Download Measuring Mass: The Inertial Balance

Name: _______________________________________
PHYC 101
Summer 2016
Newton’s First Law of Motion
http://www.physicsclassroom.com/Class/newtlaws/u2l1a.cfm
State Newton’s 1st Law of Motion
1st Law Demonstrations/Activities
 Place a coin on top of a card on top of a cup or beaker. Try to slide
card out from under the coin so that the coin lands in the cup
(beaker).
Another variation of this is to place a coin on top of a playing card on
your finger. With the index finger of your other hand, quickly hit the
card and try to knock the card out from under the coin, which should
rest on your finger. See if you can do it with several coins stacked on top of each other.
the
 Hold a 100 g mass in one hand and a 1 kg mass in the other. Hold both arms out and try to quickly move
them back and forth.
 Secure a string to a battery-operated toy car. Turn the car on and hold the string on the floor (or table if
there is enough room) so that the car moves in a circle around your hand. Pick an opportune time, let go of
the string, and observe the car as it moves away.
Place “targets” around the room or on the table and attempt to let go such that the car will move off and hit
the target.
 BE VERY CAREFUL WITH THIS ACTIVITY!!! Suspend a 500 g mass
with a piece of light thread. Tie another piece of thread to the bottom
“hook.” While holding the top thread to suspend the mass in the air,
grab the bottom thread and give it a quick pull. Record your observation.
Attach a new piece of thread to the mass and suspend it again. Now grab
the bottom thread and give it a slow steady pull until one of the threads
breaks. Record your observation.
 A potentially less expensive variation of the popular “pull the tablecloth out from under the dishes”
demonstration is to pull a sheet of ordinary paper out from under objects that are placed on it. Try this with
some objects at your table (desk).
 Place a doll or action figure in or on a toy car. Roll the car across the floor or table and let it crash into a
barrier.
Make it crash again with the doll or action figure secured to the toy car.
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Name: _______________________________________
PHYC 101
Summer 2016
 Make a modeling clay figure and place on top of a small toy car. Allow the car to roll down an incline and
then crash into a barrier. Observe what happens to the clay figure after the crash. Compare the results
when the car is released from various heights.
 Toss a tennis ball straight up in the air while running or walking rapidly across the room and. Continue
walking/running at the same speed and observe the path of the ball.
Do this again and change your speed immediately after tossing the ball. Describe what happens if you 1)
speed up, and 2) slow down, after releasing the ball.
 Tie or tape a string to a tennis ball. Place a bucket on the floor. Hold the string while running across the
room and release the string so that the ball falls into the bucket.
 Others?
Newton’s Second Law of Motion
http://www.physicsclassroom.com/Class/newtlaws/u2l3a.cfm
State Newton’s 2nd Law of Motion (Be sure to include all three parts)
The Acceleration Direction
The Relationship between Net Force, Mass, and Acceleration
 Use a 0-20 N scale to suspend a 1 kg
mass. Name two forces acting on the
mass while it is suspended in the air.
 Draw a diagram illustrating these two
forces.
 Which one of these two forces do we have control over (i.e., which one can we change)?
 Which one of these two forces stays the same in all circumstances while on earth?
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Name: _______________________________________
PHYC 101
Summer 2016
 While the mass is suspended in air by the scale (at rest and remaining at rest), we know that there
is no net force acting on the mass. This means that the scale pulls upward on the mass with the
same amount of force that the earth’s gravity pulls downward on the mass. The scale therefore
reads the same as the weight of the object.
 Hold the mass stationary in the air. Give the scale a quick upward pull in order to make the mass
move (accelerate) upward. What did you notice about the reading on the scale?
In this case, to make the mass accelerate upward, the upward force of the scale on the mass had
to be ____________________________ the downward force of the earth’s gravity on the mass
(weight of the object). This is why the scale reading was ____________________________ the
normal weight of the mass.
 Is the scale reading greater than, less than, or equal to the object’s weight while the mass is being
raised at a steady rate? ___________________
In this case, the mass moves upward at a steady rate. Its acceleration is therefore
_______________________, so the upward force of the scale on the mass had to be
_______________________ the downward force of the earth’s gravity on the mass. This is why
the scale reading was _______________________ to the normal weight of the mass.
 Hold the mass stationary in the air. Quickly lower the mass. What did you notice about the
reading on the scale?
In this case, to make the mass accelerate downward, the upward force of the scale on the mass
had to be _________________________ the downward force of the earth’s gravity on the mass.
This is why the scale showed _________________________ the normal weight of the mass.
 Is the scale reading greater than, less than, or equal to the object’s weight while the mass is being
lowered at a steady rate? ___________________
In this case, the mass moves downward at a steady rate. Its acceleration is therefore
_______________________, so the upward force of the scale on the mass had to be
_______________________ the downward force of the earth’s gravity on the mass. This is why
the scale reading was _______________________ the normal weight of the mass.
 You should be able to explain what happens to the reading on the scale while the mass is slowing
to a stop while being raised.
 You should be able to explain what happens to the reading on the scale while the mass is slowing
to a stop while being lowered.
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Name: _______________________________________
PHYC 101
Summer 2016
Newton’s Third Law of Motion
http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
State Newton’s 3rd Law of Motion
Interaction Forces
Determine the reading(s) on the scales in each of these cases. Be sure to “zero” the scales horizontally or
vertically as needed in each case.
Hook two scales (referred to as “A” and “B”) together. Pull on Scale A so that its reading is 15 N.
 What is the reading on Scale B if both scales are held motionless?
________
 What is the reading on Scale B if both scales move slowly to the left?
________
 What is the reading on Scale B if both scales move slowly to the right?
________
Hook the scales together. Describe what happens when you try to keep Scale A at 15 N and Scale B at 5 N.
Hook each scale to one end of a 1.0 m (or longer) piece of string. Pull on Scale A so that its reading is 15 N.
 What is the reading on Scale B if both scales are held motionless?
________
 What is the reading on Scale B if both scales move slowly to the left?
________
 What is the reading on Scale B if both scales move slowly to the right?
________
Have each scale pull on the string and describe what happens when you try to keep Scale A at 15 N and Scale B at
5 N.
Interaction Pairs – Example 1
You hold a ball in your hand. Name two pairs of 3rd Law interaction forces that are relevant to this situation.
1a –
1b –
2a –
2b –

Forces 1a and 1b are ______________________ equal to each other.

Forces 2a and 2b are ______________________ equal to each other.

Forces 1a and 2a are ______________________ equal to each other.
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Name: _______________________________________
PHYC 101
Summer 2016
Which of these forces determine what happens to the ball?
When the ball is stationary in your hand, the _____________ force of ____________________________________
_________________________ must be ___________________________________ the ______________ force of
___________________________________________________________________________________________.
To toss the ball upward, the _____________ force of _________________________________________________
must be __________________________________ the ______________ force of __________________________
__________________________________________________________________________.
Interaction Pairs – Example 2
You stand on the floor. Name two pairs of 3rd Law interaction forces that are relevant to this situation.
1a –
1b –
2a –
2b –

Forces 1a and 1b are ______________________ equal to each other.

Forces 2a and 2b are ______________________ equal to each other.

Forces 1a and 2a are ______________________ equal to each other.
Which of these forces determine whether or not you can jump into the air?
While you stand in contact with the floor, the _____________ force of ____________________________________
_________________________ must be ___________________________________ the ______________ force of
___________________________________________________________________________________________.
To jump upward, the _____________ force of _________________________________________________ must
be __________________________________ the ______________ force of ______________________________
___________________________________________________________________________________________.
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