A coin that is tossed straight up into the air. After it is released, it moves upward, reaches its highest point and falls back down again. Use one of the following choices (A through G) to indicate the acceleration of the coin during each of the stages of the coin’s motion described below. Take up to be the positive direction. Answer choice J if you think that none is correct.
A. The acceleration is in the negative direction and constant.
B. The acceleration is in the negative direction and increasing.
C. The acceleration is in the negative direction and decreasing.
D. The acceleration is zero.
E. The acceleration is in the positive direction and constant.
F. The acceleration is in the positive direction and increasing.
G. The acceleration is in the positive direction and decreasing. The coin is moving downward.
The correct answer and explanation is:
The correct answer is A. The acceleration is in the negative direction and constant.
Explanation:
When a coin is tossed straight up into the air, its motion can be broken into three distinct stages: moving upward, reaching its highest point, and moving downward.
- Upward Motion:
- As the coin moves upwards, gravity exerts a force pulling it downward. While the coin is rising, its velocity is decreasing due to the constant acceleration caused by gravity.
- The acceleration is constant because gravity pulls down with a constant force.
- Since gravity is pulling the coin downward and we are taking upward as the positive direction, the acceleration due to gravity is in the negative direction.
- Highest Point (at the peak):
- At the highest point, the coin momentarily stops before falling back down. This point does not represent a change in the acceleration, only a momentary stop in velocity.
- Even at the highest point, the acceleration due to gravity is still constant and directed downward, in the negative direction.
- Downward Motion:
- When the coin begins to fall back down, it accelerates due to gravity in the downward direction. Again, the acceleration is constant, and it continues to be in the negative direction.
The acceleration due to gravity near Earth’s surface is approximately 9.8 m/s² and it always acts in the downward direction. This force does not change in magnitude or direction during the entire motion of the coin, meaning that it remains constant and always directed downward (which we consider negative if we define upward as positive).
Thus, A is the correct answer because the acceleration is always constant, in the negative direction, throughout the coin’s entire motion.