Transcript Lecture 8: Forces & The Laws of Motion

Lecture 21:
REVIEW: PROBLEMS
Questions of Yesterday
1) Ball 1 is thrown vertically in the air with speed v. Ball 2 is thrown
from the same position with the same speed v but at an angle of
45o. Which ball is in the air longer?
A) Ball 1
B) Ball 2
C) they are in the air for the same amount of time
D) it depends on the magnitude of v
2) A pendulum swings back and forth in a circular arc. How does the
tension in the pendulum string at the highest point compare to the
tension at the bottom of the swing?
A) the tension is greater at the highest point
B) the tension is less at the highest point, but not zero
C) the tension is the same at both points
D) the tension is zero at the highest point
Practice Problem #1
An 5.00-g bullet is fired into a 250-g block that is initially at rest at the
edge of a table of height 2.00 m. The bullet remains in the block,
and after impact the block lands 4.00 m from the bottom of the
table.
What is the speed of the block+bullet when it hits the ground?
What is the speed of the bullet+block directly after the collision?
What is the initial speed of the bullet?
What fraction of kinetic energy is lost in the collision?
What is the impulse delivered to the bullet by the block as the bullet is
embedded in the block?
Practice Problem #2
A hungry 500-N bear walks out on a beam in an attempt to retrieve
some goodies (100 N) hanging at the end of the beam. The beam is
attached to a wall by a hinge and supported from the other end by a
cable. The beam is uniform, weighs 200 N and is 10 m long.
-Draw a free-body diagram of the beam
-When the bear is at x = 1.00 m, find the
tension in the wire and the components of the
reaction force at the hinge
-If the wire can withstand a maximum tension
of 1000 N what is the maximum distance the
bear can walk before the wire breaks?
Practice Problem #3
A 20.0 kg object is attached to a cord that is wrapped around a wheel
of radius r = 20.0 cm. The acceleration of the object down the incline
is 2.00 m/s2. Assume the axle of the wheel is frictionless.
-What is the tension in the rope?
-What is the moment of inertia of the wheel?
20.0 kg
30o
-What is the angular speed of the wheel 2.00
s after it begins to rotate, starting from rest?
-How many revolutions has the wheel made after 2.00 s?
-What is the change in kinetic energy of the system after 2.00 s?
Practice Problem #4
Two astronauts, each having a mass of 100.0-kg, are connected by a
10.0 m rope of negligible mass. They are isolated in space, moving in
circles around the point halfway between them at a speed of 5.00 m/s.
Treating the astronauts as particles…
What is the magnitude of the angular momentum and the rotational
energy of the system?
By pulling on the rope, the astronauts shorten the distance between
them to 5.00 m…
What is the new angular momentum of the system?
What are their new angular and linear speeds?
What is the new rotational energy of the system?
How much work is done by the astronauts in shortening the rope?
Practice Problem #5
A 2.00-m-tall basketball player is standing on the floor 10.0 m from the
3.00-m-high basket. If he shoots the 0.5 kg ball at an angle of 30.0 o
with the horizontal…
What is the initial speed of the ball?
How long is the ball in the air?
What is the final velocity of the ball?
If the player accelerates the ball from rest for 0.5 s before releasing the
ball, what force does the player exert on the ball while he is in contact
with it?
What force does the ball exert on the player while they are in contact?
Questions of the Day
1) Which answer(s) best describe your feelings/thoughts about
physics BEFORE you took this class?
A) physics is hard
B) physics is easy
C) physics is stupid
D) physics is cool
E) I love physics
F) I hate physics
G) I could care less about physics
2) Which of the above answers best describe your
feelings/thoughts about physics NOW?
Be honest, your grade won’t be affected by your answer!