This unit starts out differently- with brainstorming. Make sure to use teacher techniques for discussion- give students time to think, say more, and have what they are saying paraphrased. (See teacher talk moves.)
-What is force? List student ideas on the board.
-1st rule of forces: they must all be caused/applied by a physical object. Apply this rule to students' list of forces on the board. Should end up with pushes and pulls; might be uncertain about gravity and movement.
-Draw picture with arrows to show forces- a force diagram.
- First step- a schema. You may only show objects involved that are made of matter. (Can you hit someone with it or buy it at the store?) Use solid lines to connect touching objects, dotted lines for a force at a distance (only 4- Entire Earth/gravity, magnetism, electrostatics, and strong nuclear force). Air is not important unless it's not even (wind, moving, etc.).
- Give an example schema to students for holding a book up on your palm.
- For the force diagram, put a dot in the middle and draw an arrow from it to represent each force from the lines touching that object in the schema. How should the size of the arrows compare? Use the schema to make a force diagram for the book.
- Do some more practice: a bowling ball on the table, a tennis ball on the table (do with group). How do you decide the size of the arrows?
-2nd rule of forces: If the change in velocity = 0, the forces are balanced.
- Initially, this rule might just be that zero velocity means the forces are balanced.
- Use a hover disc gliding across a table at a constant speed as an example- think about the motion. The velocity is constant; acceleration is zero.
- Draw a schema and force diagram for the puck while it is moving at a constant speed. The past/future forces do not count (hand pushing or catching is not included).
- Is there any sideways force? Are the forces balanced? What does this mean about objects moving at a constant speed?
-Further examine forces:
- Do table push or just get in the way? Have a student hold things place in hands with eyes closed to demonstrate surface (normal) force. Also do with object on a spring, sponge, flexible whiteboard which will bend, and on magnets stacked on a pole.
- How do we measure forces? What units? Newtons on a spring scale. Have a student pull down with 5 N. What do I have to do holding it? I have to pull up with the same amount.
- Do a schema and force diagram for a box hanging on a string from a spring scale.
-Introduce friction: Do a schema and force diagram for a heavy book pushed across the table (at a constant speed).
- What do you notice about how it is moving? What does that mean about the forces?
- Demo: 2 whiteboard erasers with the fuzzy part rubbing on each other to show friction
- Draw a schema and force diagram. Must have 4 forces for them all to be balanced. How do you know how big to draw the arrows?
- Always ask yourself: Is there a surface pushing? Is there friction?
- But will objects move forward on their own? Ball in a car? Dry ice/hover puck on a table? Air hockey? Lunar lander simulation (not in html5 yet)? Only if there is friction slowing it down. No force is needed for motion, we just don't get this because we almost always experience friction.
-Unit 4 Worksheet 1 - Force Diagrams. Do non-angled problems first, then angled problems.
- In between the two types of problems, talk about shadow components. Use cutout arrows and a flashlight to show the shadow on a horizontal and vertical surface.
- As I change the angle, what happens to the shadow?
- How does the length of the real arrow compare the the shadow arrows? What is the longest a shadow can be- how?
- Shadow part = the horizontal and vertical components. On force diagrams when we start getting forces in more than one direction, vertical arrows balance together and horizontal arrow balance together.
- For the angled problems: does an even number of arrows = balanced and odd number = unbalanced?
- Is the angled string doing 2 things? What is it doing?
- Problem #3: A key takeaway- acceleration is in the direction of the unbalanced force.
- Does a string have the same tension through the whole thing? Do the demo in the picture below and have students vote before you turn the middle scale around.
-Lab: with random objects, spring scales, balances.
- What could we measure here? What is the relationship between mass and the force of the Earth (weight)?
- Do you want to use g or kg? (Or let them do what they want and discuss differences later.)
-Lab followup: Establish the idea of weight on Earth (it's different other places) as being a measure of the gravitational field strength on Earth.
- Are mass and weight the same thing? Just different numbers?
- Force field- area of space where an object experiences a force like the force of the entire Earth pulling on us. On Earth, the gravitational field strength is 10 N/kg. This helps us translate between mass and the force of the Earth (weight). How do you get something's weight from its mass?
- What is mass? Show students inertial mass by having students push or receive a cart with/without mass on it to feel the difference.
- The change in how you have to move it is side to side, so that's not the weight- weight is up/down.
- There is a natural tendency of objects to resist change in motion (inertia/inertial mass).
- Newton's 1st law- objects in motion will stay in motion in a straight line if the forces are balanced (and the same for at rest).
-Unit 4 Worksheet 2 - Angled force diagrams.
- Students must use the following steps that will allow them to solve problems they have not seen before:
- 1. Schema
- 2. Force diagram
- 3. Shadow components- horizontal and vertical
- 4. Check vertical and horizontal forces separately for if they are balanced or unbalanced
- 5. Solve
- May also now explain force "nicknames"- Ff for frictional force, Fn for normal force (or use perpendicular force), Fw for weight (the pull of the entire Earth), tension = string
- Students may not draw force diagrams on the picture -> they aren't the same (the arrows represent force, not string)
- Tip: If there aren't enough misconceptions on the white boards, have some groups purposefully add some wrong things you suggest to create more for discussion in class circle.
