Question

a student pushes a block from rest across a frictionless surface while the block is in front of a motion detector for 3 seconds. the block first speeds up for the 1st second, then moves at constant velocity for the 2nd second, and finally slows down during the final second. which statement best explains the motion of the block using newtons 1st law? a. there is a net force on the block during the 2nd second as a net force keeps the motion constant. b. there is a net force on the block during the 3rd second as a net force keeps the motion constant. c. there is no net force on the block during the 2nd second as zero net force keeps the motion constant. d. there is no net force on the block during the 3rd second as zero net force causes the motion to accelerate.

Explanation:

Step1: Recall Newton's 1st Law

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Step2: Analyze motion in 2nd second

The block moves at constant velocity in the 2nd second. According to Newton's 1st Law, when an object has constant - velocity motion, the net force acting on it is zero.

Step3: Analyze other options

• Option A: A net force causes acceleration, not constant motion, so it's wrong.
• Option B: In the 3rd second, the block slows down, so there is a net force acting on it in the opposite direction of motion, making this option wrong.
• Option D: Zero net force does not cause acceleration. An object slows down when there is a net force acting against its motion, so this option is wrong.

Answer:

C. There is no net force on the block during the 2nd second as zero net force keeps the motion constant.