Collisions with Dave

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Questions and Answers

According to Dave, what topic is being covered in the lesson?

  • Kinematics
  • Dynamics
  • Harmonic motion
  • Collisions (correct)

Which of the following is conserved in every collision?

  • Linear momentum (correct)
  • Kinetic energy
  • Potential energy
  • Angular momentum

In an elastic collision, what happens to the objects involved?

  • Objects completely shatter upon impact.
  • Objects transform into a different state of matter.
  • Objects bounce off each other with minimal energy loss. (correct)
  • Objects stick together and move as one mass.

Which of the following is an example of a collision that can be approximated as elastic?

<p>A soccer player kicking a ball (A)</p> Signup and view all the answers

In a perfectly inelastic collision, what happens to the objects involved?

<p>Objects stick together and move as one mass. (D)</p> Signup and view all the answers

Which of the following is an example of a perfectly inelastic collision?

<p>Two asteroids merging in space (B)</p> Signup and view all the answers

What is the primary difference between elastic and perfectly inelastic collisions?

<p>Elastic collisions conserve kinetic energy; perfectly inelastic collisions do not. (B)</p> Signup and view all the answers

When modeling car crashes as perfectly inelastic collisions, what is conserved?

<p>Total momentum (C)</p> Signup and view all the answers

In a car crash, what is kinetic energy primarily converted into?

<p>Sound, heat, and internal energy (B)</p> Signup and view all the answers

According to the discussion, where do most real-world collisions fall on the spectrum of elasticity?

<p>Somewhere between elastic and perfectly inelastic (inelastic) (B)</p> Signup and view all the answers

Why do we often approximate collisions as either perfectly elastic or perfectly inelastic?

<p>To make the calculations simpler (A)</p> Signup and view all the answers

What topic will be covered next, following the discussion of linear motion?

<p>Circular Motion (A)</p> Signup and view all the answers

Which concept is NOT directly associated with the study of linear motion as outlined?

<p>Thermodynamics (A)</p> Signup and view all the answers

How is momentum analyzed in perfectly inelastic collisions?

<p>By treating both objects as a single object after the collision. (D)</p> Signup and view all the answers

Which of the following best describes the behavior of objects in an elastic collision?

<p>They bounce off each other, conserving kinetic energy. (C)</p> Signup and view all the answers

Consider two identical cars involved in a head-on collision. If they stick together after impact, what type of collision occurred?

<p>Perfectly inelastic (B)</p> Signup and view all the answers

What is the equation to determine the final velocity, $v_{final}$, in a perfectly inelastic collision between two objects with masses $m_1$ and $m_2$ and initial velocities $v_1$ and $v_2$?

<p>$v_{final} = (m_1v_1 + m_2v_2) / (m_1 + m_2)$ (A)</p> Signup and view all the answers

How does the conservation of linear momentum apply to collisions between celestial objects?

<p>It applies regardless of the objects' size or speed. (A)</p> Signup and view all the answers

Which of the following statements accurately contrasts elastic and inelastic collisions regarding kinetic energy and momentum?

<p>Elastic collisions conserve kinetic energy and momentum; inelastic collisions conserve momentum but not kinetic energy. (A)</p> Signup and view all the answers

In the context of collisions, what is the significance of approximating real-world scenarios into idealized models (perfectly elastic or perfectly inelastic)?

<p>It simplifies complex calculations while still providing reasonably accurate estimations. (B)</p> Signup and view all the answers

Consider a scenario where two objects collide, and a significant portion of the initial kinetic energy is converted into sound and heat. Which type of collision BEST describes this scenario?

<p>An inelastic collision, where kinetic energy is transformed into other forms of energy. (B)</p> Signup and view all the answers

Two balls of equal mass undergo a head-on elastic collision. Ball A is initially moving with a velocity $v$, and ball B is at rest. What are the velocities of ball A and ball B after the collision?

<p>Ball A: 0, Ball B: $v$ (A)</p> Signup and view all the answers

A ball is dropped from a height $h$ onto a stationary, rigid floor and bounces back to a height of $0.64h$. What is the coefficient of restitution for this collision?

