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(II) Mass \(M_{\mathrm{A}}=35 \mathrm{~kg}\) and mass \(M_{\mathrm{B}}=25 \mathrm{~kg} .\) They have velocities \((\) in \(\mathrm{m} / \mathrm{s}) \overrightarrow{\mathbf{v}}_{\mathrm{A}}=12 \hat{\mathbf{i}}-16 \hat{\mathbf{j}}\) and \(\overrightarrow{\mathbf{v}}_{\mathrm{B}}=-20 \hat{\mathbf{i}}+14 \hat{\mathbf{j}}\) Determine the velocity of the center of mass of the system.

Short Answer

Expert verified
The velocity of the center of mass is \(-\frac{4}{3} \hat{\mathbf{i}} - \frac{7}{2} \hat{\mathbf{j}}\) m/s.

Step by step solution

01

Understanding the Problem

To find the velocity of the center of mass for a system of two masses, we must first understand that it is a weighted average of the velocities of the individual masses. The weights are the masses themselves.
02

Applying the Formula for Center of Mass Velocity

The formula for the velocity of the center of mass \( \vec{v}_{\text{cm}} \) of a system is given by: \[ \vec{v}_{\text{cm}} = \frac{M_A \vec{v}_A + M_B \vec{v}_B}{M_A + M_B} \] where \( M_A \) and \( M_B \) are the masses and \( \vec{v}_A \) and \( \vec{v}_B \) are their respective velocities.
03

Calculate the Weighted Velocities

First, calculate the weighted velocity for each mass. \[ M_A \vec{v}_A = 35 \text{ kg} \cdot (12 \hat{\mathbf{i}} - 16 \hat{\mathbf{j}}) = 420 \hat{\mathbf{i}} - 560 \hat{\mathbf{j}} \] \[ M_B \vec{v}_B = 25 \text{ kg} \cdot (-20 \hat{\mathbf{i}} + 14 \hat{\mathbf{j}}) = -500 \hat{\mathbf{i}} + 350 \hat{\mathbf{j}} \]
04

Sum the Weighted Velocities

Add the weighted velocities from the two masses to find the total weighted velocity. \[ M_A \vec{v}_A + M_B \vec{v}_B = (420 - 500) \hat{\mathbf{i}} + (-560 + 350) \hat{\mathbf{j}} = -80 \hat{\mathbf{i}} - 210 \hat{\mathbf{j}} \]
05

Calculate Total Mass

Sum the masses of the two objects to find the total mass of the system.\[ M_A + M_B = 35 \text{ kg} + 25 \text{ kg} = 60 \text{ kg} \]
06

Compute the Velocity of the Center of Mass

Use the total weighted velocity and total mass to find the velocity of the center of mass:\[ \vec{v}_{\text{cm}} = \frac{-80 \hat{\mathbf{i}} - 210 \hat{\mathbf{j}}}{60} = -\frac{4}{3} \hat{\mathbf{i}} - \frac{21}{6} \hat{\mathbf{j}} = -\frac{4}{3} \hat{\mathbf{i}} - \frac{7}{2} \hat{\mathbf{j}} \]

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Key Concepts

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

Weighted Average
When dealing with a set of masses, the weighted average is a crucial concept for understanding their combined effects, such as determining the center of mass velocity. Imagine you have two objects, each with its own mass and velocity. The velocity of the center of mass is not just a simple average; instead, it's a weighted average. This means that the mass of each object influences the overall result more heavily depending on how large it is.

To compute this, you take each mass's velocity and multiply it by its respective mass. These are the weighted velocities. Then, sum these weighted velocities, and divide by the total mass of all objects. Here's the essential formula once more:

\[ \vec{v}_{\text{cm}} = \frac{M_A \vec{v}_A + M_B \vec{v}_B}{M_A + M_B} \]

This approach ensures that the center of mass velocity accurately reflects the influences of each object's momentum. In our example, the larger mass (35 kg) impacts the center of mass velocity more than the smaller mass (25 kg) because it contributes more to the total momentum.
  • Weighted velocity: Product of an object's mass and its velocity.
  • Total momentum: Sum of all weighted velocities.
  • Center of mass velocity: Calculated by dividing total momentum by total mass.
Velocity
Velocity is a vector quantity that denotes the speed and direction of an object's movement. In the context of center of mass calculations, understanding velocity as a vector is vital since both direction and magnitude matter.

