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A Simple Reaction-Time Test. A meter stick is held vertically above your hand, with the lower end between your thumb and first finger. On seeing the meter stick released, you grab it with these two fingers. You can calculate your reaction time from the distance the meter stick falls, read directly from the point where your fingers grabbed it. (a) Derive a relationship for your reaction time in terms of this measured distance, \(d\) (b) If the measured distance is \(17.6 \mathrm{cm},\) what is the reaction time?

Short Answer

Expert verified
The reaction time is approximately 0.19 seconds.

Step by step solution

01

Understanding the Physics

The problem involves calculating the time it takes for the meter stick to fall a certain distance. This can be related to free-fall motion, where the only force acting is gravity. We know the acceleration due to gravity, \( g = 9.8 \, \text{m/s}^2 \).
02

Using the Kinematic Equation

To derive the relationship, use the kinematic equation for constant acceleration: \[ d = \frac{1}{2} g t^2 \] Here, \( d \) is the distance the meter stick falls, \( g \) is the acceleration due to gravity, and \( t \) is the reaction time we want to find.
03

Solving for Reaction Time

Rearrange the kinematic equation to solve for the reaction time \( t \):\[ t = \sqrt{\frac{2d}{g}} \] This equation expresses the reaction time in terms of the distance \( d \) that the meter stick falls.
04

Converting Distance Units

Before substituting the distance into the equation, convert the distance from centimeters to meters. In this case, \( d = 17.6 \, \text{cm} = 0.176 \, \text{m} \).
05

Substituting Values

Substitute the values for \( d \) and \( g \) into the equation for \( t \):\[ t = \sqrt{\frac{2 \times 0.176}{9.8}} \]
06

Calculating the Reaction Time

Calculate the value using the substituted numbers:\[ t = \sqrt{\frac{0.352}{9.8}} \approx \sqrt{0.0359183673} \approx 0.18957 \] Therefore, the reaction time is approximately \( 0.19 \, \text{seconds} \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Free-Fall Motion
Free-fall motion is an interesting concept often illustrated by real-world experiments like the one involving the meter stick drop. In free-fall motion, an object moves solely under the influence of gravity, without any other forces acting upon it, such as air resistance. This sort of motion usually happens in a vacuum, but it can be closely approximated in shorter distances where air resistance is negligible.

When a meter stick is let go and falls, it is an example of free-fall motion. This is because, after it leaves your hand, the only force acting on it is Earth's gravitational force. Because of this unconstrained motion, the object will continue to accelerate as it falls.

Free-fall is commonly used to explore the effects of gravity. It allows people to understand how objects behave under the Earth's constant gravitational pull. Experiments like these help students better grasp the simple beauty of natural laws governing our world. Of course, for precise scientific and educational purposes, the effects of air may sometimes be factored out using calculations or specially controlled conditions to fully observe free-fall dynamics.
Kinematic Equation
The kinematic equations form the backbone of dynamics and are fundamental in understanding motion, especially under constant acceleration like gravity. One of the most useful equations in kinematics is: \[ d = \frac{1}{2} g t^2 \]This equation relates the distance an object falls (\(d\)) to the time it has been falling (\(t\)) and the acceleration due to gravity (\(g\)). In the context of the exercise, this equation helps to derive the reaction time by noting the simplicity of a meter stick's straight fall.

The kinematic equation highlights how distance and time are affected by gravity. Using this formula requires an understanding of the specific variables involved:
  • \(d\): Distance, the length the meter stick falls.
  • \(t\): Time it takes for the meter stick to fall, in seconds.
  • \(g\): Acceleration due to gravity, typically \(9.8\,\text{m/s}^2\).
By rearranging this equation, you can solve for any one variable if the others are known, making it instrumental in calculations concerning motion and time derived from an object's free-fall path.
Acceleration Due to Gravity
Acceleration due to gravity, represented by the symbol \(g\), is a crucial constant in physics. Here on Earth, it is approximately \(9.8 \text{m/s}^2\). This constant determines how quickly an object will speed up as it falls solely under the force of gravity. In the exercise, this constant plays a vital role to find out how long it takes for the meter stick to fall before being caught, directly impacting the reaction time calculation. Knowing the acceleration due to gravity allows us to predict how an object will move in a free-fall scenario, where it accelerates smoothly.Understanding this constant helps students appreciate the natural accelerative force that everything experiences at the Earth’s surface. Grasping these fundamental concepts gives one a deeper understanding of how objects interact and move in physical space. It provides a foundational stone for more complex studies in physics that involve motion under various forces. Whether it’s timing a falling object or launching one skywards, the acceleration due to gravity consistently influences the dynamics, making it an invaluable core concept in physics and engineering.

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Most popular questions from this chapter

Visitors at an amusement park watch divers step off a platform 21.3 \(\mathrm{m}(70 \mathrm{ft})\) above a pool of water. According to the announcer, the divers enter the water at a speed of 56 \(\mathrm{mi} / \mathrm{h}\) \((25 \mathrm{m} / \mathrm{s}) \cdot\) Air resistance may be ignored. (a) Is the announcer correct in this claim? (b) Is it possible for a diver to leap directly upward off the board so that, missing the board on the way down, she enters the water at 25.0 \(\mathrm{m} / \mathrm{s} ?\) If so, what initial upward speed is required? Is the required initial speed physically attainable?

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