Name: __________________ Period: ___ Date: _________ Chapter 7 Notes Momentum

Key Terms: conserve

collision

elastic collision

external forces

inelastic collision

internal force

impulse

momentum

law of conservation of momentum

Possible Misconceptions: Do you believe that any of the following statements are true? They AREN'T! When you finish Chapter 7, you should understand that each of these statements is FALSE, and WHY. WRONG: The momentum of an object decreases by itself. (Objects "lose momentum".) WRONG: Impulse = Momentum WRONG: Momentum is conserved only when collisions are perfectly elastic. WRONG: Impact and Impulse are the same. WRONG: When 2 objects collide and stick together, the impact force is distributed over the entire time they are stuck together.

7.1

Momentum A.

Momentum –

B.

In physics, like in football, momentum refers to how _____________________________ __________________________________.

C.

What makes an object difficult to stop? 1. 2.

D.

______________ measures the ________________of an object - how much the object resists changes in motion. More __________ means more _____________. The momentum of an object is ____________________ proportional to its mass. Twice the _______________ means _______________ the momentum.

E.

Momentum is not the same as _____________, though. An object that is ___________ has no momentum, no matter how much ________________ it has.

F.

More ________________means more momentum. Momentum is __________________ proportional to ______________________. Twice the ________________ means twice the momentum.

G.

Momentum is a _______________ quantity. Its direction is the same as the direction of the object's velocity.

Example 1:A ball of mass 2 kg. is moving with a speed of 4 m/s. What is its momentum?

An Analogy A mood is something you have - you are happy, sad, etc. - it is a characteristic of your current state of being. In the same way, momentum is something that an object has - it is a characteristic of its current physical state. A "pop quiz" in physics is something that happens to you. In the same way, an impulse is something that happens to an object.

7.2

Impulse Changes Momentum

Just like a pop quiz in physics can affect your mood, an impulse will affect the momentum of an object. A. ImpulseB.

It is equal to _________________________________

C.

An impulse is something you _____________ to an object to cause a momentum change.

D.

When you apply a force on an object, you also exert an impulse on it. Force and impulse always go together. Impulse is _______________ proportional to the applied force. Double the force, ______________ the impulse.

D.

Impulse, however, is not the same as force.

E.

Impulse also depends on ________________ the force is applied. More time, more impulse. Impulse is ________________ proportional to the time for which the force is applied. Twice as long means _________________ the impulse.

F.

Impulse is a ______________ quantity. It has the same direction as the applied force.

G.

Impulse ________________________________. ** note: impulse is not equal to momentum, but the change in momentum.

Example 2:A force of 5 Newtons acts on a ball for 4 seconds. How much impulse was exerted on the ball?

Case 1: Increasing Momentum Examples: Getting the Largest Possible Velocity: You need the largest possible _____________________, therefore the largest ____________________. 1. 2.

Case 2: Decreasing Momentum minimizing the forces Examples: In these cases the momentum change is fixed. The value of impulse cannot be changed. You can get the same impulse by applying 1. 2. Equation:

In situations where you want the smallest possible force on an object, like in a car crash, you need to have a large impact time. This can be achieved by cushioning- airbags in the case of car crashes, type of flooring in the case of a fall and flexing knees. Moving with the punch in the case of a boxer., pulling your hand backward when catching a ball. Case 3: Another example of application of impulse-momentum theory- loose couplings between railroad cars. The friction force between the train and track isn’t large enough to start the whole train at once. But it is sufficient to start one moving. Loose couplings allow each car to start in sequence. The large impulse that is required is broken into a series of smaller impulses. 7.3

Bouncing ( maximizing the force)

Examples: A.

In bouncing, momentum is reversed, so the change in momentum is ___________

B.

Twice the momentum change, _______________________________________.

( Design and construct a case to hold an egg that is to be dropped from a 3 or 4 story bldg. Mass must be the same. Cannot have things to increase air resistance- only change time of impact)

7.4

Conservation of Momentum A. Law of Conservation of Momentum – B. External forces vs. internal forces-

Diagram: Isolated system

Example: falling rock. Momentum of the rock is __________conserved because it has an external force acting on it. ( __________________). If the rock and earth is considered, then the mom _______ conserved- _______________________! C. No experiment to date has shown a violation of the Law of Conservation of Momentum. It applies to objects moving very fast, or to particles inside the nucleus as in radioactive decay - both places where Newton's Second Law fails. 7.5

Collisions A.

The law of conservation of momentum applies to __________ types of collisions.

B.

Elastic Collision-

C.

Inelastic Collision-

Example 3: A toy car with mass 5 kg moving at a speed of 2 m/s hits another toy car with mass 10 kg, which is initially at rest. After the collision the two cars stick and move together. Calculate the speed of combined mass after the collision.

Example 4: A toy car with mass 10 kg moving at a speed of 2 m/s hits another toy car with mass 5 kg, which is initially moving in the opposite direction with a speed of 1 m/s. After the collision the two cars stick and move together. Calculate the speed of combined mass after the collision.

Example 5: A 50 kg boy and a 75 kg girl standing on a skating rink push off against each other. If the boy moves back with a speed of 5 m/s, what is the speed of the girl?

See questions p.96 7.6

Momentum vectors. A. For collision in two dimension, we can use momentum vectors to solve the problems. Momentum vector before the collision must equal the sum of the momentum vectors after the collision. Example 6: A car moving north at 5 m/s collided with another car moving west with a speed of 1- m/s. If the cars stick together after the collision, what is the direction of the mass after the collision?

Examples 7: A falling fire cracker explodes into two pieces.