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Physics 151 Week 11 Day 3
Topics: Newton’s 2nd Law and Applications
 Applying Newton’s 2nd Law
 Apparent Weight
 Free Fall
 Terminal Velocity
 Friction – Model vs. Brainstorm
Scales and Elevators (Apparent Weight)
How many of you would like to be able to control you
weight?
What is weight? What does a scale measure?
What is happening in this video?
http://www.youtube.com/watch?v=XE7MMBHslQ0
Slide 4-19
Scales and Elevators (Apparent Weight)
Consider a person with a mass of 60 kg is in an elevator
standing on a scale. The elevator is accelerating upward.
• Draw a system schema and 2 force diagrams:
One for the person and one for the scale
• What does the scale read?
• Use Newton’s 2nd law to determine what the scale
reads (This is apparent weight)
Slide 4-19
Apparent Weight
Slide 5-24
Scales and Elevators (Apparent Weight)
Suppose a person with a mass of 60 kg is in an elevator
standing on a scale.
Use the system schema and force diagrams of the scale and
the person to determine what the scale would read for the
following situations:
A. If the elevator is descending at 4.9 m/s.
B. If the elevator has a downward acceleration of
4.9 m/s/s.
C. If the elevator has an upward acceleration of
4.9 m/s/s.
Slide 4-19
If I drop a set of key and a piece of paper
If I drop a piece of paper and a set of keys,
which lands first?
Slide 4-19
Free Fall, Drag force, and Terminal Velocity
The hammer and the feather
(on the moon)
http://video.google.com/videoplay?docid=692689157225978
4994
Sky Diving and terminal velocity
Slide 4-19
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?
Slide 4-19
Discussion of U5: Worksheet 3
Problem 7
A 100 N force is applied to a 50 kg crate resting on a level
floor. The coefficient of kinetic friction is 0.25.
Is the 100 N force sufficient to cause a motionless crate to
accelerate?
Draw a force diagram, then explain why or why not based on
your calculations.
Slide 4-19
Penguin in a box
A loaded penguin in a box, together weighing 60 N, rests on a plane
inclined at 20° to the horizontal. Between the box and the plane, the
coefficient of static friction is 0.26 and the coefficient of kinetic friction
is 0.15.
1. What is the minimum magnitude of the applied force F,
parallel to the plane, that will prevent the sled from
slipping down the plane?
2.
2.
Brainstorm: What do we know about Friction Force?
1.
2.
3.
4.
5.
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Seems to relate to pushing force (Drag Force?)
- Increases with more contact surface
- Not dependent on surface area, but surface type
Car needs friction for acceleration including steering
Can determine acceleration along with other forces
2 types static and kinetic, once object starts moving crosses from Ffs to Ffk
/ also rotational/rolling Friction is smallest friction
Caused by electron repulsion between two objects
- Friction opposes (in opposite direction of) applied force
- Direction of Friction force is Opposite of direction of motion / acceleration
- Parallel to motion / Resists force of acceleration
Friction Force is a contact force that acts on an object by its surface
Converts Kinetic Energy to Thermal Energy
Friction Force depends on what is in contact (involves a coefficient )
Friction is resistance between 2 surfaces in contact / resists change
Friction does not depend on mass / related to weight
It slows things down and makes them stop (can be small/negligible)
Can occur doing motion or without motion
It acts two ways
It usually accompanies a normal force / perpendicular to normal force
It is in the negative y direction, parallel to the surface of an object
Ffs > Ffk - initially starting object moving is harder than keeping it moving
Demonstrations: What do we know about Friction Force?
2 - Increases with more contact surface
- Not dependent on surface area, but surface type
Does our model of friction depend on area?
Does real friction depend on area?
Demonstration: What do we know about Friction Force?
7 - Friction opposes (in opposite direction of) applied force
- Direction of Friction force is Opposite of direction of motion / acceleration
- Parallel to motion / Resists force of acceleration
Does friction always oppose motion?
Brainstorm: What do we know about Friction Force?
18. Ffs > Ffk - initially starting object moving is harder than keeping it moving
Example Problem
A sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s.
It then crosses a rough patch of snow which exerts a friction force
of 12 N. How far does it slide on the snow before coming to rest?
Slide 5-21
Example Problem
Burglars are trying to haul a 1000 kg safe up a frictionless ramp
to their getaway truck. The ramp is tilted at angle θ. What is the
tension in the rope if the safe is at rest? If the safe is moving up
the ramp at a steady 1 m/s? If the safe is accelerating up the
ramp at 1 m/s2? Do these answers have the expected behavior
in the limit θ → 0° and θ → 90°?
Slide 5-28
Example Problem
Macie pulls a 40 kg rolling trunk by a strap angled at 30° from
the horizontal. She pulls with a force of 40 N, and there is a 30 N
rolling friction force acting on trunk. What is the trunk’s
acceleration?
Slide 5-22