Download Lecture 3

Locomotion in a physical world

Most animals have some form of
locomotion which separate them
from plants and fungi

What is motion?

A natural event that involves
change in the position or location
of an object or organism
Components to describe motion
Position = 0
Position = 1
• Change in position
– Change in distance (d)
Final distance – initial distance = D distance or Dd
– Change in time (t)
Final time – initial time = D time or Dt
Change in position at 1 cm/ second
Moved for 1 second.
Components to describe motion
Position = 0
Position = 1
• How would you describe that the ball
moved back to its original position?
Final distance – initial distance = D distance
1 cm
−
0 cm
= − 1.0 cm
Change in position at − 1 cm/ second
Moved for 1 second. Including direction is “velocity”
Speed and velocity
• Velocity is a measure of the speed in a given direction.
– e.g. You can say the top speed of an airplane is 300 kilometers
per hour (kph). But its velocity is 300 kph in a northeast direction.
• Speed is how fast an object is going with respect to an
object.
– e.g. You can say that you travel in your car at 70 miles per hour
(mph). But your velocity is………?
• We distinguish between speed and velocity because if
you add the speeds of objects, their directions are
important.
– For example, the velocity of an airplane with respect to the
ground would vary according to the direction of the wind.
Measurement
• In order to determine how fast an object is going, you measure the
time it takes to cover a given distance. Its velocity (v) or speed
equals the distance (d) traveled divided by the time (t) it takes to go
that distance:
v=d/t
• For example, if a car went 120 miles in 2 hours, its average speed
would be the distance of 120 miles divided by the time of 2 hours
equaling 60 miles per hour (mph).
• IAvg. Velocity Va = (Vi + Vf) / 2
• If you travel from Milwaukee to Chicago (90 miles) at an average
velocity of 60 mph, it would take you 90 mi. / 60 mph = 1.5 hours to
travel the distance.
A common sense approach:
• If a ball starts at the 30cm mark and
travels at a velocity of 10cm/sec for 3
seconds, stops, then travels at a velocity
of - 5cm/sec for 2 seconds, where will it
end up?
Acceleration
• Acceleration is the increase of velocity over a period of
time. Deceleration is the decrease of velocity.
– e.g. When you start running, you accelerate (increase
your velocity) until you reach a constant speed.
• A = Vf - V I
t
change in velocity over time
When you are moving is your velocity always the
same? Acceleration is a way to describe these
changes in velocity.
A ball starts with a velocity of
0 m/sec, then accelerates smoothly
and reaches the 1 meter mark in 10
seconds. What is the acceleration of
the ball?
1m
• What is the velocity of the ball after 5
seconds of the calculated acceleration?
Now that we’ve described components of motion can you tell me what
causes motion or movement?
• Newton’s 1st Law of motion:
Inertia
– Every object retains its
state of uniform motion
unless acted upon by an
unbalanced force.
– An object at rest remains
at rest.
– An object in motion will
continue in a straight line
at a constant velocity.
FORCE!
Inertia?
• a body at rest tends to stay at rest and
a body in motion tends to stay in
motion unless acted upon by an
unbalanced force
Uniform motion?
• Changing position in a constant and unvarying
manner.
• What does this mean----identify if the following
would be an example of uniform motion
The ball sitting on the table?
Yes; uniform motion
The moon orbiting around the earth?
Yes; uniform motion
Rush hour traffic?
Not uniform motion
Can you think of forces that act upon a ball sitting on a
table? Rolling across the floor? Being lifted from the table?
Change in motion due to unbalanced
forces – Newton’s 2nd Law
• How can we calculate Force?
• F = m*a
• unit of measure = Newtons (N)
a = accleration due to gravity =
9.8 m/s2
1 N = 1 kg m
s2
Why include mass for force?
Mass affects inertia – an objects tendency
to resist a change in motion
What happens if a large object encounters a force
generated by a smaller object?
• Newton’s Third law of Motion
– Whenever 2 objects interact, the force
exerted on one object is equal to and in
the opposite direction of the force exerted
on the other object.
Why does the ball stop?
• If the ball pushes on the table; does the table push
back?
• Identify forces
– Are the forces balanced?
Force
applied
by my
hand
Force = m a
Acceleration due to gravity =
9.8 m / s2
If you were standing on the ball and took a step off which
direction would the ball be directed?
• The force you would exert might exceed the capacity
for the ball to resist a change in motion.
• You push the ball backward and the result is an
opposite reaction of your motion (force) directed
forward.
• A 50 kg person takes a single step. She
accelerates her body at 1m/s2. How
much force is involved?
• Now, think of yourself walking….do you
exert a force in the direction you are
moving, or do you exert a force in the
opposite direction? (Hint: Newton’s 3rd
law)
• So, if the person is on firm ground, and
given that the earth weighs 5.98*1024
kg, how much does she accelerate the
ground?
One last thing to consider: What
is friction?
• Friction results from the two surfaces being
pressed together closely, causing
intermolecular attractive forces between
molecules of different surfaces.
• As such, friction depends upon the nature of
the two surfaces and upon the degree to
which they are pressed together.
What you should be able to do:
• Manipulate and solve algebraic equations
and problems involving position, speed,
velocity, acceleration, mass and force.
• Identify the units of each of these quantities.
• Compare speed and velocity.
• Explain how friction and gravity affect
motion, and be able to use this in problem
solving.
• Identify forces acting on an object and
determine whether they are balanced or
unbalanced.
• Identify and use Newton’s 3 Laws of Motion
to explain the general properties of motion.