Math Problem Statement

In the context of bouncing objects, the coefficient of restitution (abbreviated as COR, symbol: e) is given as the square root of the ratio between the kinetic energy of the object after bouncing and the kinetic energy of the object before bouncing on the surface. A ball (0.600 [kg]) is dropped straight down from a height of 3.65 [m] and bounces back up. Suppose the COR between the ball and the floor is 0.750. How strong (magnitude) is the average force of the floor onto the ball if the ball was in contact with the floor for 0.0150 [s]?

Solution

To solve this problem, let's go through the steps carefully.

Step 1: Understand Given Data and the Equation for Coefficient of Restitution (COR)

The coefficient of restitution ee between two objects is given by:

e=KEfinalKEinitiale = \sqrt{\frac{KE_{\text{final}}}{KE_{\text{initial}}}}

In this scenario:

  • Mass of the ball, m=0.600 kgm = 0.600 \ \text{kg}
  • Initial height, hinitial=3.65 mh_{\text{initial}} = 3.65 \ \text{m}
  • COR, e=0.750e = 0.750
  • Contact time with the floor, Δt=0.0150 s\Delta t = 0.0150 \ \text{s}

Step 2: Calculate Initial Kinetic Energy before Impact

The ball is dropped from a height, so initially it has potential energy, which converts to kinetic energy just before impact.

Using the gravitational potential energy equation:

KEinitial=mghinitialKE_{\text{initial}} = m \cdot g \cdot h_{\text{initial}}

where:

  • g=9.81 m/s2g = 9.81 \ \text{m/s}^2 (acceleration due to gravity).

KEinitial=0.600×9.81×3.65KE_{\text{initial}} = 0.600 \times 9.81 \times 3.65

Calculating this gives:

KEinitial21.441 JKE_{\text{initial}} \approx 21.441 \ \text{J}

Step 3: Calculate Final Kinetic Energy after Rebound

Using the coefficient of restitution:

e=KEfinalKEinitiale = \sqrt{\frac{KE_{\text{final}}}{KE_{\text{initial}}}}

Squaring both sides:

e2=KEfinalKEinitiale^2 = \frac{KE_{\text{final}}}{KE_{\text{initial}}}

Solving for KEfinalKE_{\text{final}}:

KEfinal=e2KEinitialKE_{\text{final}} = e^2 \cdot KE_{\text{initial}}

Substitute e=0.750e = 0.750 and KEinitial=21.441 JKE_{\text{initial}} = 21.441 \ \text{J}:

KEfinal=(0.750)2×21.441KE_{\text{final}} = (0.750)^2 \times 21.441 KEfinal12.082 JKE_{\text{final}} \approx 12.082 \ \text{J}

Step 4: Calculate Change in Kinetic Energy (Impulse)

The change in kinetic energy, which corresponds to the change in momentum, is:

ΔKE=KEinitialKEfinal\Delta KE = KE_{\text{initial}} - KE_{\text{final}}

ΔKE=21.44112.082\Delta KE = 21.441 - 12.082

ΔKE9.359 J\Delta KE \approx 9.359 \ \text{J}

Step 5: Calculate the Impulse and Average Force

Impulse JJ is defined as the change in momentum, which is also equal to the average force FF multiplied by the contact time Δt\Delta t:

J=FΔtJ = F \cdot \Delta t

The impulse can also be expressed as the change in velocity:

  1. Find initial velocity ( v_{\text{initial}} \

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Math Problem Analysis

Mathematical Concepts

Physics of bouncing objects
Kinetic energy
Coefficient of restitution
Impulse and force

Formulas

Coefficient of restitution: e = sqrt(KE_final / KE_initial)
Kinetic energy: KE = m * g * h
Impulse and average force: J = F * Δt

Theorems

Energy conservation in kinetic and potential forms

Suitable Grade Level

Grades 11-12