# A block is sitting on a table within a train moving at constant speed.

1.A block is sitting on a table within a train moving at constant speed. Consider the observations of a person on the train considering the block and of a person standing at rest on a station platform as the train passes. Which one of the following statements is correct?

a. The person on the train would say that the block is in equilibrium, but not static equilibrium; and the external observer would say that the block is in static equilibrium

b. The person on the train would say that the block is in equilibrium, but the external observer would say that the block is not in equilibrium

c. The person on the train would say that the block is in static equilibrium, but the external observer would say that the block is in equilibrium, but not static equilibrium

d. Both the person on the train and the external observer would agree that the block is not in equilibrium

e. Both the person on the train and the external observer would agree that the block is in equilibrium, but not static equilibrium.

2. Consider the following: (i) the book is at rest, (ii) the book is moving at a constant velocity, (iii) the book is moving with a constant acceleration. Under which of these conditions is the book in equilibrium?

a. (iii) only b. (ii) and (iii) only c. (i) and (ii) only d. (ii) only e.  (i) only

3. At the circus, a clown balances a step ladder on his forehead. A few people in the audience notice that he is continually moving to keep the ladder from falling off his forehead. Why is this movement necessary?

a. This movement is not necessary. The clown is trying to make this look harder than it really is for entertainment value. The ladder will easily balance in the clown’s forehead b. By rocking the ladder on his forehead, the ladder will be more stable than if it were stationary. This is similar to riding a bicycle. You can easily balance a bicycle when it’s rolling, but not when it’s stationary. c. The clown is trying to apply a torque to the ladder in the direction opposite to other torques on the ladder d. The clown is trying to keep the center of mass of the ladder directly above his head so that the torque due to the gravitational force is zero N-m.

4. Two rods are made of brass and have the same length. The cross section of one of the rods is circular, with a diameter 2a. The other rod has a square cross section, where each side of the square is a length 2a. One end of the rods is attached to an immovable fixture which allows the rods to hang vertically. To the free end of each rod, a block of mass m is attached. Which rod, if either, will stretch more after the block is attached?

a. The one with the square cross section will stretch more b. The one with the circular cross section will stretch more c.   Both will stretch by the same amount. d. One cannot say which will stretch more without knowing the numerical values of a and m

5.A 1.0-kg weight is suspended from the middle of a 1.3-m long rope that has one end fixed to a wall. What force would you have to exert on the other end of the rope to pull it perfectly horizontal?

a. 49 N b. 0.98N c. 980N d. 0.49N

e. It cannot be done

e. It cannot be done