|
Mr. Rogers AP Physics C Study Guide -- Newton's Laws
|
|
|
|
S F = ma |
S Fx = 0,
S Fy = 0,
S Fz = 0
for a = 0 |
Components of weight force Fw on a slope
with angle b
Fwn = Fw cos b
Fwp = Fw sin b
|
|
|
Definitions of "g"
- unit of acceleration = 9.8 m/s2
- acceleration due to gravity (freefall only)
- measure of gravity field strength
|
Conditions for Static Equilibrium
- a = 0
- v = constant (often zero)
- the sum of the forces in each dimension = 0
|
Newton's Laws
- 1st: establishes inertial frame of
reference
- 2nd: F = ma
- 3rd: Forces always appear in pairs acting in opposite directions
on 2 different objects.
|
Free body diagrams (FBD):
- Shows only forces from the outside world that act on a body.
- Shows only a single force from an action/reaction pair.
- Never shows internal forces.
- Acceleration or velocity may be placed beside the FBD but never on
it.
|
|
|
Problem Solving Tips: Newton's
Laws |
|
Use of "g": Note that "g" is usually used as measure of gravity
field strength in dynamics problems. It is rarely used as the
acceleration due to gravity. |
bathroom scales reading = normal force, NOT the weight force.
Likewise the normal force not the gravity force creates the sensation of
weight. |
Ropes:
- Can pull but can never push
- Ropes are modeled as massless
- Tension in an ideal rope is the same throughout the rope
|
Action reaction pairs:
forces
always come in pairs that are equal in magnitude and opposite in
direction. For a person standing on the floor, the action reaction
pair for the Earth's gravity force acting on the person is the
person's gravity force acting on the Earth.
The normal force is not
the action reaction pair of the gravity or weight force. |
|
|
Dynamics |
Statics |
-
Elevator Problem
- Bob has a mass = 100 kg. He has been told that
he can lose weight by descending in an elevator. He places a bathroom
scale in the elevator, stands on it, and presses the down button causing
him to descend at an acceleration of 4 m/s2. What does
the bathroom scale read on the way down?
|
- Buzzard Problem
- Two telephone
poles are separated by a distance of 200 m. A perfectly horizontal
wire with a mass of zero is stretched between them. A 2 kg buzzard
lands on the mid point of the wire and causes it to deflect by 1 cm.
Find the tension in the wire.
|
-
Harness Type Elevator Problem
- Bob still has a mass = 100 kg. He
decides to install an elevator (of sorts in his house). The "elevator
consists of a rope running over a pulley attached to the ceiling. To
operate the make shift elevator, Bob ties the one of rope around his
waist and pulls downward on the other dangling rope. This action makes
him accelerate upward at a rate of 3 m/s2. What is the
tension in the rope?
|
-
Scale Problems
- What will each of the spring scales read?
Assume that the pulleys are massless and frictionless and the ropes and
scales massless.
|
|
| freebody
diagrams |
Static conditions |
|
| inertial frame
of reference |
|
|
| |
|
|
|