+
ward
'
s
science
Page 2
Claim-Evidence-Reasoning: (CER) Shoe Bottoms
Teacher Key
Part I: Background
1. You and your friends are talking about physics class during lunch (obviously…) and your friend says, "yeah, well since
both horizontal and vertical forces are balanced in the free body diagram the object can't be moving!…" How would
you respond?
In the diagram, all forces are balanced. Without any other information, according to Newton's 2nd Law, we have to
assume that it is either moving at constant velocity OR still.
2. And then, out of nowhere your friends says, "…well, you see, a plane accelerates forward because of the net force provided
by the engines, but when you accelerate forward while walking there is no net force…" How would you respond?
When I push my shoe against the ground (while walking) my body most definitely accelerates forward, but my shoe
remains still. If you only take into account the free-body diagram of the shoe, you would notice balanced forces, but if
my body accelerates forward it's free-body diagram would exhibit a new force forward—it's the law!
3. Assuming you already didn't lose your lunch. Your other friend asks you a couple questions. How would you respond?
a) "What is normal force?"
Normal force is the force exerted perpendicularly away from a surface. In the case of a still shoe it's the force exerted
by the floor on the shoe to balance gravitational force.
b) "I don't understand what coefficient of friction means…"
The coefficient of friction is a ration of two forces, friction and normal. It is a unit less number that loosely represents
how "rough" the interaction is between two surfaces.
c) "If a moving object has 10N applied force forward and 20N of frictional force backward can you finish this sentence
concerning the motion of the object?"
The moving object will slow down to a stop and stay still. Reminder: Frictional forces resist motion in a given
direction…hence, they will not cause on object to speed up.
Situation:
You are designing a new pair of all-purpose shoe soles for a client. You have never made shoes before, so let's start
at square-one.
