-Idea: Give lab practicum challenges at the beginning of units, but do them at the end. This unit is landing a certain mass hanging from a modified Atwood machine into a moving constant speed buggy.
Review: What happens if forces are balanced? (constant velocity- may or may not be moving)
SO- what happens if force is unbalanced? (change in velocity- acceleration)
-Have students demonstrate how a constant unbalanced force affects an object (feel it!):
- Push a hover puck continually for a distance
- Pull a car with a constant 1 N of force on a track
- Hit a bowling ball with a mallet every second
- Rocket sledder interactive
- Also try with more mass and see if the student thinks it's easier or harder to keep the force the same (more mass = more inertia = less change in velocity)
- Draw a force diagram of the demo that the class did
-What factors could you change to affect acceleration? 3 labs on the relationship between force, mass, ramp angle and the acceleration: (make sure to talk about constants!)
- What is the relationship between force and acceleration? (If doing this with a modified Atwood machine, you must take the hanging mass being removed each trial and add it to the car so the total mass of the system remains the same.)
- What is the relationship between mass and acceleration?
- Do a followup lab to see if the mass is 1 kg and the force is 1 N, what is the acceleration? (m/s/s)
- Summary: Newton's 2nd Law- a=F/m, where F is the unbalanced force
-Possible lab follow-up discussions:
- What is (inertial) mass? Have students push 2 cars with their eyes closed and see if they can tell which has more mass
- What is a kilogram? A quantitative measurement of resistance to acceleration or change in motion
- What is a Newton? Amount of force needed to accelerate a 1 kg massed object at 1 m/s/s
-Unit 5 Worksheet 1 - Unbalanced forces in an elevator. Extension: Elevator ride interactive.
- You're on the ground floor and going up in an elevator. What happens at the beginning when you start moving (accelerate up), the middle (constant speed), and the end when you are slowing down (accelerate down).
- What forces are on you in an elevator? (Earth and floor) What is the floor pushes more? (accelerate up- what could this mean?) What if the Earth pulls more? (accelerate down- what could this mean?) Can the earth pull with a different amount? (The floor just pushes with less)
- Before going over the worksheet: Thinking about riding in an elevator, when do you feel heavier? Normal? Lighter? How were you moving? Which way was the acceleration? Which way was the unbalanced force?
- If you feel heavy, then the force of the floor is greater than the Earth. If you feel light, then the force of the floor is less than the Earth.
- Our bodies are accelerometers!
- Whiteboard the worksheet
-What is an operational definition of weight? Which part of the force diagram is the weight- the floor (scale) or the Earth? What is the weight affected by or is it a property of an object? Weight is the measure of the pull of the gravitational body.
-Friction experiments: A block being pulled across a board at a constant velocity. If the velocity if constant, what do we know about the forces? How would we tell the size of the frictional force? (Must pull straight and horizontally parallel to the surface. Use blocks with wood sides and felt sides.)
- Student predict what a force vs time graph of me pulling a block from rest and then at a constant speed looks like. Demonstrate it with the force sensor on the screen and talk about what the graph looks like and why- compare it to the motion.
- Have students draw force diagrams for different times on the graph
- Is there a difference between starting/static friction and sliding/kinetic friction?
-What do you think we could vary to affect the frictional force?
- What is the relationship between surface (normal) force and the frictional force? (Have all students do this one first and then compare between all groups. Use 2 different surfaces between groups.) Include both static and kinetic friction so the graph has 2 lines on it.
- Each group then does one of the following:
- What is the relationship between (constant) speed and frictional force?
- What is the relationship between surface roughness and frictional force? (How will you graph this?)
- What is the relationship between surface area of contact and the frictional force? (Must keep mass constant!)
- What is the relationship between pull angle and the frictional force? (maybe do this one)
- Summary: Equation of lines are Ff = u*Fn
-Unit 5 Worksheet 2 - Unbalanced forces story problems
-2 cars with a string between them pulled with 2 N: what would the tension in each string be? (introducing the concept of a system)
- Draw schema and a force diagram for each car
- Draw a force diagram for the whole system as well
- Test setup on an actual track to see if the calculations are correct according to the force sensors
-A 500 kg (0.5kg) car on track with pulley and hanging 100 g (0.1 kg) mass. What will happen to the car when we release it?
- Draw schema and a force diagram for each car
- Draw a force diagram for the whole system as well
- What will the acceleration be?
-Atwood machine- a "frictionless" pulley with a mass hanging on each side
- With equal masses on each side and one higher than the other- what happens if I let go? If i start it moving and then let go?
- With a larger mass hanging higher- what happens if I let go? What is push the smaller one downward? Can you figure out the tension in the spring?
-Unit 5 Worksheet 3 - Unbalanced forces with angles
-Unit 5 Worksheet 4 - Coefficient of friction
-Demo: Drop a coffee filter upside down- what do you notice?
- Is it a constant speed? The whole time?
- What must the forces be like? What's balancing the Earth's pull?
- What about it I drop a single filter and 2 filters inside each other? When do they hit the ground?
-Lab Practicum: Land a mass hanging from a modified Atwood machine into a moving buggy. (Each group has a different mass hanging and maybe a different buggy.)
-Scenario discussion problem to lead into the next unit: If person is standing on top of a building and drops a ball, what happens?
- What is free fall acceleration? Can you give a for every statement for 10 m/s/s?
- With up/down motion, what should we call positive and negative directions?
- Make a "t-chart" of the time, velocity, and distance
- Graph the velocity vs. the time. What does the area between the line and zero mean? The slope?
- Why is the displacement in the first second not 10 m? What is the average speed for that first second?
- Why does the distance traveled each second keep increasing?
(Next unit, start off by looking at the same scenario, but for the ball being throw horizontally from the roof.)
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