Transcript Week 8 (Physics Lecture #7)

Warm-Up I: Think about This & Discuss with Neighbors
Consider a trip on an elevator. You begin at rest at the bottom, start to
move up, move at constant speed for a short while, then slow down to a
stop at the top. Then, starting from rest at the top, you start to move
down, move at constant speed for a short while, then slow down to a stop
at the bottom. You’ve likely taken many such trips. Try to remember such
a trip. Do you feel heavier, lighter, or normal during such a trip? At which
parts? Discuss with your neighbors.
Warm-Up II: Think about This & Discuss with Neighbors
Consider driving in a car at constant speed over a hill. You’ve likely done
this at some point. At the top of the hill, do you feel heavier, lighter, or
normal? What if you are driving faster? Discuss with your neighbors
Wed. May 18 – Physics Lecture #7
Forces, Motion, and Gravity
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Announcements
Forces and Motion
Circular Motion
Gravity
Gravity and Circular Motion
Uncredited images from OpenStax College Physics
A ball flying in the air affected only by gravity has an
acceleration that points down towards the ground as shown in
the figure.
What is the direction of the velocity of the ball at the instant
shown?
1
4
2
5 Can’t be
determined
3

a
Consider the semi-circular portion of track shown. A marble
follows the track, and exits at point P as shown in a top-down,
bird’s eye view.
Which of the
sketched paths is
the best prediction
for the path the
marble will follow
after exiting the
track?
3
2
4
1
5
P
A net force of 10 N is applied to an object, resulting in an acceleration
of 2 m/s2. What is the object’s mass?
1. 0.2 kg
2. 2 kg
3. 5 kg
4. 10 kg
5. 20 kg
6. None of the above
You push straight upward with a force of 150 N on a 10 kg object.
Determine the acceleration of the object.
You push straight upward with a force of 50 N on a 10 kg object.
Determine the acceleration of the object.
You are standing in an elevator. At some instant, the magnitude of the
normal force of the floor pushing up on you is greater than the magnitude
of the gravity force acting down on you. At this instant, what can you say
about the direction of your motion?
1. You are moving up.
2. You are moving down.
3. You are not moving.
4. Not enough information.
Follow the black spot, red spot, and blue spot.
1. Black spot (innermost)
2. Red spot
3. Blue spot (outermost)
4. Same
5. Can’t answer
a) Which spot has the higher speed?
b) Which spot has the higher angular speed?
c) Which spot has the higher acceleration?
Ladybug Revolution https://phet.colorado.edu/en/simulation/rotation
The figure shows a bird’s-eye view of particles attached to a string
moving in horizontal circles on a table-top at constant speed.
a) Which particle has the largest acceleration (in the event of a tie, hold up
as many cards as needed)?
b) Which particle has the largest net force acting on it (in the event of a
tie, hold up as many cards as needed)?
A stunt pilot performs a loop-the-loop at constant speed in a vertical
circle. The diagram shows the airplane at the top and the bottom of the
loop. Where does the pilot feel the larger force of the seat pushing on
her?
1. At the top of the loop
2. At the bottom of the loop
3. Same at top and bottom of loop
4. Not enough information
Consider
Wed.
May two
18 –identical
Physicsobjects,
Lecture #7
each of the same mass m, a
distance r apart. If each mass is
doubled, and the distance also
doubles, by how much does the
gravitational force change?
1. No change
2. Increases 2x
3. Increases 4x
4. Decreases 2x
5. Decreases 4x
6. Not enough info
Determine the acceleration due to gravity at the surface
of the earth (mass 5.97 x 1024 kg, radius 6.37 x 106 m).
The earth (mass 5.97 x 1024 kg, radius 6.37 x 106 m) and
moon (mass 7.35 x 1022 kg, radius 1.74 x 106 m) are a
distance 3.84 x 108 m apart.
a) Determine the force of gravity between the earth and the
moon.
b) Assuming a circular orbit with a stationary earth,
determine the period of the moon’s orbit.
Geosynchronous satellites are placed in a circular orbit around the earth
(mass 5.97 x 1024 kg, radius 6.37 x 106 m) at such a distance that their
orbital period is exactly equal to 24 hours.
a) Why might this be useful?
b) How high above the surface of the earth must such a satellite orbit?
c) How fast must such a satellite orbit?