Download What a Drag!

HPP Activity A35.v1
What a Drag!
You have now dealt with conservative forces, forces that conserve the total mechanical energy of
a system. The gravitational force and elastic forces are good examples. What about forces like
the friction force? They do work but is the work they do stored as potential energy somewhere?
If so, where? If not, then where did the energy go?
Exploration
GE 1.
Let's head back to the human-sized cart one more time. Set up the cart with
significant friction and practice pushing someone on the cart at a constant
speed in front of the motion detector. Now connect the force plate with
handles and use it to push the cart and volunteer at a constant speed.
1. What force is required to keep the cart moving at a constant speed? How
big is the friction force in this case? Why?
2. How much work did the pusher do in moving the cart/person 2.0 m?
3. How much work did friction do as the cart/person moved through the
same distance as in question 2? Be careful to get the correct sign of the work
done! What is the angle between the friction force and the displacement of the
cart?
4. What is the total work done on the cart?
5. What is the change in gravitational potential energy of the cart?
6. What is the change in kinetic energy of the cart? (Only consider what
happens to the cart while it is being pushed with a constant force.)
7. What is the change in total mechanical energy of the cart/person?
Activity Guide
© 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity A35.v1
2
8. Now that you know the friction force, give the cart/person system a big
push and let them coast to a stop. How far did they travel while stopping?
9. How much work did the friction force do in stopping the cart? Was it
positive or negative?
10. What was the change in mechanical energy for the cart/person? We need
an actual value here!
11. Compare your answers from 9 and 10.
Invention
GE 2.
The work done by the friction force is always negative and often causes a
decrease in the total mechanical energy of an object. Work done by pushes
like the one propelling the cart/person above can either increase or decrease
the total energy of a system of objects. We call these forces non-conservative
because they do not conserve the total mechanical energy of a system of
objects. We can generally write:
Wnc=ΔKE+ΔPE
where Wnc is the work done by non-conservative forces like friction and the
push in the trials above. If no non-conservative forces act then the total
mechanical energy of the system remains constant.
In GE 1 when the cart/rider moved at a constant velocity, chemical potential
energy allowed the student to push the cart and rider (i.e. do work on the
cart/rider). Friction did an equal amount of negative work on the cart/rider so
that there was no change in the total mechanical energy of the cart/rider.
Application
GE 3.
Activity Guide
© 2010 The Humanized Physics Project
HPP Activity A35.v1
3
You've seen the total mechanical energy decrease to zero when only friction
was present (GE 1: 8) but what happens if we increase the force that was used
in GE 1: 1? Let's try to double the force used in that case.
Push the cart/person with twice the force that it took to move the cart at a
constant velocity. You may have to try several times to get it right. Try to
keep the force as constant and as close to double the previous force as
possible. Collect both force and motion data for at least 2.0 m.
1. What happened to the cart/person? Describe their motion.
2. How big was the pushing force in this case?
3. How much work was done on the person/cart by this force in moving a
distance of 2.0 meters?
4. What was the change in mechanical energy of the cart/person? Show your
calculations!
5. Subtract your answer in 4 from your answer in 3 and record it here. What
is the meaning of this difference?
6. Compare the work done by friction in GE 1: 2 to your answer to 5. Was
the work done by friction the same in GE 1 as it was in this case? Explain.
Activity Guide
© 2010 The Humanized Physics Project