Physics 105, Fall Term, 2009
Did you complete the reading assignment?
1) An object will
continue in its state of motion (moving or at rest) unless acted upon by a
2) The net force on an object is the
product of the mass of the object and its acceleration.
3) Every force is opposed by a force that is
equal in magnitude but opposite in direction.
All science is based on Newton's 3 laws of motion. Before Newton described
nature in these laws, "science" was referred to as natural philosophy and was
based on the belief of philosophers' ideas on how elements desire to interact
with each other. Now, several hundred years after Newton, we observe the
behavior of nature as reactions to forces applied to them.
Go to the applet at:
http://phet.colorado.edu/sims/forces-1d/forces-1d_en.jnlp where we will
observe Newton's laws.
Click the friction 'Off' bubble and check the 'Barriers' check box on the right
side of the applet.
Before applying a force to the file cabinet, watch it for a while as it is at
rest. Does it change its state of motion?
Now apply a large force to the cabinet for a short moment by clicking on it and
quickly dragging it with the mouse to the right or left and then releasing the
mouse. For the short period of time that the motion is applied, does its state
of motion change?
After the file cabinet is released from the force applied to it, does its state
of motion change?
Eventually the file cabinet stops when it hits the brick wall. Why, in physics
terms, does the file cabinet stop moving?
The wall is applying a force on the file cabinet which causes it to change its state of motion from moving to being at rest.
Newton's 2nd Law
Now instead of manually applying force with the mouse, set a constant
application of force by typing in a force in the 'Applied Force' box on the left
side of the applet and then pressing the 'Go' button. Select the 'Graph Applied Force' and 'Graph Acceleration'
buttons. You can also select which object you are applying the force to by
selecting them from the right side of the applet. To reset the position and
graphs press the 'Clear' button on the left side of the applet.
Apply a constant 100 N force to each object and watch its motion and the applied
force graphs and acceleration graphs. Which object has the most applied force on
it according to the graph?
All the same
Which object has the greatest acceleration according to what you see and what is
on the graph?
Explain your result from the question above.
All objects have the same amount of force acting upon them. The text book has the least mass. Since F=ma, and the text book has the least mass, it must accelerate the fastest under the same force.
What is the acceleration of each object after they hit the wall? What is the net
force on the object?
Acceleration is zero since it is not moving. If an object is not accelerating, there is a net force of zero acting on it.
Newton's 3rd Law
When an object is being pushed against a wall, the force being applied on that
object is now being applied on the wall. If this is the case, why does the wall
The wall does not move because there is an equal force pushing it in the opposite direction.
Using the values you did for the activities for Newton's 2nd law, how much force
does the wall put on the object when it is being pushed into the wall?
How much force does the object put on the person pushing it while it is still
moving (before it hits the wall)?
The comments entered in these last two boxes go into a big anonymous data file that I will use to guide the lectures of the day. That means two things:
1) If you have a specific question or concern and would like an individual
answer you will need to come by the office or send an email.
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Was there anything that you didn't understand in the reading assignment?
What was confusing to you?
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