An object will slow down if there is no net force.

A spaceship located in deep space fires its rockets for 10 s, after which the thrust is turned off. Compare the velocity of the spaceship just after the rockets are turned off to its velocity one minute later.

Correct Answer

A block is sitting on a tabletop when a hand pushes it, giving it an initial velocity v. The block slides across the table before coming to rest. Draw a vector representing the net force on the block for each of the following times:

a. As the hand is in contact with the block.

b. While the block is sliding across the table.

c. A few moments after the block has stopped sliding

 

 Correct Answer

Students who suffer from this misconception will claim that the rocket ship must slow down when the rockets are turned off, since there is no force to keep it moving. In the block question, they will answer that there is no net force at time b, since the block has no active agent exerting a force on the block.

 

Related misconceptions/student difficulties that may arise in these questions:

A constant force accelerates a body, until the body uses up all the power of the force.

The net force must be in the direction of motion, so objects will travel along a line in that direction.