Which has a greater momentum: a heavy truck at rest or a moving skateboard? |
The moving skateboard. |

Distinguish between force and impulse. |
Impulse is force times a time interval. |

An impulse can be increased by |
increasing the force or increasing the time interval. |

For the same force, why does a long cannon impart more speed to a cannonball than a short cannon? |
The force is applied for a longer time in the long cannon. |

How is the impulse -momentum relationship related to Newton’s second law? |
F=ma=mΔv/Δt, so Ft=Δ(mv) |

To impart the greatest momentum to an object, you must: |
exert the greatest force over the longest time. |

When you are struck by a moving object, is it favorable that the object makes contact with you over a short time or over a long time? |
Long contact time decreases the magnitude of the average force and is favorable. |

Why is a force that is applied for a short time more effective in karate? |
The average force is increased. |

Why is it advantageous to roll with the punch in boxing? |
Rolling with the punch increases contact time, which decreases the force. |

Which undergoes the greatest change in momentum (if all of the baseballs have the same speed just before being caught and just after being thrown)? In the preceding question, which case requires the greatest impulse? |
For both questions: A baseball that is caught and then thrown back |

Can you produce a net impulse on an automobile if you sit inside and push on the dashboard? |
No, never. |

Is it correct to say that, if no net impulse is exerted on a system, then no change in the momentum of the system will occur? |
Yes, always. |

What does it mean to say that momentum is conserved? |
It means momentum does not change. |

When a cannonball is fired, the momentum of the system (cannon + cannonball) is conserved if…? |
the momentum of the cannon is equal to the magnitude of the momentum of the cannon ball and points in the opposite direction. |

In which is momentum conserved: an elastic collision or an inelastic collision? |
Momentum is conserved in elastic and inelastic collisions. |

Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A? |
The speed of car B is equal to the initial speed of A. |

If the equally massive cars of the preceding question stick together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A? |
Their speed is half the initial speed of car A. |

Suppose a ball of putty moving horizontally with 1 kg·m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg·m/s of momentum. What is the magnitude of their combined momentum? |
1.41 kg·m/s |

In the preceding question, what is the total momentum of the balls of putty before and after the collision? |
1.41 kg·m/s before and after. |

# chp 6 – homework questions

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