Download Phys_21_J5_Forces_Friction_Pulleys

Newton’s Laws
And Friction
In this laboratory you will investigate some aspects of Newton’s Laws and friction we
have discussed in class.
Friction:
a) Consider a block being pulled by a string on a horizontal surface with friction as shown
in the figure. Assuming the block is pulled with the string being kept parallel to the table
surface draw a free-body diagram that illustrates all the forces acting on the block. If the
block is being pulled at a constant speed how do the applied force F and friction force f
compare in direction and magnitude? What about the contact force from the table N and
the weight of the block W?
F
b) Using the spring-scale provided, while the block is on its largest surface, determine the
maximum static friction that can exist between the block and table. Now pull the mass at a
slow steady rate and determine the kinetic friction that exists between the block and table.
How does the maximum static friction compare with the kinetic friction?
Maximum Static Friction =
Kinetic Friction =
F
c) Using the results of (b) predict what force will be required to pull the block at a constant
speed when its mass is doubled. What force would be necessary if its mass were tripled or
quadrupled?
Prediction:
d) Determine the mass of the block and test your predictions to part (c) by loading the
appropriate masses on top of the block. How do your observations compare with your
predictions?
F
Observations:
e) The “coefficient of friction”  is defined by f =  N where f is the appropriate friction
(e.g. kinetic or static) and N is the normal force between the two surfaces. From your data
what is the coefficient of friction that characterizes the wood-track surface?
f) In the friction law f =  N the normal force between the surfaces is used rather than the
weight. Can you explain why this is distinction is important? If one were to plot the
applied force F versus N what would the graph look like?
g) Consider the dependence of the friction force on the contact area between the block and
table surfaces. By tilting the block on its side and pulling it at constant speed determine the
friction force that acts when: the block is pulled by itself, doubled in mass, tripled in mass,
and quadrupled in mass. Compare your results for the coefficient of kinetic friction with
those of part (d). Does the coefficient of friction appear independent of area?
Observations:
F
h) Suppose the block is placed on an inclined plane of angle  as shown in the figure. Draw
a free-body diagram for the block assuming it is sliding down the incline at constant speed.

i) Establish a coordinate system with axes perpendicular and parallel to the incline.
Resolve each force into components and write down two relationships that express the fact
that the block is moving down the incline with constant speed.
j) Solve the equations of part (i) and determine a relationship between the coefficient of
friction  and the angle of the incline. Does your result depend on the block’s mass or
gravity? If not can you explain why?
k) Incline the track so that the block travels down it with constant velocity. Measure the
inclination (“angle of repose”) of the track and from this determine the coefficient of
friction. How does this value of  compare with your previous value?
Pulleys:
a) Consider the following problem: Suppose a 500g mass is hung from a spring scale as
shown in figure 1. Predict what reading (Newtons) the spring scale will read. What force F
must you exert to hold up the mass and how does this compare with the tension in the
string and scale reading?
F
Prediction:
b) Suppose instead that a string is placed over the pulley and attached to another 500g
mass as shown in the figure. Predict what the readings will be on each of the spring scales
shown in the figure.
Prediction:
c) Obtain a spring scale from the front of the room and test your predictions to parts (a)
and (b). If your result is off for the top scale consider taking into account the mass of the
pulley and other scale.
Observation:
F
d) Suppose that a 500g mass is hung from a pulley and the other end of the cord is held
fixed as shown in the figure. Predict the reading of the scale. How does this reading
compare with the force F2?
F1
Prediction:
F2
e) Suppose that instead the other end of the string is attached to the mass as shown in the
figure predict the reading of the scale in this case.
F
Prediction:
f) Set up the appropriate configuration and test your predictions to (d) and (e). Comment
on the agreement between your predictions and observations.
Observations:
g) Finally consider the pulley arrangement shown in the figure. How does the force F
compare with the weight of the mass (assume the pulleys have negligible weight)?
F
h) Can you devise an apparatus for which the force F will be 1/3 or 1/4 the weight of the
hanging mass?