Chapter 2: Kinematics in one dimension



PY 211 General Physics

Spring 2002 Midterm 1

February 11, 2002 (8:00 pm – 9:30 pm)

Last Name:__________________ First Name:___________________

BU ID:______________________

Lecture section (circle one): B1 - Heintz C1 - Chamon D1 - Mohanty

1) Do not turn over this cover sheet until instructed to do so.

2) Enter your name, ID number, and circle your lecture section above.

3) No talking is allowed during this exam.

4) Do not detach pages from this exam packet.

5) Show all steps in your solution! If you need more space for calculations, use the back of the page preceding the question; for example, calculations for problem 3 should be done on the back of page containing question 2.

6) Put all final answers in boxes.

7) No notes or books other than your one-page summary sheet are allowed! You should have only pencils, pens, and a calculator.

Problem 1:(check the correct answer) [20 points]

a) A train moves along a long straight track. The graph shows

the position as a function of time for this train. The graph

shows that the train

← speeds up all the time.

← slows down all the time.

← speeds up part of the time and slows down part of the time.

← moves at a constant velocity.

b) The graph shows below the position as a function of time for

two trains running on parallel tracks. Which is true?

← at time t both trains have the same velocity.

← both trains speed up all the time.

← both trains have the same velocity at some time before t.

← somewhere on the graph both trains have the same acceleration.

c) Imagine a ball, suspended from a rubber band. The ball moves up and down contracting and extending the rubber band. At its lowest point, the ball’s

← velocity and acceleration are zero.

← velocity is nonzero but its acceleration is zero.

← acceleration is nonzero but its velocity is zero.

← velocity and acceleration are both nonzero

.

Problem 2: (check the correct answer) [20 points]

a) You are a passenger in a car and not wearing your seatbelt. Without changing its speed, the car makes a sharp left turn, and you find yourself colliding with the right hand door. Which is correct?

← before and after the collision, there is a rightward force pushing you into the door.

← starting at the time of collision, the door exerts a leftward force on you.

← both of the above.

← neither of the above.

b) A constant force acts for a short time interval on a cart initially at rest on an air track and gives the cart a certain final speed. Suppose we repeat the experiment with the cart initially moving with constant speed in direction of the force. After we exert the same force for the same time interval, the increase in the speed of the cart

← is equal to two times its initial speed

← is equal to the square of its initial speed

← is equal to four times its initial speed

← is the same as when it started from rest

← cannot be determined from the information given.

c) In the 17th century, Otto von Güricke, a physicist in Magdeburg, fitted two hollow bronze hemispheres together and removed the air from the resulting sphere with a pump. Two eight-horse teams could not pull the halves apart even though the hemispheres fell apart when air was readmitted. Suppose von Güricke had tied both teams of horses to one side and bolted the other side to a heavy tree trunk. Then the tension on the hemispheres would be

← twice

← the same as

← half

what it was before.

Problem 3:

A projectile of mass m is launched horizontally from the edge of a cliff of height 250m, with the objective of landing past the edge of another cliff of height 100m located a distance 200m away. Earth's gravitational acceleration points vertically downward.

a) How long does the projectile take to hit a point on the ground above the 100m cliff? [6 points]

b) What is the minimum speed [pic] necessary for the projectile to just land at the edge of the 100m high-cliff and not fall in the valley? [6 points]

c) What is the projectile velocity vector at impact when it is launched with the minimum speed [pic]? [4 points]

d) What is the projectile speed at impact when it is launched with the minimum speed [pic]? [4 points]

Problem 4:

A block is set in horizontal motion over a surface, with an initial velocity [pic]. The kinetic friction coefficient between the block and the surface is[pic]. Gravity acts vertically downward.

a) Draw the free body diagram for the block and clearly show all the forces acting on it. [4 points]

b) Determine the distance d traveled on the surface before the block comes to a complete stop. Express your answer in terms of [pic] and g. [6 points]

c) What is the minimum speed [pic]so that the block clears at least 1.0m if the friction coefficient is [pic]? [6 points]

(d) How much time does the block take to stop under the conditions in part (c)? [4 points]

Problem 5:

The block shown in the figure below has mass m = 8.0 kg and lies on a plane tilted an angle θ to the horizontal. There is friction between the block and the inclined plane. Gravity acts vertically downward.

[pic]

(a) When the block is initially at rest, it starts sliding when the angle θ reaches 45o. Calculate the static friction coefficient μs. [6 points]

(b) If the coefficient of kinetic friction μk = 0.7, calculate the x and y components of the total force on the block when θ = 60o. [6 points]

(c) If the block starts at rest, determine how long it takes to travel 0.50 m along the incline. Assume the coefficient of kinetic friction μk = 0.7, and θ = 60o. [8 points]

Problem 1 Score: _________________

Problem 2 Score: _________________

Problem 3 Score: _________________

Problem 4 Score: _________________

Problem 5 Score: _________________

TOTAL: _________________

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position

time

position

time

250m

200m

100m

g

[pic]

t

................
................

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