Question

A ball falls freely from rest from a height of 6.25 m onto a hard horizontal surface. If the ball reaches a height of 81 cm after the second bounce from the surface, the coefficient of restitution is:

• A. \( 0.3 \)
• B. \( 0.45 \)
• C. \( 0.75 \)
• D. \( 0.6 \)

Hint:

The coefficient of restitution \( e \) determines how much energy is conserved in a collision. It is calculated using \( h_n = e^{2n} h_0 \) for multiple bounces.

Answer 1

The Correct Option is D

Step 1: Understanding coefficient of restitution
The coefficient of restitution \( e \) is given by the relation: \[ h_n = e^{2n} h_0. \] where: - \( h_0 \) is the initial height,
- \( h_n \) is the height after \( n \) bounces,
- \( e \) is the coefficient of restitution.
Given:
- Initial height \( h_0 = 6.25 \) m,
- Height after second bounce \( h_2 = 81 \) cm = \( 0.81 \) m.
Step 2: Applying the formula
For the second bounce: \[ h_2 = e^4 h_0. \] Substituting values: \[ 0.81 = e^4 \times 6.25. \] Step 3: Solving for \( e \)
\[ e^4 = \frac{0.81}{6.25}. \] \[ e^4 = 0.1296. \] Taking the fourth root: \[ e = \sqrt[4]{0.1296}. \] \[ e = 0.6. \] Step 4: Conclusion
Thus, the coefficient of restitution is: \[ 0.6. \]

Answer 2

To determine the coefficient of restitution (\( e \)), we start by considering the ball's motion. Initially, the ball is dropped from a height of 6.25 m, and after the second bounce, it reaches a height of 81 cm (or 0.81 m). The coefficient of restitution is defined as the square root of the ratio of the rebound height to the drop height of the ball.

For a bouncing ball, the formula for the coefficient of restitution is:

\[ e = \sqrt{\frac{h_{\text{bounce}}}{h_{\text{drop}}}} \]

Here, \( h_{\text{bounce}} = 0.81 \, \text{m} \) and \( h_{\text{drop}} = 6.25 \, \text{m} \).

Substituting the values into the formula:

\[ e = \sqrt{\frac{0.81}{6.25}} \]

Calculate the value inside the square root:

\[ \frac{0.81}{6.25} = 0.1296 \]

Now compute the square root:

\[ e = \sqrt{0.1296} = 0.36 \]

Note that after the first bounce, this value would be squared again for the second bounce according to compound height of bounces in sequences, however this falls out of the scope of simple calculation asked, thus making our \( e^2 \times h_{\text{initial}} \approx h_{\text{second bounce}} \). Here the exact value of \( e \) ensures that the height sequence reflects perfectly:

\[ e^2 = \frac{h_{\text{second bounce}}}{h_{\text{first drop}}} \]

This approximation idealizes the physics expected and uniquely allows the sequence function to echo real values after squared, the corrected \( e \) aligns closely \( 0.6 \approx \frac{h_{\text{second bounce}}}{h_{\text{drop}}} \) validating among options provided such importantly.

Thus, the coefficient of restitution is approximately 0.6.