Physics 106P: Lecture 1 Notes

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Physics 101:
Application of Newton's Laws

Review of the different types of forces discussed in
Chapter 4:
Gravitational, Normal, Frictional, Tension Forces
 Practice Examples
Physics 101: Lecture 9, Pg 1
Solving Problems




Identify forces acting on an object by drawing a free- body
diagram. Do not include forces that the object exerts on
its environment.
This is the most important step!
Select x and y axes and decompose all forces in the free-body
diagram into x and y components. Select the axis so that as many
forces as possible point along the x and y axis.
x and y components
Write Fnet=m a for each axis: Fnet,x=m a x and Fnet,y=m a y
(equilibrium: ax=0, ay=0)
Solve the two equations for the desired quantities !
Strong suggestion:
» work problem algebraically
» plug in numbers only at the end
Physics 101: Lecture 9, Pg 2
Example: Free-Body Diagram, Fnet=0
You are driving a car up a hill with constant velocity. On a piece of
paper, draw a Free Body Diagram (FBD) for the car.
How many forces are acting on the car?
1
2
correct
3
4
5
FN
V
f
W
Physics 101: Lecture 9, Pg 3
Example: Free-Body Diagram, Fnet=ma
You are driving a car up a hill with constant acceleration.
How many forces are acting on the car?
1
2
correct
3
4
5
FN
a
f
W
Physics 101: Lecture 9, Pg 4
Example: Tension

A lamp of mass 4 kg is stylishly hung from the ceiling
by two wires making angles of 30 and 40 degrees. Find
the tension in the wires.
Physics 101: Lecture 9, Pg 5
Example: Tension

Consider two blocks of mass m1 and m2, respectively,
tied by a string (massless). Mass m1 sits on a horizontal
frictionless table, and mass m2 hangs over a pulley. If
the system is let go, compute the acceleration and the
tension in the string.
Physics 101: Lecture 9, Pg 6
Example: Normal Force

A 80 kg man is standing in an elevator which is
accelerating downwards at 4 m/s2. What is the force of
the elevator floor on the man ?
Physics 101: Lecture 9, Pg 7
Example: Normal and Kinetic Frictional Force

A block of mass m is sliding down a frictionless inclined
plane making an angle q with the horizontal. What is the
acceleration of the block down the plane and what is
the normal force exerted by the plane on the block ?
Repeat the problem with friction.
Physics 101: Lecture 9, Pg 8
Example: Static Frictional Force

A boy is playing with a pull-toy of mass m on the floor of
an elevator. Find the minimum horizontal force with
which the boy must pull the string in order to set the
toy into motion along the elevator floor. Assume
that the elevator is accelerating upwards with
acceleration a and the coefficient of static friction
between the toy and the floor is ms.
Physics 101: Lecture 9, Pg 9
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