2 nd Law of Thermodynamics Lecturer: Professor Stephen T. Thornton.
Kinds of Forces Lecturer: Professor Stephen T. Thornton.
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Transcript of Kinds of Forces Lecturer: Professor Stephen T. Thornton.
Reading Quiz: A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be frictionless. Which of these sketches is the correct free-body diagram for this puck?
A B C
Reading Quiz: A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be frictionless. Which of these sketches is the correct free-body diagram for this puck? No net force, because of constant velocity.
A B C
Conceptual QuizConceptual QuizA) the force pushing the stone forward
finally stopped pushing on it
B) no net force acted on the stone
C) a net force acted on it all along
D) the stone simply “ran out of steam”
E) the stone has a natural tendency to be at rest
You kick a smooth flat
stone out on a frozen
pond. The stone slides,
slows down, and
eventually stops. You
conclude that:
After the stone was kicked, no force was pushing
it along! However, there must have been some some
forceforce acting on the stone to slow it down and stop to slow it down and stop
itit. This would be friction!!
Conceptual QuizConceptual QuizA) the force pushing the stone forward
finally stopped pushing on it
B) no net force acted on the stone
C) a net force acted on it all along
D) the stone simply “ran out of steam”
E) the stone has a natural tendency to be at rest
You kick a smooth flat
stone out on a frozen
pond. The stone slides,
slows down, and
eventually stops. You
conclude that:
Weight—the Force of Gravity; and the Normal
Force
a) 0
b) 40.0 N 0
c) 40.0 N 0
y N
y N
y N
F F mg ma
F F mg
F F mg
What happens when a person pulls upward on the box in the previous example with a force greater than the box’s weight, say 100.0 N?
There is no normal force!
A. The normal force from the table.B. The gravitational force the apple exerts on the Earth.C. The gravitational force the apple exerts on the table.D. The normal force the apple exerts on the table.
Conceptual Quiz
B)
A. The normal force from the table.B. The gravitational force the apple exerts on the Earth.C. The gravitational force the apple exerts on the table.D. The normal force the apple exerts on the table.
Tension in a Heavy Rope
Heavy rope:
Light rope:
3 2 1T T T
3 2 1T T T
We usually consider light ropes.
g
= mg
mass m
Elevator and counterweight (Atwood’s machine)
TI prefer T rather than F
E C
Magnitudes equal
a a
FT must be greater than mcg.
What is wrong with right diagram?
Conceptual Quiz: We have a 1.0 kg mass hanging from the string. The string is wrapped around a pulley so the string is horizontal. If we separate the horizontal string and insert a spring scale, what will the scale read?
A) 0
B) 1 N
C) 5.9 N
D) 9.8 N
Answer: D
The gravitational force pulling on the string is (1 kg)(9.8 m/s2) = 9.8 N. The tension in the string must equal this, and it is constant throughout. We measure the tension by inserting the spring. It must measure 9.8 N.
Conceptual Quiz: We have 1.0 kg masses hanging from two pulleys. We unhook the horizontal string and insert a spring scale. What will the spring scale read for the tension in the horizontal string?
A) 0 N
B) 9.8 N
C) 19.6 N
Answer: B
It doesn’t matter whether the right hand string is attached to the pole or to a pulley with a hanging 1.0 kg mass.
Solving Problems with Newton’sLaws: Free-Body Diagrams
1. Draw a sketch.
2. For one object, draw a free-body diagram, showing all the forces acting on the object. Make the magnitudes and directions as accurate as you can. Label each force. If there are multiple objects, draw a separate diagram for each one.
3. Resolve vectors into components.
4. Apply Newton’s second law to each component.
5. Solve.
Hanging Object. An object is hanging by a string from your rearview mirror. While you are accelerating at a constant rate from rest to 28 m/s in 6.0 s, what angle does the string make with the vertical?