A 10 kg box is sliding on friction-less ice at 6 m/s. It hits and sticks to a stationary 10 kg box. What is the speed of the two blocks as they move away stuck together?
Answer
Conceptually
Since the combined mass is twice the original mass, the speed must be 1/2 of the original speed, so the two blocks are going 1/2 of 6, or 3 m/s.
Mathematically
Momentum initial must equal the final momentum
Initial momentum is 10 kg * 6 m/s = 60 kgm/s
Final momentum must be 60 kgm/s
p = m*v so…
60 kgm/s = 20 kg * speed final
(60 kgm/s) / (20 kg) = 3 m/s.
Today
Last in-class build day.
If you do not finish today, you must come in at lunch or 7th period to finish your car.
Cars are due by Friday, when we will test them out.
Don’t forget the photo assignment due next Wednesday.
Have your Mousetrap car out to Page 4 so I can stamp that you have completed that page.
Calculate the mass of a mousetrap car that has a speed of 2 m/s and a momentum of 1 kg*m/s. Calculate the speed of a ball that has a mass of 0.5 kg and a momentum of 2 kg*m/s. Write out the equations then solve them.
Answers
Part 1: Mass = 0.5 kg
Part 2: Speed = 4 m/s.
Classwork
Continue working on your car. You should have it complete by today. Work on Page 5, through Number 14.
Honors students will need to take a photo of a physics phenomena and describe the physics principles demonstrated by the photo.
Due date
Photos and essays must be submitted electronically to your teacher by 11:59pm, Wednesday March 11th.
Grading
Emphasis is on the composition of your photo and the accuracy of your physics explanation. 60% of your grade will be based on how well your photo captures a physics phenomena, and 40% of your grade will based on the quality and accuracy of your essay.
Guidelines
Photos
Sample natural photo
There are two categories of photos. Your can decide which category you enter.
(1) Natural photos are those that involve everyday situations that may demonstrate a variety of physics concepts.
(2) Contrived photos are those that are set up to show a particular physics concept or related set of concepts.
Original photos may be cropped for better composition (as long as the final product is still an 8” x 10” or 8½” x 11” print). Minimal manipulation of digital images with the intention of accurately rendering the subject is acceptable e.g.: small adjustments to brightness, color, color balance and/or sharpness, is acceptable. This is not a contest to see how well you can manipulate photos; emphasis is on the composition of your photo and the accuracy of your physics explanation.
All photos must be taken with a digital camera, and the original image must be emailed to your teacher. In order to avoid plagiarism, you must also submit an additional photo ‘selfie’ showing you in the photo (take this right before or after you take your contest photo).
In your own words, define the word momentum. Next, give an example of two objects where one has more momentum than the other.
Today you will be learning about momentum. Most people have a natural grasp of what momentum means…so remember that as we begin our study. You may have some mistakes in your knowledge, and you may not know the math formula, but I bet you could describe two things that have different momenta (the plural of momentum is momenta).
Class work
Complete the warm up.
While watching the videos, take notes on the handout. The teacher can pause it if you need it (handout is at the bottom for students who are not in class today).
Read pages 374-377 in the textbook. Answer the questions on the handout as you read.
Complete the practice problems.
Video 1
Watch the first 3:41 minutes of this video.
Video 2
Play this video all the way through.
Book reading
Read pages 374-377 in the textbook, then answer the rest of the questions and problems on the worksheet.
Video 3
If there is extra time in class, this video introduces momentum in collisions.
Hint: A “vector quantity” just means you have to pay attention to the direction. You’ll spend more time on vectors later in math, but all you need to remember is that you can have positive or negative momentum if they are going in opposite directions.
Elastic collisions are ones where the two objects bounce off each other and don’t stick. Inelastic collisions are where they stick together and move as one object after the collision.
One page one, describe the choices you made for each part of your car, and explain why you chose that style.
On page two, make a few sketches showing how your car will be put together.
Timeline for project
Wednesday: Learning about momentum, continue finding parts for your car at home.
Thursday: Build day 1. Today you will put together your car in a ‘rough draft’ style: you build it, but pay attention to changes you could make as you test it out, then revise your model.
Friday: Build day 2: Today you will need to test your rough draft car to ensure it makes it five meters. You can also tweek your design as needed.
Monday: Learning about momentum. You can work on your car outside of class if needed.
Tuesday: Official test for your ‘final version’ car.
Look at your notes sheet with your table partners. Add one thing to your notes that your partners had but you didn’t, and write this in your warmup section as well.
