Conservation of Momentum

1. Define momentum 2 ways.
   1. How hard it is to stop an object
   2. P ≡ mv

2. State that momentum is a vector.

3. Explain what conservation of momentum means. The sum of the momentums of the particles immediately before an event (see objective 4) is equal to the sum of the momentums of the particles immediately after the event.

4. Name two types of situations where momentum is conserved.
   1. Explosions - things flying apart
   2. Collisions - things flying together

5. State one of the key reasons momentum can be conserved. - It's a vector!

6. Solve 1 dimensional "explosion" problems using conservation of momentum.
   • Bubba shoots a rifle
   • Bubba shoots a rail gun
   • Bubba pushes Nancy

7. Solve 1 dimensional collision problems using conservation of momentum when the colliding objects stick together.
   • Bubba is hit by a projectile
   • Meteors collide and stick together

Homefun (formative/summative assessment): Read sections 9.1

Formative Assessment: Physics Investigation

Follow up Questions

Impulse

8. Define impulse mathematically 2 different ways, in a special case, and explain in general terms what it indicates.

• (imp) ≡ ΔP
• The area under the force vs. time curve
• (imp) = Ft , note this is a special case when F = a constant
• Roughly speaking: an indication of how much effect a force will have on changing an object's momentum

9. State that impulse follows Newton's 3rd law.

10. Solve impulse problems.

    car crashes

area under the F vs t curve

 

Homefun (formative/summative assessment) problems1, 2, 3 p. 233.

Angular Momentum

11. State the mathematical definition of angular momentum. L = I w
12. State that angular momentum can only be changed by creating a torque (twisting action) on an object for a period of time. This torque for a period of time is an angular impulse. t (Δt) = ΔL
13. Explain why an object in orbit has constant angular momentum. The gravity force acts through the object's center of mass, hence cannot create a torque on it.
14. Given a satellite in elliptical orbit, derive the highest velocity given the lowest velocity and the distance from the planet at each point. Relevance: Constant angular momentum explains why comets speed up as they approach the Sun.
15. Solve spinning ice skater problems. Spinning ice skaters can be modeled as having constant angular momentum.

Homefun (formative/summative assessment):Section Review Problems 7, 8 ,9, 11, 12; page 235

2D Collisions

16. Describe the difference between elastic and inelastic collisions.

Note: kinetic energy is the energy of motion

K = 1/2 mv^2

where: m = mass, v = velocity

Collision Type	Conservation  Momentum	Converts Some Kinetic Energy to Heat.	Particles Stick
Elastic	Yes	No	No
Inelastic	Yes	Yes	Yes

 

17. State that the only truely elastic collision occurs between molecules.

18. Describe 2D elastic collisions When a moving mass collides with a stationary mass of the same size in a glancing elastic collision, the momentum vectors form a 90 degree angle after the collision.

• the y-components of the two momentum vectors cancel
• the sum of the x-components of the 2 momentum vectors afterwards= the momemtum before

 

19. Calculate the final velocity for 2D inelastic collisions.

Homefun (formative/summative assessment): problems 22, 23, page 243

Review of Objectives 1- 19 (1-3 days)

Formative Assessments:

1. Work review problems at the board.

2. Work practice problems.

Metacognition Problem Solving Question: Can I still work the problems done in class, several hours or days later? Some amount of repetition on the exact same problems is necessary to lock in learning. It is often better to thoroughly understand a single example of a problem type than to work example after example understanding none of them completely.

Relevance: Good test preparation is essential to performance in physics class.

Homefun (formative/summative assessment): problems 81, 89, 91, 59 pages 193-194; problems turn in on the day stapled to the back of the test.

Summative Assessment: Unit exam objectives 1-23