PHYSICS 103: Lecture 7
Agenda for Today:
• Review HW Solutions
• Applications of Newton’s Laws
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
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Air resistance
Friction
Ramps
Example problems
NEWTON’S LAWS OF MOTION
FIRST LAW:
An object at rest will remain at rest and an object in
motion will remain in motion unless acted upon by an
external force.
SECOND LAW:
The net force on an object is equal to the product of
that object’s mass times its acceleration. The
acceleration is in the same direction as the force.
F=m.a
THIRD LAW:
For every force that one object exerts on a second
object, there is an equal but oppositely directed force
that the second object exerts on the first object.
The force of air resistance R acting on a sky diver
increases as the velocity increases.
R gets larger as the velocity increases
Fnet  mg  R
a
m
m
R
g
m
ag
If R=W then you don’t accelerate =>
you reach terminal velocity
Copyright © 2001 by The McGraw-Hill Companies, Inc. All rights reserved.
Test your understanding
Question: A man and a woman are parachuting from the
same altitude. If the man is twice as heavy as the woman, and
they have identical parachutes, who gets to the ground first?
Question: A skydiver jumps from a helicopter. As she falls
faster and faster through the air, does her acceleration
increase, decrease, or remain the same?
FRICTION
Question: When I slide a block across a table, why does
it come to a stop?
FRICTION
All surfaces are not
perfectly flat
• Opposes relative motion of two surfaces
• Acts to bring two surfaces to one velocity
• Is a reactionary force
• Comes in two kinds : static and sliding
FRICTION
Static Friction:
-acts to prevent objects from starting to slide -forces vary from zero to an upper limit
Sliding or Kinetic Friction:
-acts to stop objects that are already sliding
- forces have a fixed magnitude
Ff
FN
Fg
NO Friction
F
Static Friction
Static Friction
Static Friction
Kinetic Friction
FRICTION
Increases:
• When you push surfaces more tightly together
• When you make surfaces rougher
Static Friction: Ff = ms FN
Kinetic Friction:Ff = mk FN
mk < ms
Peak static force is greater than sliding force
Test your understanding
• Why is it hard to move forward in your car on ice?
• Why does sand on ice help?
• Would you rather be in a porsche or buick in snow storm?
• Does skidding make you come to a stop faster or slower?
• How do anti-lock brakes work?
Ramps
•Why can you lift a very heavy object with a ramp?
• Why is it easier to lower an object than raise it?
• Why is it more difficult to ride a bike up a steep slope than
a more gradual one?
Ff
FN
FN
Fg
q
Fg
We already learned that when all forces balance there is no motion
Unbalanced forces induce acceleration
Forces on a ramp
FN = Fg cosq
a// = g sinq
q
Fg q
Question: What is the acceleration of a 30 kg block sitting on a
frictionless ramp with a 30 degree incline if I apply a 200 N
force along the ramp?
FN = Fg cosq
We know: m = 30 kg
a// = g sinq
q
Fg = mg = 30 (9.8) N = 294 N
Fg q
a = Fnet/m = 53/30 m/s2 = 1.77 m/s2
Example Problems Distributed in Class: Test your
understanding
Main Points from Today’s Lecture
• Applications of Newton’s Laws
 Air resistance
You should understand that there is a force acting against
gravity causing your acceleration to be less than g. This
force increases with velocity until it equals your weight.
You then reach terminal velocity
 Friction
You should understand that friction is a reactionary force
that opposes motion. It comes in two kinds (static and
kinetic) and it is a function of how rough a surface is and
the normal force.
 Ramps
You should understand how to find the net force on an
object on a ramp to find the acceleration
 Example problems