<p>0.80 (C)</p> Signup and view all the answers

Two cars approach an intersection. Car A (1000 kg) is traveling east at 20 m/s, and Car B (1500 kg) is traveling north at 10 m/s. They collide inelastically and move together as one mass after the collision. What is the approximate speed of the combined mass immediately after the collision?

<p>10.0 m/s (A)</p> Signup and view all the answers

A bullet of mass $m$ is fired into a stationary wooden block of mass $M$ resting on a frictionless surface. The bullet becomes embedded in the block, and they move together with a speed $v$. What was the initial speed $u$ of the bullet?

<p>$u = ((M+m)/m)v$ (C)</p> Signup and view all the answers

Two objects of equal mass, A and B, collide. The collision is perfectly elastic. Object A is initially moving with velocity $v_A = 10 m/s$ and object B is at rest. What is the relative velocity of object A with respect to object B after collision?

<p>-10 m/s (C)</p> Signup and view all the answers

Object A (mass $m_A$) moving at velocity $v$ collides elastically with object B (mass $m_B$) which is at rest. After the collision, object A bounces back with a velocity of $-v/3$. What is the ratio of $m_B$ to $m_A$, i.e., $m_B/m_A$?

<p>2 (A)</p> Signup and view all the answers

A system consists of two particles with masses $m_1$ and $m_2$. At $t = 0$, $m_1$ is at the origin and $m_2$ is at position $r$. The two particles undergo perfectly inelastic collision. What is the center of mass of the system?

<p>$r_{cm} = (m_2r) / (m_1 + m_2)$ (C)</p> Signup and view all the answers

Two identical billiard balls collide. Before the collision, one ball is moving at 3.0 m/s and the other is stationary. After the collision, the first ball is stationary. What is the velocity of the second ball after the collision, assuming the collision is elastic and neglecting any rotational motion?

<p>3.0 m/s (B)</p> Signup and view all the answers

A 2.0 kg block moving at 5.0 m/s collides with a 3.0 kg block initially at rest. After the collision, the 2.0 kg block recoils at 1.0 m/s. What is the velocity of the 3.0 kg block after the collision, assuming the surface is frictionless?

<p>4.0 m/s (B)</p> Signup and view all the answers

A rubber ball with a mass of 0.15 kg is dropped from a height of 1.2 m onto a hard floor. If the coefficient of restitution between the ball and the floor is 0.7, how high will the ball bounce on the first bounce?

<p>0.59 m (B)</p> Signup and view all the answers

A billiard ball of mass $m$ collides with an identical stationary ball. After the collision, the first ball moves at an angle of 30 degrees with respect to its original direction, and the second ball moves at an angle of -60 degrees with respect to the first ball's original direction. If the initial velocity of the first ball was $v$, what are the speeds of the two balls after the collision?

<p>$v_1 = (v \sqrt{3}) / 2$, $v_2 = v/2$ (C)</p> Signup and view all the answers

Consider a perfectly inelastic collision in one dimension between an object of mass $m_1$ moving with initial velocity $v_0$ and an object of mass $m_2$ at rest. What fraction of the initial kinetic energy is lost in the collision?

<p>$m_2 / (m_1 + m_2)$ (B)</p> Signup and view all the answers

Imagine a collision between two equal-mass objects in a closed system, where one object is initially moving at a velocity v, and the other is at rest. If the collision is perfectly elastic, what is the total kinetic energy of the system after the collision, in terms of the initial kinetic energy KE?

<p><em>KE</em> (C)</p> Signup and view all the answers

Two astronauts in space are initially at rest with respect to each other. Astronaut A, with a mass of 70 kg, pushes off of Astronaut B, with a mass of 90 kg. After the push, Astronaut A is moving away at a speed of 0.5 m/s. What is the speed of Astronaut B?

<p>0.39 m/s (D)</p> Signup and view all the answers

Two balls with different masses, $m_1$ and $m_2$ ($m_1 < m_2$), are dropped from the same height. Assuming that the collision with the ground is perfectly elastic, which ball will reach a greater height on the first bounce?

