HPP Activity20.v1
May the force be with you: Newton's 2nd Law
Exploration
Go to the station with the large, human-sized cart. Place the drag device under it. Get a friend to
make you move at constant velocity.
GE 1.
1. What must your friend do to keep you moving at constant velocity?
2. What does Newton's first law suggest about the net force on the cart in this
situation?
3. Is your answer consistent with the fact that your friend must push you?
Remove the drag device from the large cart. Obtain a large spring scale.
GE 2.
1. If you get on the cart and let your friend pull you so that there is a constant
force on you, what do you think the motion will look like?
2. Give it a try and describe your motion here.
Since it is difficult to make precise measurements on the large cart we will use
a model of this situation: a small cart with weighted line attached to it
providing a constant pulling force. There should be a station with such a cart
set up with a force probe and motion detector ready to take data.
3. Repeat the experiment with the small cart. What does the velocity graph
look like with this constant force applied?
Invention
We need to develop a mathematical model for how the force on an object depends on its
acceleration. You can use the small low-friction cart with hanging masses attached to it to
provide a constant force. By varying the hanging mass you can vary the force. Note that the
force probe measures force in Newtons, which is the SI unit.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity20.v1
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GE 3.
1. Collect data on force and acceleration for several values of hanging mass.
Record your data here.
a [m/ss]
F [N]
2. Use Excel to make a graph of Force as a function of acceleration. Paste
here.
3. Describe the relationship between force and acceleration using words.
4. Describe the relationship between force and acceleration symbolically, that
is, with an equation.
Newton's second law of motion specifies the relationship between net force on an object and the
resulting acceleration. Check with your instructor, then state Newton's second law of motion
here.
Newton's Second Law of Motion:
Application
GE 4.
Suppose you exert a known constant force on the small cart and measure its
acceleration. Now you double the mass of the cart and exert the same force on
the cart.
1. What do you predict will happen to the acceleration? Be quantitative.
2. Try the experiment with the cart. Record your data here.
Original mass of cart:
Activity Guide
 2002 The Humanized Physics Project
HPP Activity20.v1
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Force:
Acceleration:
New mass of cart:
New acceleration:
3. Are the results consistent with Newton's second law?
Activity Guide
 2002 The Humanized Physics Project