Thursday, October 05, 2006
Topic 1: Motion
What are the 3 ways to describe motion?
o Position, velocity, and acceleration
How can we describe position?
o We use a vector, because it can tell us direction and distance (from some defined start
What are the 2 parts of velocity?
o How fast we are going (speed)
o What direction we are going in (direction)
What is acceleration?
o It is how fast (and in what manner) our velocity is changing
What is Newton's first law?
o An object that is not subject to any net outside forces moves at constant velocity
What is Newton's second law?
o The force exerted on an object is equal to the product of the object's mass times its
o The acceleration is in the same direction as the force
What is Newton's third law?
For every force that one object exerts on a second object, there is an equal but
oppositely directed force that the second object exerts on the first object
Gravity and Componential Motion
What is gravity? What determines the effect we will experience due to gravity?
o It is a downward force produced by the earth which acts on us, pulling us down
The gravitational effect we experience is always an acceleration of ~10 m/s2 --
REGARDLESS of our mass
This means that (neglecting air resistance) a feather falls just as fast as a brick
Why doesn't gravity cause us to fall through the floor?
o Because of the "normal force": an upward force produced by the surface we are
standing on that counteracts the gravitational force so that we don't move
Describe the motion of a projectile launched at some angle with respect to the horizontal.
o We can break up its initial velocity into horizontal and velocity components
Its horizontal velocity will never change, because there are no forces present to
change it (recall Newton's First Law)
Its vertical velocity will be subject to gravity, so its upward progress will slow,
eventually become zero, then start to head downwards
If you wanted to shoot a monkey in a tree, where should you aim? (Assume that the monkey will
drop and fall downwards as soon as the shot is fired).
o You should aim straight at the monkey (not above it or below it), because once the
bullet is in the air and the monkey has dropped, both objects are subject to the same
force (only gravity), and so they will meet at some point
Work and Energy What is the difference between kinetic and potential energy?
o Kinetic energy is the energy of movement: the amount of this one possesses is related
to one's velocity
o Potential energy is stored: the amount of this one possesses is related to its ability to
Discuss energy conversions.
o Energy cannot be created or destroyed, but it can be converted from one form into
o The types of conversions which occur and their efficiencies affect the things we see in
the real world
For example, some materials bounce better than others because they do a
better job of converting kinetic energy to elastic potential (while others just
convert kinetic into heat)
Discuss work, in particular the way it is related to potential energy.
o To do work on something is to exert a force on it and cause it to move some distance in
the direction of that force
o Work is related to potential energy because we can give an object potential energy by
doing work on it
For example, lifting a ball from the ground to a table
Imagine a scenario where (with friction present) you are pushing a block horizontally across the
floor. Describe the different forces acting on the box.
o There is gravity acting downwards, and the normal force acting upwards
o There is the human pushing force acting in the box's forward direction, and friction
acting opposite to that
The pushing force must exc