<p>Both balls will reach the same height. (D)</p> Signup and view all the answers

An object of mass $m$ collides with a stationary object of mass $3m$. After the collision, the objects stick together. What percentage of the initial kinetic energy is lost due to this collision?

<p>75% (C)</p> Signup and view all the answers

A car of mass $m$ is traveling at a speed $v$ when it rear-ends an identical car, also of mass $m$, that is at rest. Assuming the collision is perfectly inelastic, what is the change in kinetic energy of the system as a result of the collision?

<p>$-mv^2 / 4$ (D)</p> Signup and view all the answers

Consider an elastic collision between two particles of masses $m_1$ and $m_2$ in one dimension. Before the collision, mass $m_1$ has a velocity $v$ and mass $m_2$ is at rest. After the collision, what is the velocity of mass $m_2$ if $m_1 = m_2$?

<p>$v$ (C)</p> Signup and view all the answers

A small ball of mass $m$ is dropped onto a very large and heavy ball that is fixed in place. If the collision is perfectly elastic, what is the approximate change in the momentum of the large ball?

<p>0 (C)</p> Signup and view all the answers

Flashcards

What is a collision?

A collision occurs when a moving object contacts another object.

Conservation of Linear Momentum

In every collision, the total momentum of the system remains constant, meaning it is neither lost nor gained.

Elastic Collision

A collision where objects remain separate, kinetic energy and momentum are conserved.

Perfectly Inelastic Collision

Collisions where objects stick together upon impact; only momentum is conserved.

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Car Crashes (as Inelastic Collisions)

Total momentum is maintained, but kinetic energy transforms into sound, heat, and deformation.

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Inelastic Collisions

Collisions fall between perfectly elastic and perfectly inelastic; kinetic energy is lost, but objects don't necessarily stick together.

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Study Notes

  • The professor is Dave, and the lesson is about collisions.

Defining Collisions

  • Collisions occur when a moving object contacts another object.
  • Collisions apply to:
    • Balls on a pool table
    • Tiny molecules
    • Large celestial objects (asteroids, planets)

Conservation of Linear Momentum

  • In every collision, there is a conservation of linear momentum.
  • Conservation of linear momentum manifests differently based on the collision type.

Elastic Collisions

  • Involve objects that remain separate after impact.
  • Both total kinetic energy and total momentum are conserved.
  • Objects bounce off each other with minimal energy loss.
  • Atom and molecule collisions can be approximated as elastic (as with ideal gas).
  • Many collisions are nearly elastic:
    • A soccer player kicking a ball
    • The player's foot and the ball separate completely after impact.
  • Some kinetic energy is lost as heat and sound in nearly elastic collisions.

Perfectly Inelastic Collisions

  • Two separate objects collide and move together as one mass.
  • Celestial bodies (asteroids) may merge this way.
  • Planets, including Earth, began forming this way.
  • Momentum is easy to analyze because both objects can be treated as a single object after the collision:
    • m1v1 + m2v2 = (m1 + m2)v_final

Car Crashes

  • Car crashes can be modeled using the perfectly inelastic collision.
  • Adding the masses of the cars and combining velocity vectors reveals what happens after the collision.
  • Total momentum is conserved, but total kinetic energy is not.
  • Kinetic energy converts into:
    • Sound energy ("crashing" sound)
    • Heat energy
    • Internal energy (causing deformation)
  • A perfectly elastic collision is like an elastic band returning to its original state and shape after being stretched.

Inelastic Collisions

  • The amount of kinetic energy lost depends on the objects involved and various other factors.
  • No collision is fully elastic or perfectly inelastic.
  • Collisions exist somewhere between the two extremes of the spectrum, an inelastic collision.
  • We can usually approximate a collision as one of the two extremes to simplify calculations.

Linear Motion Conclusion

  • The study covered linear motion from:
    • Kinematics
    • Dynamics
    • Harmonic motion
    • Momentum
  • Next up is a move to Circular Motion

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