SPH 3U
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1
Normal Force and Elevators
a book is traveling in 4 different elevators. Find the apparent weight that a
bathroom scale would register (find the normal force acting on the book) in
the following 4 different situations
2
3
4
v=0
a=
m = 5 kg
v = constant
a=
m = 5 kg
a = - 1m/s2
a = 3 m/s2
m = 5 kg
m = 5 kg
Practice Problems:
1) A 60 gram stack of paper experiences a force due to air resistance of 0.30
N. Calculate
a) force due to gravity b) net force
c) acceleration
(Ans: a) 0.588 N
b) 0.288 N
c) 4.8 m/s2)
2) A person (70 kg) in an elevator experiences a normal force of 750 N.
Calculate the net force acting on the person and his acceleration.
(Ans: 64 N, 0.91 m/s2)
3) Barney the Dinosaur (80 kg) falls off his parade car and hits the surface,
coming to a stop in 40 milliseconds. If his velocity just prior to contact with the
ground was 5 m/s, calculate
a) acceleration
b) net force c) normal force
2
(Ans: a) -125 m/s [D] b) -10000 N [D] c) 10 784 N[U])
Tension
Example 1: Spiderman (that web slinging Superhero) climbs up a web. If the
maximum tension the web can support is 1500N, what is the maximum
acceleration Spidey can move along the web if his mass is 100 kg?
Example 2: A grocery bag is lifted out of the trunk of a car with an acceleration
of 3.0 m/s2. The bag (with its contents) has a mass of 5.0 kg. Will the bag break
if the maximum tension force the handles of the bag can withstand is 80 N?
Friction and Free Body Diagrams
friction acts on objects that are in _________ with other objects, _______
or opposes relative motion between object  brings object to a stop
- friction acts when:
1) brakes slow down a car
2) boat travels through water 3) person walks
on pavement
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if there was no friction, then a person would not be able to walk on pavement
(person would simply slide and slip, as if he/she was trying to walk on ice)
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the friction force is parallel to the surfaces that are in contact and opposes
motion
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the force due to friction is proportional to mass (its harder to push a heavy
rock than a lighter rock)
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the force due to friction is also dependent on the surfaces (its harder to
push a textbook across sandpaper than ice)
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force of friction is caused by small imperfections in the smoothness of
surfaces
In general, there are two types of friction forces:
a) ________ friction - both objects are stopped and a force is being applied to
one of the objects.
The magnitude of the frictional force is given as :
Ff = s|Fn|
b) ___________ friction - objects are moving relative to each other
The magnitude of the frictional force is given as
Ff = k|Fn|
 (pronounced m-yoo) is called the coefficient of friction, and depends on the
properties of the two surfaces in contact.
In general, s > k (it is harder to move an object that is stopped than to keep an
object moving).
Example 1: Push an object across a floor, resistance is due to friction, decrease
the resistance by tilting the object so that there is less contact between the
surfaces.
A)
B)
Example 2: A car seat has a 5.0 kg smooth ball and a 5.0 kg dumbbell sitting on
it. The car is moving at constant velocity when suddenly it comes to a stop.
Why does the ball roll forward, but the dumbbell remains at rest?
Example 3: Find the acceleration of an object that has a mass of 2.0 kg, and
experiences an applied force of 10 N [E]. The coefficient of static friction is
0.10.
- solve this problem into x and y parts (components)
x
y
Practice Problems:
1) The coefficient of friction is 0.75 acting between a piece of rubber (mass is
0.5 kg). Calculate the force of gravity acting on the object, the normal force
and the maximum force due to friction.
(Ans: 4.9 N, 4.9 N, 3.675 N)
2) A 5 kg block is initially stationary when a force of 10 N causes it to
accelerate forward at 0.5 m/s2. Calculate:
a) force due to gravity
b) normal force
c) net force
d) force due to friction
e) coefficient of friction
(Ans: a) 49 N
b) 49 N
c) 2.5 N
d) 7.5 N
e) 0.153)
2) A 1 kg superpuck is initially sliding across the ice (no applied force) and has a
coefficient of friction of 0.25. a) Calculate the acceleration of the puck.
b) If the mass of the puck doubles, calculate the acceleration. (Ans: 2.45 m/s2, -2.45 m/s2)