A student attaches a string to the block of dry ice on the table, and pulls steadily on the block.MULTIPLE CHOICE: Which of the following forces act on the block? A. towing force B. a normal force...

A student attaches a string to the block of dry ice on the table, and pulls steadily on the block.

MULTIPLE CHOICE:

Which of the following forces act on the block? A. towing force B. a normal force C. kinetic friction D. the force of gravity

Which of the following describes the motion of the block while it is on the table? the block A. moves at constant speed B. slows down gradually to a stop C. speeds up for a bit, then moves at constant speed D. accelerates constantly

Eventually, the block reaches the edge of the table. After the block leaves the table, which of the following forces act on the block? A. the force of motion B. normal force C. kinetic friction D. the force of gravity

Asked on by juniormax

2 Answers | Add Yours

neela's profile pic

neela | High School Teacher | (Level 3) Valedictorian

Posted on

The force of towing makes the block move against the friction of the ice with the surface of the table, which is proportional to the normal weight force the ice block. The gravitational force is balanced by the normal reactoon of the table.The choice is A.

Since he pulls the block sreadily, there are three possibilities. His steady force is just equal to the the kinetic friction of the ice with the surface of the table in which case,choice C holds. If he applies a steady force less than the kinetic friction , the ice bloc does not move, no such a choice given. If his steady pull exceeds the kinetic friction of the ice bloc with the table surface , then the choice at D holds.

The students pulls the block steadily on the table.When the block reahes the edge,the pulling is only due to gravity. So the only force acting on the block is gravity of the earth, the choice being at D.

krishna-agrawala's profile pic

krishna-agrawala | College Teacher | (Level 3) Valedictorian

Posted on

The question says that the students pull the string steadily. WE assume that this means that the string is being puled at steady rate. Also we will need to assume that the effect to evaporation of dry ice into carbon dioxide is negligible.

Based on these assumptions the answerer

are as follows:

Forces acting on the block:

All the four forces, that is towing force, a normal force, kinetic friction, andthe force of gravity will act on the block. However the resultant force will be zero as towing force will be cancelled out by the kinetic friction, and force of gravity will be cancelled out by a normal force which is equal and opposite of gravitational force.

Motion of the block while it is on the table:

The block will move at constant speed equal to the rate at which the string is being pulled. However is must be noted that when the student just starts pulling on the block, the rate of pulling for a split second will be accelerating from zero to steady state. We are not considering this initial situation for any part of the question.

Forces acting on the block after it leaves the table:

The force of gravity. The block will fall with accelerating speed under the influence of this force.

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