Q3 Physics: Momentum

8.1: Does momentum apply to objects at rest?

8.1: Inertia applies to both objects at rest and in motion, however, when discussing momentum, we are only concerned with objects in motion.

8.1: What is momentum?

8.1: Momentum is inertia in motion, and specifically in motion.

8.1: What is the basic concept to momentum (when concerned with collisions)?

8.1: The Big concept for momentum is that it is conserved for all collisions as long as external forces don't interfere.

8.1: How does one increase or decrease momentum?

8.1: To change the momentum of an object, change it's mass, velocity, or both. Because you can't really change an objects mass in reality while in momentum (technically you can), most problems will be concerned with changing velocity.

8.1: Can small objects have the same momentum has a large object?

8.1: Yes. If an object is fast enough, it can reach the same momentum as a large object going at a slow velocity. Remember, it's dependent on mass AND velocity multiplied together.

8.1: What is the SI unit of momentum?

8.1: kg • m/s, or kilogram meters per second

8.2: What does the change in momentum depend on?

8.2: A change in momentum depends on the force that acts and the length of time it acts. This is otherwise known as impulse.

8.2: What is the relation between impulse and momentum?

8.2: Impulse is dependent on momentum. The greater one's change in momentum, the greater the impulse.

8.2: What is impulse?

8.2: Impulse is the change in momentum. Similar to the difference between velocity and acceleration.

8.2: What is the impulse equation?

8.2: impulse = Ft = Δmv where, F = Force t = time Δ = change in m = mass v = velocity

8.3: What happens to impulse if an object bounces?

8.3: Impulses are greater when an object bounces.

8.3: Who invented the Pelton Wheel and how does the Pelton Wheel make use of impulse to its advantage?

8.3: Lester Allan Pelton invented the impulse type water turbine in the 1870s. The wheel's "pegs" have a cup-style to them that allows water to curl in and exit out. Because bouncing increases the force (thus, increasing impulse), the wheel will generate more energy than if it were to simply receive the "dead weight" of the water.

8.3: Which of the two requires a greater impulse; the impulse required to bring an object to a stop, or to bring it to a stop and then to throw it back again?

8.3: The impulse required to bring an object to a stop and then to "throw it back again" is greater than the impulse required merely to bring the object to a stop.

8.3: Suppose you catch a falling pot with your hands. Is there a greater impulse when simply catching it or catching and throwing it up again?

8.3: The latter.

8.4: What are some system examples that best represent the law of conservation of momentum?

8.4: Examples include: Molecular forces within a basketball have no effect on the momentum of the basketball. A push against the dashboard from inside does not affect the momentum of a car. These are internal forces. They come in balanced pairs that cancel within the object.

8.4: In order to create momentum, what must happen to an object that wouldn't violate the law of conservation of momentum?

8.4: In order to change the momentum of a system, a force or impulse must be exerted on the object externally (key word: externally).

8.4: Suppose there is a cannon firing a cannonball. What is it's net momentum before and after firing? Why doesn't the cannon recoil at the same velocity that the cannonball is firing in?

8.4: It's net momentum before firing is zero. After firing, the net momentum is still zero because the momentum of the cannon is equal and opposite to the momentum of the cannonball. The reason why the cannon doesn't recoil at the same velocity is because of the wide difference in mass and velocity. The cannon has a lot of mass, resulting in less of a recoil.

8.4: Is momentum a vector or scalar quantity?

8.4: Momentum has both direction and magnitude, making it a vector quantity.

8.4: What does the law of conservation of momentum state?

8.4: The law of conservation of momentum states that, in the absence of an external force, the momentum of a system remains unchanged.

8.4: What is the net momentum before and after a change if all the forces acting in the system are internal? Examples?

8.4: The net momentum of the system before and after the event is the same. Some examples include: - Atomic nuclei undergoing radioactive decay - Stars exploding - Cars colliding

8.5: Given the following situation, solve the net momentum before and after the collision: Two carts are rolling on a track. Assume identical masses. The first cart is moving at a velocity of 4m/s. After the collision, the two carts, because of the fact that this is an inelastic collision, stick together.

8.5: NET MOMENTUM before collision = NET MOMENTUM after collision (1m)(4 m/s) + (1m)(0 m/s) = (2m)(1/2v after) Remember; momentum is always conserved. Thus, the net momentum before and after will be the same. In order to make up for the mass increasing my 2, it's velocity is decreased by half.

8.5: What is an elastic collision? Is bouncing in an elastic collision perfect?

8.5: Elastic collisions occur when objects collide without being permanently deformed and without generating heat. As such, collisions during elastic collisions bounce perfectly off each other. Because there's friction, heat generation, deformities, etc. in the real world, elastic collisions are near-impossible, apart from sub-atomic levels.

8.5: Perfectly elastic collisions are common only on a microscopic level. What is one example?

8.5: Electrically charged particles bouncing off each other generate practically no heat (they don't really "touch" in the sense of the word).

8.5: In inelastic collisions, does the momentum conservation law hold true? What is it's equation?

8.5: Even in the event of inelastic collisions, the momentum conservation law holds true. The equation is NET MOMENTUM before collision = NET MOMENTUM after collision

8.5: In the absence of external forces, when objects collide, what happens to the net momentum of both objects before and after collision? What equation represents this?

8.5: In the absence of external forces, net momentum both before and after the collision would be equal. NET MOMENTUM before collision = NET MOMENTUM after collision

8.5: What is an inelastic collision? Is bouncing in an elastic collision perfect?

8.5: Inelastic collisions occur when colliding objects become distorted and generate heat during the collision. As such, collisions during inelastic collisions bounce imperfectly.

8.6: True or false? Momentum is not conserved when interacting objects don't move in a straight line.

8.6: False; momentum is conserved, even when interacting objects don't move in a straight line.

8.6: Is the vector sum of momenta the same before and after a collision?

8.6: Yes; the vector sum of momenta is the same both before and after a collision has occurred.

8.1: What is the momentum equation?

p = mv where, p = momentum m = mass v = velocity If direction is not an important factor, velocity becomes speed.