-Unit 4 Worksheet 3 - Various force diagrams. Do the same as above for Worksheet 2.
- Demo: an object suspended from the wall and from a spring scale (may use a pulley). Show that as you pull it more horizontal, the force on the spring scale gets bigger (and if using a pulley, the string gets shorter).
-Lab: 2 cars of a track with push force sensors on them. Have one car with more mass than the other. Try different scenarios and to see: which car will exert more force? Take data for trials from each scenario.
- Moving same speed at each other
- Moving toward each other; one fast, one slow
- Moving in the same direction; on is faster and overtakes the other
- One is stopped and the other one hits it
- Keeping adding new tasks for each group as they go (add more mass to one car and redo scenarios, can you come up with a scenario where one has more force?, add a spring, etc.)
-Force pairs demos: Question for each- are the forces the same? Is the result (acceleration) the same? When is it or is not the same?
- Force plates- show how they work by having someone jump on it and do pushups on it and analyze the graphs produced
- Force plates- 2 people push and we sees who can push harder
- Youtube video- visualizing Newton's 3rd law with colliding carts. Do the metals bend the same? Is the force the same?
- Instruct me how to walk. Too hard? Instruct me how to jump.
- Did I accelerate?
- How did I accelerate? I can't push on myself. Did the floor push on me?
- Did I push more or did the floor push harder? Remember what we just did!
- Why did I move more?
- What way was each for between the floor and me?
- Now tell me how to walk again.
- Which way did I push?
- How did I move forward?
- What are other forces interactions that cause locomotion? (cars, space ships, canoe, swimming, helicopter- use toy for demo to show what happens if spun different directions!)
- If I want to move forward which way do I push?
- Newton's 3rd Law: All forces come in pairs that are equal in strength, opposite in direction, and act on the other object.
- Fa on b = Fb on a
- They are pairs if you can flip flop the label
-2 cars with a string between them on a track and I pull on one of them. The second car has more mass.
- Draw a schema and 2 force diagrams- one for each car.
- Show the force pairs between the two diagrams.
- Should unbalanced force be in the direction of the acceleration?
- No friction (acceleration) #3, 6, 9
- Friction (constant velocity) #1, 4, 7
- Idea you might come up with for a future lab: does the mass of an object affect its friction?
-Set a known mass/weight object on the table.
- How much does this weigh?
- Draw a schema and force diagram. How many forces are on the diagram?
- What direction are the forces in? Which is longer?
- Are these force pairs? Have the class split into sides by yes or no and argue. Students switch sides as they are convinced.
- Summary: Force pairs cannot be on the same force diagrams, because they are on separate objects and force diagrams only who the force(s) on 1 object.
-Lab for higher students: Car on an inclined track to show students shadow components of gravity after they use spring scale to get the car's weight. Students hold the car on the ramp with a force sensor to get the force of the pull up the ramp also (see notes).
- Draw a force diagram to scale with the shadow components of gravity. Is there a way to check if the numbers you get in the triangle fit together?
- You can have students compare the angle of the ramp to the angle of the shadow components. (Have each group do a different angles- 23, 28, 37, 53)
- For the normal force, put a marker vertically on the ramp and show how its direction changes as you incline the ramp. What do you notice? It's always perpendicular.
- Cool demo: Incline the ramp from horizontal to vertical with the force sensor holding the car in it's location on the ramp to see how the force on the graph produced changes (goes from 0 N at horizontal to 6 N at vertical for a 6 N object).
- Can you put strings in place of the surface (normal) force with the string pulling it up the ramp and have the car stay in place if you remove the ramp?
Fun teacher note- easy to use right triangles:
- 3, 4, 5
- 5, 12, 13
- 7, 24, 25
- 9, 40, 41
- 11, 60, 61
How it actually worked out in class, Fall 2015:
-Day 1: Measure free fall acceleration on Earth (from last unit)
-Day 2: Discuss free fall acceleration (data table and motion map); Mass in kg vs. N on spring scale lab (both are 10!) data collection and graph
-Day 3: For every statement for kg vs. N lab and discuss; Forces list/discussion, intro schema/force diagram and 1st rule of forces- caused by physical object
-Day 4: Further develop force diagrams:
*review schema/force diagram of object held in palm of hand
*show pulling on an object with a spring scale each direction is equal force; 2nd rule of forces- if v=0, forces are balanced (same size/opposite direction) (will modify this rule later)
*schema and diagram for two different masses objects in my hand; whiteboard and compare between groups and two objects
*does the table apply force to an object on it? objects placed in hands and on a whiteboard between tables
*example of a heavy object on the big scale- do schema and force diagram for object with numbers
*schema and diagram for scale which is held by one person and pulled by another (take reading of N on scale also); whiteboard and discuss number/size of arrows
-Day 5:
*schema and force diagram of hover puck moving across table; whiteboard and discuss
*modify 2nd rule of forces- moving objects can also have balanced forces; 0 acceleration = balanced forces
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