Physics Chapter 6

Front 1

momentum

Back 1

The product of the mass of an object and its velocity, momentum = mass x velocity (or speed) = mv. p.84

Front 2

impulse

Back 2

The product of the force acting on an object and the time during which it acts. Impulse = force x time interval = Ft. p.85

Front 3

relationship of impulse and momentum

Back 3

Impulse is equal to the change in the momentum of the object that the impulse acts upon. In symbol notation, Ft = ^mv. p.89

Front 4

law of conservation of momentum

Back 4

In the absence of an external force, the momentum of a system remains unchanged. Hence, the momentum before an event involving only internal forces is equal to the momentum after the event: mv(before event) = mv(after event). p.90

Front 5

elastic collision

Back 5

A collision in which colliding objects rebound without lasting deformation or the generation of heat. p.91

Front 6

inelastic collision

Back 6

A collision in which the colliding objects become distorted, generate heat, and possibly stick together. p.91

Front 7

Which has more momentum, a 1-ton car moving at 100 km/h or a 2-ton truck moving at 50 km/h?

Back 7

Both have the same momentum (1 ton x 100 km/h = 2 ton x 50 km/h). p.85

Front 8

Does a moving object have impulse?

Back 8

No, impulse is not something an object "has," like momentum. Impulse is what an object can "provide" or what it can "experience" when it interacts with some other object. An object cannot possess impulse just as it cannot possess force. p.85

Front 9

Does a moving object have momentum?

Back 9

Yes, but, like velocity, in a relative sense - that is, with respect to a frame of reference, usually Earth's surface. The momentum possessed by a moving object with respect to a stationary point on Earth may be quite different from the momentum it possess with respect to another moving object. p.85

Front 10

For the same force, which cannon imparts a greater impulse to a cannonball - a long cannon or a short one?

Back 10

The long cannon will impart a greater impulse because the force acts over a longer time. (A greater impulse produces a greater change in momentum, so a long cannon will impart more speed to a cannonball than a short cannon.) p.85

Front 11

If a boxer is able to increase the duration of impact 3 times as long by riding with the punch, by how much will the force of impact be reduced?

Back 11

The force of impact will be only a third of what it would have been if he hadn't pulled back. p.88

Front 12

If the boxer instead moves "into" the punch so as to decrease the duration of impact by half, by how much will the force of impact be increased?

Back 12

The force of impact will be 2 times greater than it would have been if he had held his head still. Impacts of this kind of account for many knockouts. p.88

Front 13

A boxer being hit with a punch contrives to extend time for best results, whereas a karate expert delivers a force in short time for best results. Isn't there a contradiction here?

Back 13

There is no contradiction because the best results for each are quite different. The best result for the boxer is reduced force, accomplished by maximizing time, and the best result for the karate expert is increased force delivered in minimum time. p.88

Front 14

When does impulse equal momentum?

Back 14

Generally, impulse equals a "change" in momentum. If the initial momentum of an object is zero when the impulse is applied, then impulse = final momentum. And, if an object is brought to rest, impulse = initial momentum. p.88

Front 15

In reference to Figure 6.9:
a. How does the force that Cassy exerts on the bricks compare with the force exerted on her hand?
b. How will the impulse resulting from the impact differ if her hand bounces back upon striking the bricks?

Back 15

a. In accord with Newton's third law, the forces will be equal. Only the resilience of the human hand and the training she has undergone to toughen her hand allow this feat to be performed without broken bones.
b. The impulse will be greater if her hand bounces from the bricks upon impact. If the time of impact is not correspondingly increased, a greater force is then exerted on the bricks (and her hand!). p.89

Front 16

Newton's second law states that, if no net force is exerted on a system, no acceleration occurs. Does it follow that no change in momentum occurs?

Back 16

Yes, because no acceleration means that no change occurs in velocity or in momentum (mass x velocity). Another line of reasoning is simply that no net force means there is no net impulse and thus no change in momentum. p.90

Front 17

Newton's third law states that the force a cannon exerts on a cannonball is equal and opposite to the force the cannonball exerts on the cannon. Does it follow that the impulse the cannon exerts on the cannonball is equal and opposite to the impulse the cannonball exerts on the cannon?

Back 17

Yes, because the interaction between both occurs during the same time interval. Since time is equal and the forces are equal and opposite, the impulses, Ft, are also equal and opposite. Impulse is a vector quantity and can be cancelled. p.90

Front 18

Consider the air track in Figure 6.16. Suppose a gliding cart with a mass of 0.5 kg bumps into, and sticks to, a stationary cart that has a mass of 1.5 kg. If the speed of the gliding cart before impact is v(before), how fast will the coupled carts glide after collision?

Back 18

According to momentum conservation, the momentum of the 0.5-kg cart before the collision = momentum of both carts stuck together afterwards:
0.5V(before) = (0.5 + 1.5)V(after)
V(after) = 0.5V(before)/(0.5 + 1.5) = 0.5V(before)/2 = V(before)/4
This makes sense, because four times as much mass will be moving after the collision, so the coupled carts will glide more slowly. The same momentum means four times the mass glides 1/4 as fast. p.93