Physics 1A
Lecture 4A
"More than anything else... any guy here would
love to have a monkey. A pet monkey."
-- Dane Cook
More info on Quiz 1
Quiz 1 on Friday during regular class hours.
10 questions
Multiple choice with a twist
The correct answer is never among the possible
choices. You have to pick the closest answer to
what you think is right.
Graphing calculator allowed.
Closed book. One page cheat sheet that you write.
Projectile Motion
Example
A hunter tries to shoot a rare monkey
hanging from a tree with a dart gun. The
hunter has the monkey in his sights. But the
monkey notices the hunter and drops from
the branch exactly when he hears the hunter
blow. Will the hunter hit the monkey or miss
the target?
In class Question:
A) The dart hit the monkey.
B) The dart miss above the monkey.
C) The dart miss below the monkey.
Projectile Motion
Answer
Monkey and the
dart fell at the
same rate, under
the same
influence of
gravity.
So the dart will
hit the monkey.
What follows is not on
this weeks quiz!
Relative Motion
Relative motion is about relating measurements
of two different observers.
Usually we make measurements with respect to
a stationary frame (such as the ground).
But sometimes it may be useful to use a moving
frame of reference instead of a stationary one.
There are no specific equations to learn to solve
relative velocity problems.
But you will have to use previous equations and
be very careful with your notation.
Relative Motion
Keeping track of your subscripts will be the key
to relative motion.
Let’s say that there are two cars (A and B)
moving with different speeds in different
directions.
An observer stationary with respect to the
Earth (E) is where we normally take our
measurements.
But we may need to take measurements of A
with respect to B (or vice versa).
Relative Motion
The position of car A as measured by E is:
The position of car B as measured by E is:
The position of car A as measured by car B is:
We can relate these
positions by the
following equation:
We can then relate
velocities by:
Relative Motion
Example
A boat’s speed in still water is 20.0km/hr. If
the boat is to travel directly across a river
whose current has a speed of 12.0 km/hr, at
what upstream angle must the boat head?
Answer
First, you must define a coordinate system.
Let’s say that the boat wants to travel
directly north and that the river current
flows directly west.
Relative
Motion
Answer
According to the equation:
vbs = vbw + vws
Let’s make a quick diagram of the situation:
vws
N
W
vbs
θ
vbw
The boat will point 36.9o
East of North
Forces
What is a force? How would you define it?
What are some examples of forces?
Forces are pushes or pulls that may or may not
affect the motion of an object.
Forces are vectors (magnitude and direction).
Usually we think of forces as occurring between
two objects that are in contact with one another.
But this is not always the case. Gravity, for
example, is a force that occurs between two
objects not in contact (Earth and a ball in the air).
Forces
We say that forces can either be contact forces or
field forces (action at a distance).
Forces
We have that stated that there are many forces in
the universe.
But, as best as we know today, they can be broken
down into 4 fundamental forces which make up the
multitude of forces we experience in everyday life.
The Four Fundamental Forces of Nature:
1) Strong Nuclear Force
2) Electromagnetic Force
3) Weak Nuclear Force
4) Gravitational Force
Forces
Sir Isaac Newton was the first to introduce the
idea of forces (F) to explain the motion of objects.
He came up with three important laws which
govern how we view forces and motion.
Newton’s First Law of Motion:
An object continues in its state of motion at a
constant speed along a straight line, unless
compelled to change that state by a net force.
Where a net force is a vector sum of all forces on
an object (ΣF).
Forces
In order to help us understand Newton’s Laws we
will turn to the help of force diagrams (also called
free-body diagrams).
With a force diagram you represent the object
experiencing the motion as a dot and draw all the
force vectors acting on that object.
For example, for a chair sitting in the middle of a
room the force diagram would be:
Fground on chair
Fgravity on chair
chair
By Newton’s 1st Law:
ΣF=0
Inertia
Some things are harder to change the motion
of compared to others.
For example, if a baseball is thrown at you it
is rather easy to stop its motion.
But if a bowling ball is thrown at you (with
the same velocity as the baseball) it is much
harder to stop its motion.
Why is one harder to stop than another?
Inertia
Inertia is the natural tendency of an object to
remain at rest or in motion at a constant speed
along a straight line.
Inertia is a scalar.
Note: inertia is not a force, but merely a tendency.
Inertia is measured by the quantity of mass (kg).
Something that has a mass of 1,000kg has 1,000
times more inertia than something that has a mass
of 1kg.
Newton’s 2nd Law
Newton’s Second Law of Motion:
When a net external force (ΣF) acts on a mass m,
an acceleration, a, will result per the following
formula:
or
For Next Time (FNT)
Keep on working on Chapter 3 HW.
review homework in preparation
for quiz on Friday!
Keep on Reading Chapter 4.