Whenever an interaction occurs in a system, forces occur in equal and opposite pairs. Which of the following do not always occur in equal and opposite pairs? 1. Impulses. 2. Accelerations. 3. Momentum changes. 4. All of these occur in equal and opposite pairs. 5. None of these do. Ch 6-1

Whenever an interaction occurs in a system, forces occur in equal and opposite pairs. Which of the following do not always occur in equal and opposite pairs? 1. Impulses. 2. Accelerations. 3. Momentum changes. 4. All of these occur in equal and opposite pairs. 5. None of these do. Ch 6-1 Answer: 2 Because time for each interaction part is the same, impulses and momentum changes also occur in equal and opposite pairs. But not necessarily accelerations, because the masses of the interaction may differ. Consider equal and opposite forces acting on masses of different magnitude.

Which would be more damaging? 1. Driving into a massive concrete wall. 2. Driving at the same speed into a head- on collision with an identical car traveling toward you at the same speed. 3. They are equivalent. Ch 6-2

Which would be more damaging? 1. Driving into a massive concrete wall. 2. Driving at the same speed into a head- on collision with an identical car traveling toward you at the same speed. 3. They are equivalent. Ch 6-2 Answer: 3 Your car decelerates to a dead stop either way. The dead stop is easy to see when hitting the wall, and a little thought will show the same is true when hitting the car. If the oncoming car were traveling more slowly, with less momentum, you’d keep going after the collision with more “give,” and less damage (to you). But if the oncoming car had more momentum than you, it would keep going and you’d snap into a sudden reverse with greater damage. Identical cars at equal speeds means equal momenta—zero before, zero after collision.

Strictly speaking, when a gun is fired, compared with the momentum of the recoiling gun, the opposite momentum of the bullet is 1. less. 2. more. 3. the same. Ch 6-4 Thanks to David G. Willey.

Strictly speaking, when a gun is fired, compared with the momentum of the recoiling gun, the opposite momentum of the bullet is 1. less. 2. more. 3. the same. Ch 6-4 Thanks to David G. Willey Answer: 1 Why? Because more than just a bullet comes out of the barrel when a gun is fired. The gas, formed when the powder in the cartridge burns, pushes the bullet along the barrel, and this gas too has appreciable mass and exits at high speed. More than negligible momentum is given to the gases. So, momentum of recoiling gun = momentum of bullet + momentum of gases More than one person has been accidentally killed by a “blank” fired at close range!

1. to the left (backward). 2. to the right (forward). 3. not at all. An ice sailcraft is stalled on a frozen lake on a windless day. A large fan blows air into the sail. If the wind produced by the fan strikes and bounces backward from the sail, the sailcraft will move Ch 6-5 1. to the left (backward). 2. to the right (forward). 3. not at all.

1. to the left (backward). 2. to the right (forward). 3. not at all. An ice sailcraft is stalled on a frozen lake on a windless day. A large fan blows air into the sail. If the wind produced by the fan strikes and bounces backward from the sail, the sailcraft will move Ch 6-5 Answer: 2 You might think the sailcraft wouldn't move— that the force of wind impact on the sail would be balanced by the reaction force on the fan— which would be true if the wind came to an abrupt halt upon striking the sail. But it doesn’t. The wind bounces from the sail and produces a greater force on the sail than if it merely stopped (like any collision, more force is required to reverse the direction of something than to merely start or stop it). So there is a net force on the sailcraft and a forward acceleration. Or consider impulse and momentum. The impulse on the sail is greater than the impulse on the fan. Why? Because the air undergoes more change in momentum bouncing from the sail than starting from the fan. Note there are two force pairs to consider: (1) the fan-air force pair, and (2) the air-sail force pair. Because of bouncing, the air-sail pair is greater. Solid vectors show forces exerted on the sailcraft; dashed vectors show forces exerted on the air. The net force on the sailcraft is forward, to the right. 1. to the left (backward). 2. to the right (forward). 3. not at all.