AP Physics Unit 2 - two blocks on a table

Poll

A block of mass M is set on a rough table and is connected to a string that passes over an ideal pulley. In Case 1, the free end of the string is connected to a hanging 10 kg mass.

In Case 2, the hanging object is removed and a person grabs the free end of the string and pulls with a constant force equal to 98 N, the weight of the hanging object in Case 1.

In both cases, the block is released from rest the same distance from the right edge of the table. With which statement do you agree?

• Block M in Case 1 will reach the right edge of the table first.

• Block M in Case 2 will reach the right edge of the table first.

• Both blocks will reach the right edge of the table at the same time.

​Let's analyze the situation to see which answer is correct...

AP Physics 1 - Unit 2 (Dynamics)​

Draw

The dots below represent each object in Case 1. Draw the forces acting on those objects after the system is released. Use the grids to draw longer arrows to represent stronger forces. Assume m < M. Recall that the system is accelerating.

Multiple Choice

Ari and Bella observe the demonstration and notice that the blocks accelerate in both cases. However, in Case 2 the block reaches the edge of the table in less time even though the force exerted on the string in this case is the same as the weight of the hanging object in Case 1. How can this be?

• This can't be. In both cases 98 N of force are exerted on the string so both blocks should have the same acceleration and reach the edge of the table at the same time.

• This is because the tension in the string in Case 1 is less than in Case 2.

• This is because Newton's 3rd law states that block M exerts an equal but opposite force on the table as the table does on the block.

Multiple Select

Apply Newton's 2nd Law to Case 1 with both the hanging block, m, and the block on the table, M, in the system. Select all forces that must be considered when applying Newton's 2nd law in the direction of acceleration.

• F_friction

• F_{Earth on M}

• F_{Earth on m}

• Tension

• F_{Normal on M}

Multiple Select

Apply Newton's 2nd Law to Case 1 with both the hanging block, m, and the block on the table, M, in the system. What masses must be included in the calculation?

• M

• m

• Table

• Pulley

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​a = (F_{E on m} + F_friction)/(m + M)

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​Newton's 2nd Law for Case 1

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Multiple Choice

How would Newton's 2nd Law for Case 2 differ, where the system is mass, M?

• It wouldn't...you end up with the same equation.

• The same net force is applied as in Case 1, but the mass of the system is different.

• A different net force is applied, but the mass of the system is the same as Case 1.

• Not enough information to solve.