Transcript Powerpoint for today

Newton's Laws of Motion
Newton 0th Law
Objects are dumb - They have no memory of the past and cannot predict the
future. Objects only know what is acting directly on them right now
Newton's 1st Law
An object that is at rest will remain at rest and an object that is moving will continue
to move in a straight line with constant speed, if and only if the sum of the forces
acting on that object is zero.
Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the
object
Newton's 3rd Law
Recall that a force is an interaction between two objects. If object A exerts a force
on object B then object B exerts a force on object that is in the opposite direction,
equal in magnitude, and of the same type.
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Gravitational Force & Coordinate Systems
A. Fg,y = -- mg
B. Fg,y = -- mg sin(theta)
C. Fg,y = mg cos(theta)
D. Fg,y = -- mg cos(theta)
E.Can’t tell what Fg,y is
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Samantha the Skier
Samantha is skiing at Taos on an intermediate slope that
makes a
25 degree angle with the horizontal.
A. Assuming Friction is too small to be accounted for,
what is Samantha’s acceleration?
B. After Samantha has gone 100 m, what is her velocity?
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Newton’s Second Law
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Newton 2 Demonstrations
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Brainstorm:
What do we know about Friction?
1.
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Static & Kinetic Friction - Part I
Demonstration
Explain the graph, i.e. why does it look this way?
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Static & Kinetic Friction - Part II
Describe what is happening to the forces on the box and the
effect of the forces on the motion of the box from the pictures.
QuickTime™ and a
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are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
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Static & Kinetic Friction - Part III
Below is graph of the friction force exerted by the table on
the box.
A. Label times a-f that match the free-body diagrams in the
previous problem.
B. If the mass of the box is 3.0 kg, the maximum Ffs is 10 N,
and Ffk has an average of 6.0 N, find the coefficents of
static and kinetic friction.
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Coefficients of Friction
What can you deduce/generalize about friction forces
from this table?
Describe 3-4 real world situations that can be explained
by this table
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Parking on a Hill
A. If you park on a hill with a 10 degree slope with the car
held by the parking brake, what is the magnitude of the
frictional force that holds your car in place?
B. The coefficient of static friction between your car's
wheels and the road when wet is 0.30. What is the
largest angle slope on which you can park your car in the
rain so that it will not slide down the hill?
C. The coefficient of kinetic friction between your wheels
and the wet road surface is 0.25. If someone gave your
your car a push on the wet hill and it started sliding
down, what would its acceleration be?
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