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IV. [20 pts total] Blocks A, B and C are being pushed to the left
across a frictionless table with a constant horizontal force. Block
A has mass 2M, block B has mass 3M, and block C has mass M.
The three blocks move through a distance d.
B
A
C
A. [6 pts] Draw separate qualitatively correct free-body
Frictionless table
diagrams for each of the three blocks. Make sure that the
label for each force in your free-body diagrams contains the following information:
• the object exerting the force
• the object on which the force is exerted
• the type of force (normal, magnetic, etc.)
B. [4 pts] Is the force exerted on block B by block C greater than, less than, or equal to the force on
block C by block B? Explain.
The force on block C by block B is equal to the force on block B by block C. These two forces
are a Newton’s 3rd Law pair: their magnitudes are equal, but their directions are opposite.
C. Block B is now replaced by block D, which has a mass
much greater than the mass of block B. The hand is still
pushing with the same force.
i.
[5 pts] Has the magnitude of the acceleration of
block A increased, decreased, or remained the same
after block B is replaced with block D? Explain.
A
D
C
Frictionless table
The hand is pushing with the same force as before on the new system of blocks. The new system has
a larger mass, so the acceleration of the system is smaller than before. Since all the blocks are
moving together, the acceleration of block A, which is equal to that of the system, is also smaller
than before.
ii. [5 pts] Has the net force on block A increased, decreased, or remained the same? Explain.
The mass of block A has not changed but the acceleration of block A has decreased. Therefore by
Newton’s second law the net force on block A must also have decreased.
Physics 121A, Summer 2007
Exam 2
25 July, 2007
ME-UWA121A073-E2(FRC,N23,TNS)sol.doc