Download mg - UF Physics

Applying Newton’s
Laws, Weight
Applying Newton’s Laws
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Book-Proposed Strategy:
Decide what object will have Newton’s 2nd law applied to it
Identify all the external forces acting on that object
Draw a FBD to show all the forces acting on the object
Choose a coordinate system. If the direction of the net force is
known, choose axes so that the net force (and acceleration)
are along one of the axes
Find the net force by adding the forces as vectors
Use Newton’s second law to relate the net force to the
acceleration
Relate the acceleration to the change in the velocity vector
during a time interval of interest
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Problem 1: Pushing a crate at an angle
A crate, of mass 100 kg, is being
pushed forward across a
horizontal surface. The
coefficient of friction between
the crate and the surface is 0.2.
The force is being applied to the
crate at a 30 degree angle with
respect to the vertical axis.
What is the minimum force at
which the crate will move from
its prone position? [g = 10 m/s2]
∘
30
F
100 kg
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Problem 2: Line of Blocks
Five blocks are lined up on a frictionless surface. Their
masses are, left to right, 10, 20, 30, 40 and 50 kg. The 10
kg block is being pushed to the right with a force of
150 N. The system of blocks is accelerating without
separation. What is the force with which the 30 kg
weight is pushing on the 40 kg weight?
|F |= 150 N
10 kg 20 kg 30 kg 40 kg 50 kg
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Concept: Inertial Reference Frames
Newton’s first law defines a set of reference frames for
which the observed physics will be the same.
All systems which move with constant velocities with
respect to each other will record the same velocity
changes when performing measurements:
An inertial system is an idealized construction. In our
calculations, we will assume that a fixed observer on
the Earth’s surface is in an inertial reference frame.
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Concept: Apparent Weight
Classic application of non-inertial reference frames is
an elevator accelerating or decelerating. Objects inside
the elevator appear to be gaining or losing weight
depending on the direction of the acceleration.
Newton’s 2nd law for the object:
a
N
y
Apparent Weight:
x
mg
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Problem 3: Apparent Weight
Luke stands on a scale in an elevator that has a
constant acceleration upward. The scale reads 960 N.
When Luke picks up a box of mass 20 kg, the scale
reads 1200 N. (the acceleration stays the same.)
(a) Find the acceleration of the elevator
(b) Find Luke’s weight
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Problem 4: Edge Pulley
Consider the system on the
picture. The 100kg block is
initially held in place. The
coefficient of friction between
the 100 kg block and the surface
is 0.2. Ignore the mass of the
pulley and string. [g = 10 m/s2].
100 kg
(a) If the 100kg block is released, does
the system move?
(b) If so, what is the acceleration of the system?
50 kg
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Concept: Air Resistance
• air resistance: the force with which air opposes
movement of objects through air
• air resistance is velocity dependent - the higher the
velocity of the object, the higher the air resistance
=αv
• a simple model is that in which F
• consequence: terminal velocity during freefall
air, resist
object
Fair
Fair
Fair
mg
mg
mg
v = vterm
v > vterm
v < vterm
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Concept: Four Fundamental Forces
• Electromagnetic Force
• electricity, magnetism, atoms, light
• Gravitational Force
• planets, galaxies
• Weak Force
• nuclear decay - keeps the stars “burning”
• Strong Force
• proton, neutron
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