not change even
if the force of gravity
on its changes. Mass is a
useful physical property for
describing and measuring
matter. Mass is a measurement
of inertia. The SI unit of mass is
Kilograms.
Weight is a measure
Of the force of gravity
on you. Weight is a
useful measurement
of how much matter
an object has. Wt = mg
Mass and Weight
• Your mass will remain the same – it’s how
much stuff that makes you up
• Weight is affected by the force of gravity –
so on different planets, your mass will
remain the same – but your weight will
vary
•
(Sounds like a good diet plan for me)
• Weight (Wt or W) = mass x gravity
Forces and Motion
• Isaac Newton quantified the Laws of
Motion in his 1687 work “Mathematical
Principles of Natural Philosophy”
• He equated motion to
forces and how they
interact on objects
• He developed three
laws
Newton's 1st Law of Motion
• 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.
• Sometimes referred to as the “Law of Inertia."
– Inertia is the state of rest or resisting a force that may cause
motion or a change in velocity
– Galileo First theorized about this using a ball on an incline
• There are two parts: one which predicts the behavior of
stationary objects; and the other which predicts the
behavior of moving objects .
The two parts are summarized in the following diagram.
For example:
• You and the car are traveling the same
velocity – and if you brake suddenly – the
car stops but you continue forward until
the seatbelt stops you!
• If there were no friction – such as air
resistance, gravity, etc. – then an object
will continue in motion forever at the same
velocity! This is how things move in
space!
Example of 1st Law of Motion
– Pendulum is a example for Newton's first law
• This thing is often called a Newton’s Cradle
– Once its in motion its always in motion.
– Once its at rest its always at rest.
Newton's 2nd Law of Motion
• The second law states that the acceleration of
an object is dependent upon two variables - the
net force acting upon the object and the mass of
the object.
• It explains the relation of force, mass &
acceleration.
• Force=mass x acceleration (F = ma)
• The net force on an object is equal to the
product of its acceleration and its mass.
Force
• Force is measured in the SI unit called a
Newton (N)
– 1 N = 1 kg x 1 m / s2
1 N = .225 lbs
1 lb. = 4.448 N
• Forces usually are in equilibrium (balanced)
Force
• Is a push or pull
• It can be divided into two subsets:
unbalanced and balanced
• Unbalanced force can cause an object to
start or stop moving; or change its
acceleration, velocity or direction
• A balanced force is equal forces on an
object that will not change the object’s
motion
Forces in balance. . .
• The shuttle on the
launch pad is in a state of
force equilibrium – the forces
are balanced
• The shuttle is in inertia or rest
with v1 = 0
Fnet
Fgravity/Weight
When forces become
unbalanced – such as
when the thrust is applied. . .
• Then you have
lift-off
• The applied force
causes acceleration
• The two SRB have a
total of 23,600,000 N of
force
(they separate at 45 km
and are recovered and
reused)
Acceleration
• a = Δv
Δt
• This means the acceleration rate is the change
of velocity (v2 – v1) divided by the change in time
(t2 – t1).
• Any of these variables can be a zero
• Acceleration can be either negative or positive!
• Acceleration due to gravity (g) is basically the
same concept, except it is up or down!
Acceleration Due to Gravity
• agrav or just plain a, has a value of 9.80665
m/s2
– We’ll round this off to 9.81 m/s2
• Use 10 for guesstimating!
– This value is for earth – your home
planet may vary!
– Believe it or not – agrav at the equator is
9.7804 m/s2 and at the poles it is 9.8321
m/s2
• Can you figure out why?
Free Fall Acceleration
• If v1 (initial velocity is zero or the object is
at rest then falls):
– V2 = gt
– V2 = √2gh
– H = ½ gt2
– H = v2 t
2
If v1 does not equal zero. . .
• V2 = v1 + g t
• V22 = v12 + 2 g h
• H = v1t + ½ g t2
• H = v 2 + v1 t
2
Momentum (ρ)
• Momentum is the product of an object’s
mass and velocity
• It is directly proportional to mass and
velocity
• It’s the tendency for an object to keep in
motion
–p = mv
– F t = m v; where F t is the impulse or
change in momentum
Newton’s 3rd Law of Motion
• A force is a push or a pull upon an object
which results from its interaction with
another object.
• A rocket launching is a prime example of
this law
• Action force and
reaction force
Newton’s 3rd Law. . .
• Basically – the law means that for every
action there is an equal and opposite
reaction
• A rocket launch – the Fthrust downwards
(action) forces the rocket upwards
(reaction) against the Fgravity
• Of course, there are factors such as drag
(friction) which must be overcome
Now – it’s time for. . .
Some Rocket Science!!!!!