In our original task with masses A and B, both objects have specific velocity vectors. These vectors are specified in the i-j coordinate system, which separates movement into horizontal (\( \,\mathbf{i} \,\)) and vertical (\( \,\mathbf{j} \,\)) components. For example, mass A's velocity is 12 units in the i direction and -16 units in the j direction. Here's how you put it to use:
  • Horizontal component: Part of the velocity along the horizontal axis (i).
  • Vertical component: Part of the velocity along the vertical axis (j).
  • Resultant velocity: Combination of both horizontal and vertical components to give the actual movement vector.
Because each object has a different velocity, when calculating the velocity of the center of mass, we need to manage these vector operations to ensure we correctly handle both components. This approach allows us to determine the resultant direction and speed for the system's overall motion. Understanding velocity in this detailed framework helps students grasp how each motion component creatively influences the system's collective behavior.
Mass
Mass is a fundamental property of matter representing the amount of matter in an object. In physics problems involving systems of objects, like in the exercise, mass is directly linked to a system's momentum and impacts the motion of the center of mass.

Each object's mass is used to determine how much influence that object will have on the system's overall behavior. The larger the mass, the greater its impact on the velocity of the center of mass. Consequently, the center of mass moves closer to the heavier object in space if the system were visualized
  • Mass and momentum: Heavier objects contribute more to overall momentum.
  • Total mass: Sum of all individual masses in the system.
  • Impact on motion: Massive objects alter the path and speed of the center of mass more significantly.
In our scenario, mass A is considerably larger than mass B. Thus, its velocity will weigh more heavily on the final velocity of the center of mass due to its larger mass. Understanding how mass distributes and affects motion is key for grasping these types of physics challenges. It underscores the importance of each object's mass in calculating systemic properties like the center of mass.

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Most popular questions from this chapter

(III) A 280 -kg flatcar \(25 \mathrm{~m}\) long is moving with a speed of \(6.0 \mathrm{~m} / \mathrm{s}\) along horizontal frictionless rails. A \(95-\mathrm{kg}\) worker starts walking from one end of the car to the other in the direction of motion, with speed \(2.0 \mathrm{~m} / \mathrm{s}\) with respect to the car. In the time it takes for him to reach the other end, how far has the flatcar moved?

(II) \((a)\) A molecule of mass \(m\) and speed \(v\) strikes a wall at right angles and rebounds back with the same speed. If the collision time is \(\Delta t,\) what is the average force on the wall during the collision? \((b)\) If molecules, all of this type, strike the wall at intervals a time \(t\) apart (on the average) what is the average force on the wall averaged over a long time?

(III) ( \(a\) ) Calculate the impulse experienced when a \(65-\mathrm{kg}\) person lands on firm ground after jumping from a height of \(3.0 \mathrm{~m} .\) (b) Estimate the average force exerted on the person's feet by the ground if the landing is stiff-legged, and again \((c)\) with bent legs. With stiff legs, assume the body moves \(1.0 \mathrm{~cm}\) during impact, and when the legs are bent, about \(50 \mathrm{~cm} .\) [Hint: The average net force on her which is related to impulse, is the vector sum of gravity and the force exerted by the ground.

(III) Prove that in the elastic collision of two objects of identical mass, with one being a target initially at rest, the angle between their final velocity vectors is always \(90^{\circ} .\)

An astronaut of mass \(210 \mathrm{~kg}\) including his suit and jet pack wants to acquire a velocity of \(2.0 \mathrm{~m} / \mathrm{s}\) to move back toward his space shuttle. Assuming the jet pack can eject gas with a velocity of \(35 \mathrm{~m} / \mathrm{s},\) what mass of gas will need to be ejected?

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