Download Core Lab 4 Newton`s Second Law of Motion - eLearning

San Diego Unified School District
Physics 1,2 Core Laboratory Activity
Lab 4 Newton’s Second Law of Motion
California Standard Addressed
PH1. Newton’s laws predict the motion of most objects. As a basis for understanding this
concept:
1. c. Students know how to apply the law F = ma to solve one-dimensional motion problems
that involve constant forces (Newton’s second law).
SDUSD Student Performance Outcomes
Students will determine the acceleration of an object in an Atwood’s and Modified Atwood’s
machine.
Students will determine the net force exerted on an object in an Atwood’s and Modified
Atwood’s machine.
Students will graph and interpret the relationship between net force, mass and acceleration.
Name
Date
Period
Engage
On an aircraft carrier, aircraft are attached to catapults, which give
them the necessary acceleration to go from a standing position to 165
miles per hour (265.5 kph) in just two seconds.
1. What does the catapult exert on the plane?
2. Prepare a free body diagram for a plane accelerated by an aircraft
carrier catapult.
3. Sketch the appearance of a position vs. time, velocity vs. time and
acceleration vs. time graph for the jet (assume uniform acceleration).
4. If the mass of the plane were to be increased (ex. extra fuel tanks
attached to wings), what affect do you think this would have on the
acceleration of the plane?
5. If the catapult were to be given extra steam (increasing the force it exerts on the plane), what affect do you think
this would have on the acceleration of the plane?
1
Explore
Consider this situation.
The smart pulley (linear) sensor and computer can provide us specific information about the acceleration of the cart
and mass system.
We will use this information to develop a graph of force vs. acceleration.
The shape of this graph will tell us about the relationship between force and acceleration.
◊ Prepare the set up illustrated above.
◊ Carefully place the photogate over the low-friction pulley so that the infrared beam crosses the spokes of the pulley.
◊ Use a paperclip as a mass hanger.
◊ Transfer the masses to the hanger for each set up.
Number of
masses
transferred
to hanger
Mass on
hanger
(kg)
Tension
(weight of
masses on
hanger)
(N)
Trial 1
Acceleration
(from
graphical
display)
(m/s2)
Trial 2
Acceleration
(from
graphical
display)
(m/s2)
Trial 3
Acceleration
(from
graphical
display)
(m/s2)
Average
Acceleration
of system
(m/s2)
1
2
3
4
5
Plot Acceleration (x axis) vs.
Force (y axis) on the grid below.
It is important to note that the hanging mass is part of an
accelerated system.
The entire system includes the cart, the string, the
paperclips, the masses resting in the cart and the
masses on the hanger.
When one part of this system moves with a given
acceleration the entire system moves with that
acceleration.
Mass of the car
+Mass of the hanger
+Mass of all of the masses
= Mass of the system
6. What is the mass of the system?
2
7. Determine the slope of the line.
8. Compare the slope of the line to the mass of the system.
9. If the force increases, what happens to the acceleration of the system?
Consider this situation.
◊ Prepare the set up illustrated above.
◊ Carefully place the photogate over the low-friction pulley so that the infrared beam crosses the spokes of the pulley.
◊ Use a paperclip as a mass hanger.
◊ Place masses on the cart.
Number
of
masses
placed
on cart
Total Mass of
System
Mass of the car
+Mass of the hanger
+Mass of all of the masses
= Mass of the system
Trial 1
Acceleration
(m/s2)
Trial 2
Acceleration
(m/s2)
Trial 3
Acceleration
(m/s2)
Average
Acceleration
of system
(m/s2)
(kg)
1
2
3
4
5
Plot Acceleration (x axis) vs. Force (y axis) on the grid below.
10. If the mass of a system increases for a
constant net force exerted on the system, what
happens to the acceleration of the system?
11. If the net force exerted on a system of
constant mass increases, the acceleration will
increase. If the net force exerted on a system is
held constant, but the mass of the system is
increased, the acceleration will decrease. These
statements summarize…
3
Explain
12. Three students in your class have different ideas about the magnitude of forces and changes in velocity. Carefully
read the student arguments and decide which statement is best supported by the evidence.
Student A
“ The size of an unbalanced force has no affect on the amount of acceleration experienced by an object. All that
matters is that the force is unbalanced. If there is an unbalanced force, the object will experience acceleration.”
Student B
“ The bigger the pull or the push, the bigger the change in motion experienced by an object. There is a linear
relationship between the size of the exerted force and the acceleration experienced by an object.”
Student C
“The amount of mass to be moved is also important. If the mass is increased and the force is kept steady, then the
acceleration will decrease.”
Isaac Newton was the first to recognize that the acceleration of an object is directly proportional to the net force
exerted on an object. This linear relationship, Newton’s Second Law of Motion, is defined in the following way:
Fnet  m• a
Another way of stating this law is that the acceleration of an object is directly proportional to the net force exerted on
it.
A biography of Newton can be found at:
http://www.maths.tcd.ie/pub/HistMath/People/Newton/RouseBall/RB_Newton.html
4
Elaborate
Consider the set up illustrated below. The nuts have identical
masses and the pulley is frictionless.
16. Conduct the experiment and sketch the
displayed graphs below.
13. Prepare a free-body diagram for nut A.
14. If you were to give nut B a gentle downward tug, and then let
the system move, what kind of motion do you predict nut A would
have?
15. Predict the appearance of these graphs for nut A after the tug.
17. What kind of motion did nut A demonstrate
after the tug?
18. After the tug, was a net force exerted on the
system? How do you know?
19. Suppose one of the masses was larger than
the other, what kind of motion would you expect
to see? Why?
5
Consider the set up illustrated below. The nuts have identical
masses and the pulley is frictionless. The paperclip has a small,
but significant mass.
23. Conduct the experiment and sketch the
displayed graphs below.
20. Prepare a free-body diagram for nut A.
21. If you were to release nut B, what kind of motion do you
predict nut A would have?
22. Predict the appearance of these graphs for nut A after nut B
is released.
24. What kind of motion did nut A have after nut
B as released?
25. Measure the mass of the nuts and record
that value here.
26. Measure the mass of the paperclip, and
calculate the weight of the paperclip.
27. Using the weight of the paperclip as the net
force, and the sum of the masses of the nuts and
paperclip as the total mass of the system,
calculate the acceleration of the system using
Newton’s second law.
28. Compare this value to the displayed value
from the computer graph.
6
Evaluate
29. Summarize Newton’s Second Law.
This is a frictionless system. Select the
best fee-body diagram for this situation.
a)
This is a frictionless system. What would
an x vs.t graph look like for this system?
b)
c)
a)
c)
b)
This is a frictionless system. Determine
the acceleration rate for the block.
d)
e)
e)
d)
b) 0.5m/s2
a) 0.5 m/s
c) 2.0 m/s
d) 2.0m/s2
Select the best a free-body diagram for
this situation.
a)
What would an x vs.t graph look like for
this system?
b)
c)
a)
c)
b)
Determine the acceleration rate for the
block.
d)
e)
e)
d)
a) 0.5 m/s
b) 0.5m/s2
c) 2.0 m/s
d) 2.0m/s2
a)
b)
c)
d)
Determine the Net Force Exerted on the
system.
a) 30 N
b) 10 N
c) 20 N
d) 0 N
Determine the acceleration of the
system.
a) 1.0 m/s2
m/s2
b) 0.5 m/s2
c) 5.0 m/s2
d) 10.0
Determine the tension exerted on the 1
kg mass.
a) 30 N
b) 10 N
c) 20 N
d) 40 N
Select the best free-body diagram for the
3kg block.
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