Lecture 21: REVIEW: PROBLEMS

Questions of Yesterday1) 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 1B) Ball 2C) they are in the air for the same amount of timeD) 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 pointB) the tension is less at the highest point, but not zeroC) the tension is the same at both pointsD) the tension is zero at the highest point

Practice Problem #1An 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 #2A 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 #3A 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?

-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?

20.0 kg

30o

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 #4

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 30.0o 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?

Practice Problem #5

Questions of the Day

1) Which answer(s) best describe your feelings/thoughts about physics BEFORE you took this class?

A) physics is hardB) physics is easyC) physics is stupidD) physics is coolE) I love physicsF) I hate physicsG) 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!