Sierzega/Ferri: Linear Dynamics 10
Friction Can Be Rough
10.1 Observe and Find a Pattern
One object will be pulled along three different surfaces at a constant velocity. Observe the
spring scale reading for each experiment and draw a sketch, motion diagram, and a force
diagram.
Surface:______________
Surface:______________
Surface:______________
Spring scale reading:_______
Spring scale reading:_______
Spring scale reading:_______
Sketch:
Sketch:
Sketch:
Motion Diagram:
Motion Diagram:
Motion Diagram:
Force Diagram:
Force Diagram:
Force Diagram:
ΣF equation for the horizontal
direction:
ΣF equation for the horizontal
direction:
ΣF equation for the horizontal
direction:
a) What is different about the three surfaces?
b) Did each surface exert the same amount of force horizontally?
Sierzega/Ferri: Linear Dynamics 10
Did You Know?
The term friction refers to the resistive force the surface exerts on a moving object. This force is
exerted on an object in the direction opposite to its motion.
10.2 Reason
Carefully observe what happens to your feet when you walk. Analyze what happens when you
walk using motion diagrams and force diagrams. Consider the beginning and the end of each
step separately. Does the friction force exerted by the surface on your feet help you walk or is it
better if there were no friction?
10.3 Represent and Reason
A person pulls a rope, which in turn pulls a crate across a horizontal, rough surface, as shown
below.
Three motion diagrams are shown below for the crate (with velocity arrows only). In the table
that follows, construct a force diagrams for the crate. (hint: pay attention to the length of the
arrows!)
(a)
(b)
(c)
(a)
(b)
(c)
Sierzega/Ferri: Linear Dynamics 10
10.4 Represent
Nick pulls on a cart, exerting a 50-N force to the right. There is a 35-N friction force exerted to
the left. The cart has a mass of 20 kg.
a) In which direction is the object moving? How do you know?
b) In which direction is the object accelerating? How do you know?
c) Draw a force diagram and a motion diagram for the cart. Are they consistent? Explain.
d) Write a number statement for the total force exerted on the cart.
e) Determine the acceleration of the cart.
f) What would happen to the acceleration if the cart’s mass were to double?