CONFLICTING CONTENTIONS (Ball Toss) conceptual
Question:-Mr. Avtar tossed a ball up in the air. The class observed the ball rise up to a greatest height
then fall back into Mr. Avtar’ hand. He then asked the class, “What is the acceleration of the
ball at the apex of the motion, which is the highest point reached by the ball?”
[NOTE: This first problem of the workbook presents an example of a very complete and
thorough solution to a conceptual exercise. By the end of the course, you will improve on your
ability to ‘reason through’ an answer to a conceptual question in this way. But don’t be afraid to
start at the beginning with simpler answers.]
Alonzo, Daniel, and Beth said the acceleration was zero. Mary and Frank said it was constant
and pointing downward. Cathy and John said it wasn’t zero, but it wasn’t constant either.
Which group do you agree with, or do you have a different opinion? Record your prediction and
your reasons for it before continuing with this exercise.
The Reasons
Alonzo said the acceleration of the ball was zero because it was momentarily at rest at the
highest point of the motion.
Frank said the acceleration was constant and downward because the acceleration of gravity is
always constant and gravity was acting on the ball.
Beth said the acceleration was zero because it was negative when it was rising, since it was
slowing down, and positive when it was falling, since it was speeding up, so it had to pass
through zero at the apex of the motion.
John agreed with Beth that the acceleration was changing, but it was positive on the way up and
negative on the way down because that was the direction the ball was moving. Therefore, it isn’t
possible to measure the acceleration of the ball when it is at rest.
Which of these explanations are clearly wrong, and how do you know? Which one of the three
explanations is correct?
The Answer
The acceleration of the ball is not zero at the apex of the motion. It is, in fact, downward and
constant through the full motion of the ball, just as Frank said.
Alonzo said that the velocity was zero at the apex, therefore the acceleration was zero. What
was wrong with this Alonzo’s reasoning?
Both Beth and John said the acceleration was not constant but changing in different ways. What
was wrong with their reasoning?
Frank is right: the acceleration of gravity is downward and constant, but his answer refers to a
theory that is known to be true. Can you provide a better explanation for the correct answer
based on observation alone?
The Solution
Key Principles
According to the Concept Review definitions (1a-c), velocity measures a change in displacement
and acceleration measures a change in velocity of an object.
According to Principle 3c, positive and negative acceleration depend on both the changing speed
and also the direction of motion
The Facts of the Problem
The ball is rising upward and slowing down before the highest point. It is momentarily zero at
the apex. It then moves faster and downward after it reaches the apex.
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then fall back into Mr. Avtar’ hand. He then asked the class, “What is the acceleration of the
ball at the apex of the motion, which is the highest point reached by the ball?”
[NOTE: This first problem of the workbook presents an example of a very complete and
thorough solution to a conceptual exercise. By the end of the course, you will improve on your
ability to ‘reason through’ an answer to a conceptual question in this way. But don’t be afraid to
start at the beginning with simpler answers.]
Alonzo, Daniel, and Beth said the acceleration was zero. Mary and Frank said it was constant
and pointing downward. Cathy and John said it wasn’t zero, but it wasn’t constant either.
Which group do you agree with, or do you have a different opinion? Record your prediction and
your reasons for it before continuing with this exercise.
The Reasons
Alonzo said the acceleration of the ball was zero because it was momentarily at rest at the
highest point of the motion.
Frank said the acceleration was constant and downward because the acceleration of gravity is
always constant and gravity was acting on the ball.
Beth said the acceleration was zero because it was negative when it was rising, since it was
slowing down, and positive when it was falling, since it was speeding up, so it had to pass
through zero at the apex of the motion.
John agreed with Beth that the acceleration was changing, but it was positive on the way up and
negative on the way down because that was the direction the ball was moving. Therefore, it isn’t
possible to measure the acceleration of the ball when it is at rest.
Which of these explanations are clearly wrong, and how do you know? Which one of the three
explanations is correct?
The Answer
The acceleration of the ball is not zero at the apex of the motion. It is, in fact, downward and
constant through the full motion of the ball, just as Frank said.
Alonzo said that the velocity was zero at the apex, therefore the acceleration was zero. What
was wrong with this Alonzo’s reasoning?
Both Beth and John said the acceleration was not constant but changing in different ways. What
was wrong with their reasoning?
Frank is right: the acceleration of gravity is downward and constant, but his answer refers to a
theory that is known to be true. Can you provide a better explanation for the correct answer
based on observation alone?
The Solution
Key Principles
According to the Concept Review definitions (1a-c), velocity measures a change in displacement
and acceleration measures a change in velocity of an object.
According to Principle 3c, positive and negative acceleration depend on both the changing speed
and also the direction of motion
The Facts of the Problem
The ball is rising upward and slowing down before the highest point. It is momentarily zero at
the apex. It then moves faster and downward after it reaches the apex.
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