Question

scenario
angela is standing on a very low - friction skateboard while blake pushes her away from the motion detector, which is set to record velocity as a function of time. in trial 1, blake pushes softly. in trial 2, blake pushes harder, and in trial 3, blake pushes angela the hardest.
analyze data
part a: use the data provided by the motion detector to find a pattern between the change in angelas motion and the interaction with blake. fill in the blanks.
when blake pushes harder, the slope of the velocity vs. time line ____________
since the slope __________ the acceleration is __________
when blake stops pushing, the slope of the velocity vs. time line becomes __________. since the slope is now ________, this means that the acceleration is also __________.
using representations
part b: the students then repeat the experiment, this time with angela holding two textbooks and then four textbooks. sketch a diagram of the velocity vs. time for angela with two and then four textbooks. blake pushes with the same force every time. the velocity of angela without textbooks is already sketched.
argumentation
part c: the following statement is written to describe what will happen after the first 5 seconds when blake is no longer pushing. cross out any incorrect statements and explain why they are incorrect. (use the checklist to help you check your own writing! after you have written your answer, make sure that you can check off the statements in the list, or revise your answer!)
“after blake stops pushing, angela will travel at a constant speed for a few seconds before she runs out of force. then she will decelerate and stop.”
checklist:
i answered the question directly.
i stated a law of physics that is always true.
i connected the law or laws of physics to the specific circumstances of the situation.
i used physics vocabulary (mass, acceleration, velocity, constant, changing).

Explanation:

Step1: Analyze slope - force relationship

The greater the force Blake applies, the greater the acceleration of Angela. Since the slope of a velocity - time graph represents acceleration, when Blake pushes harder, the slope of the velocity - time line increases.

Step2: Recall acceleration - slope link

The slope of the velocity - time graph is equal to acceleration. So when the slope is larger, the acceleration is larger.

Step3: Analyze post - push situation

When Blake stops pushing, there is no net force acting on Angela (low - friction surface). According to Newton's first law, an object in motion will stay in motion with a constant velocity. So the slope of the velocity - time line becomes 0, and since slope equals acceleration, the acceleration is 0.

Step4: Sketch velocity - time graph for different masses

When Blake pushes with the same force, according to Newton's second law $F = ma$ (where $F$ is force, $m$ is mass and $a$ is acceleration), as the mass (number of textbooks) increases, the acceleration decreases. So the slope of the velocity - time graph for 2 textbooks and 4 textbooks will be less than the slope for no textbooks, and they will all become horizontal (slope = 0) when Blake stops pushing.

Step5: Correct the statement

The statement "After Blake stops pushing, Angela will travel at a constant speed for a few seconds before she runs out of force. Then she will decelerate and stop." is incorrect. There is no such thing as "running out of force". According to Newton's first law, in the absence of a net force (low - friction surface), Angela will continue to move at a constant velocity indefinitely once Blake stops pushing.

Answer:

PART A: increases; larger; 0; 0; 0
PART B: Draw two lines with slopes less than the "No Textbooks" line for two and four textbooks respectively, and all lines become horizontal at $t$ (when Blake stops pushing).
PART C: Cross out the entire statement. Explanation: There is no "running out of force". According to Newton's first law, in the absence of a net force (low - friction surface), Angela will move at a constant velocity once Blake stops pushing.