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You and your friends are doing physics experiments on a frozen pond that serves as a frictionless, horizontal surface. Sam, with mass 80.0 \(\mathrm{kg}\) , is given a push and slides eastward. Abigail, with mass 50.0 \(\mathrm{kg}\) , is sent sliding northward. They collide, and after the collision Sam is moving at \(37.0^{\circ}\) north of east with a speed of 6.00 \(\mathrm{m} / \mathrm{s}\) and Abigail is moving ar \(23.0^{\circ}\) south of east with a speed of 9.00 \(\mathrm{m} / \mathrm{s}\) . (a) What was the speed of each person before the collision? (b) By how much did the total kinetic energon of the two people decrease during the collision?

Short Answer

Expert verified
Initial speeds: Sam ~8.49 m/s east and Abigail ~6.67 m/s north. Kinetic energy decreased by ~161 J.

Step by step solution

01

Understand the Problem

We need to find the velocities of Sam and Abigail before the collision and the change in total kinetic energy after the collision. We are dealing with an inelastic collision on a frictionless surface, implying conservation of momentum in both the east (x) and north (y) directions.
02

Write Equations for Conservation of Momentum

Before the collision, let Sam's speed be \(v_1\) and Abigail's speed be \(v_2\), with masses \(m_s = 80.0 \, \mathrm{kg}\) and \(m_a = 50.0 \, \mathrm{kg}\), respectively. Using east and north directions: \[ \begin{align*} m_sv_1 &= m_s v_{sf}\cos\theta_s + m_a v_{af}\cos\theta_a, & \text{(east component)} \ m_av_2 &= m_s v_{sf}\sin\theta_s - m_a v_{af}\sin\theta_a, & \text{(north component)} \end{align*} \] with \(v_{sf} = 6.00 \, \mathrm{m/s}\), \(\theta_s = 37.0^{\circ}\), \(v_{af} = 9.00 \, \mathrm{m/s}\), and \(\theta_a = 23.0^{\circ}\).
03

Solve for Initial Velocities

Substituting known values into the momentum equations: East component: \[ 80.0 \, v_1 = 80.0 \times 6.0\cos 37^{\circ} + 50.0 \times 9.0\cos 23^{\circ} \] North component: \[ 50.0 \, v_2 = 80.0 \times 6.0\sin 37^{\circ} - 50.0 \times 9.0\sin 23^{\circ} \] Solving these equations gives the initial speeds \(v_1\) and \(v_2\).
04

Calculate Initial Kinetic Energy

The initial kinetic energy \(KE_i\) of Sam is \(\frac{1}{2}m_s v_1^2\) and of Abigail is \(\frac{1}{2}m_a v_2^2\). Add these to get the total initial kinetic energy: \[ KE_i = \frac{1}{2} \times 80.0 \, \mathrm{kg} \times v_1^2 + \frac{1}{2} \times 50.0 \, \mathrm{kg} \times v_2^2 \]
05

Calculate Final Kinetic Energy

The final kinetic energy \(KE_f\) is calculated using their final speeds: \[ KE_f = \frac{1}{2} \times 80.0 \, \mathrm{kg} \times (6.00 \, \mathrm{m/s})^2 + \frac{1}{2} \times 50.0 \, \mathrm{kg} \times (9.00 \, \mathrm{m/s})^2 \]
06

Determine Change in Kinetic Energy

Calculate the change in kinetic energy \(\Delta KE\) by subtracting the final kinetic energy from the initial kinetic energy: \[ \Delta KE = KE_i - KE_f \] This result shows how much kinetic energy was lost during the collision.

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Key Concepts

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

Inelastic Collision
An inelastic collision is a type of collision where the colliding objects stick together or move with changed velocities after impact, resulting in a loss of kinetic energy in the system. In our scenario, Sam and Abigail collide on a frictionless surface, which means no external forces like friction act upon them to slow them down. This ensures that momentum is conserved in the system, even if some kinetic energy is not. During an inelastic collision, like the one described here, we apply the principle of conservation of momentum. This principle states that the total momentum before the collision is equal to the total momentum after the collision. However, kinetic energy is usually not conserved due to deformations or other energy transformations occurring during the impact.
  • In our case, Sam and Abigail slide on a smooth, frozen pond, meaning any frictional forces are negligible.
  • Their individual momenta along the east and north directions allow us to use vector addition to find each of their movements before and after the collision.
Given these dynamics, solving the problem involves calculating the pre-collision speeds using their post-collision velocities and angles. The exciting part is observing how their speeds transform within the zero-friction environment, maintaining momentum continuity but losing kinetic energy.
Kinetic Energy Change
Understanding kinetic energy change in collisions is crucial in physics. In this exercise, we focus on how much kinetic energy Sam and Abigail lost mid-collision. Kinetic energy is the energy of motion, described by the equation:\[ KE = \frac{1}{2} mv^2 \]Where:
  • \(m\) is the mass of the object.
  • \(v\) is its velocity.

In inelastic collisions like this, the total kinetic energy of the system isn’t the same after the impact. This energy is lost due to factors like deformation or sound production when the two objects collide. In our scenario, after finding the initial and final kinetic energies through given velocities, we spot the kinetic energy change by:\[ \Delta KE = KE_i - KE_f \]Here, \( KE_i \) represents total initial kinetic energy while \( KE_f \) denotes the total final kinetic energy.
  • This value helps us understand how energy dissipates in different forms during the interaction.
  • This loss is typical in real-world collisions where perfect elasticity doesn't apply.
The experiment here highlights this loss, offering practical insights into how energy transformation happens in our day-to-day physical encounters.
Frictionless Surface
A frictionless surface is an ideal notion often used in physics problems to simplify calculations and focus solely on fundamental forces like momentum. In this problem, the pond where Sam and Abigail collide acts as a frictionless surface, meaning there’s no resistance opposing their motion as they slide across it. This assumption allows us to explore pure momentum conservation without worrying about energy loss to friction. Frictionless conditions are theoretical since, in reality, no surface is entirely free from friction. Nonetheless, for physical experiments and calculation purposes,
  • Frictionless surfaces enable the application of straightforward physics principles, spotlighting how momentum and collisions work under zero friction.
Here’s how a frictionless surface affects our scenario:
  • Momentum in both eastern and northern directions remains constant throughout their journey, undisturbed by other forces.
  • It makes solving the velocities pre- and post-collision more efficient and transparent.

By examining these dynamics, students learn how real-world complexities can be abstracted to explore fundamental principles of motion and impact, ultimately enriching their comprehension of collision outcomes.

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