Forces
Day / Lesson / Topics / In Class / Homework65 / What is a Force? / Idea of force / Elastics, spring scales / Video: Fundamental Forces
Assign Song / Video Project
66 / What is the Effect of a Force? / Force causes acceleration / Dynamics carts, elastics spring scales, tickertape timer / Video: Feynman and Inertia
67 / What are the Effects of Many Forces? / Combining forces, net force / Dynamics carts, spring scales
Double Atwood machine demo / Video: Inertia Hip-Hop
68 / Free Body Diagrams / Newton’s 1st law, free body diagrams / Develop 1st law
Hover-puck demo / Read: “Newton’s First Law of Motion”, pg. 59-63
Handbook: Homework – Free Body Diagrams
Video: First Law
Video: Forces in Space!
69 / The Force of Gravity
Normal Force / Force of gravity, gravitational field strength, Fg = mg
Normal force / Take up FBD Howo
Masses, spring scales
Metersticks, masses, spring scales / Read: “Gravitational Force on Earth’s Surface”, pg. 84-88
Problems: pg. 85 #2-5, pg, 86 #6-8, pg. 87 #10
Video: Big Bang Theory
Video: Gravity – Newton to Einstein
Read: “Forces we Experience Daily”, pg. 55-56
Problems: Homework – Normal Forces
Video: Normal Force
Video: Normal Force
70 / Force, Mass and Motion
Force, Mass and Motion – part 2
Newton’s Second Law Problem Solving / Force and mass affect acceleration
Newton’s 2nd law, Fnet = ma, definition of force, definition of Newton, inertia
Force problem solving techniques / Dynamics carts, masses, tickertape timers, pulleys
Finish activity
Develop 2nd law / Video: Cars and Inertia
Read: “Newton’s Second Law of Motion”, pg. 71-73
Problems: pg. 73 #1,2,4,6
Lesson: Newton’s Second Law
Video: 2nd Law in Space!
Problems: finish handbook questions and pg. 81 #16
Lesson: Finding the Net Force
Video: Misconceptions About Freefall
71 / Forces as Interactions
Newton’s 3rd law / Forces are interactions
Newton’s 3rd Law,
Force pairs / Two carts on track
Computer force probes, two carts on track
Develop Newton’s 3rd Law / Video: 3rd Law in Space!
Video: Newton’s Third Law
Read: “Newton’s 3rd Law of Motion”, pg. 74-77
Problems: Homework – Newton’s Third Law
72 / Friction / Kinetic and static friction
Coefficient of friction, Ff = mFn / Friction boxes, masses, spring scales
Finish activity
Friction problem solving / Read: “Effects of Friction”, pg. 96-103
Video: Brake Test
Video: Chevy Volt Test
Problems: pg 97 #1, pg. 100#1, pg. 103 #3,4,5,6 pg. 105#7
Lesson: Static and Kinetic Friction
Lesson: Friction Problem
73 / Gravity / Orbit, tides, universal gravitation / R+MN 3.1 P#2,3,4,5,9*,10,11 Q#2,9
3.2 P#3,4,5,7,9 Q#1,2
74 / Review / Review: Newton’s Laws
Problems: pg. 107 #1,2,5,
Chpt 2 Rev #2,3,4,7,10,11,12,14,16
Chpt 3 Rev #6-10,14-16
Unit 1 Review # 13,20,22,24,25,26
Representations of Forces and Motion
75 / Test
SPH3U: What is a Force? – Day 65
A: What is force?
Force is a commonly used word in everyday speech. Our goal is to understand the scientific role of force in the workings of our world!
1. Describe what a force is as if you were talking to a friend’s younger brother or sister. Give some suitable examples of forces.
B. Units of Force
Make sure every member of your group tries this activity! You will need: 3 identical elastics, a 20 N spring scale, and a ruler.
1. Loop one elastic around your two pointer fingers. Separate your fingers until the rubber band has a bit of stretch. You have now created your standard unit of force. Describe a method using the ruler that will allow other people to create the same amount of force.
2. Give your unit of force a name (often in honour of its discoverer).
3. Describe the feelings of force experienced by each finger while the elastic is stretched. How do the feelings of each finger compare?
4. Rest your fingers and try it again using two elastics now. Be sure to use the same standard distance. Describe how the sensation of force has changed. Can you estimate how much has it changed? Explain.
5. One day on a hike you find an interesting, heavy rock. Explain how you could use your collection of elastic bands and your method from question B#1 to measure the weight of the rock.
* Check your answers with your teacher before continuing.
C. Measuring Force
Rather than carry around a bag of elastics and ruler, we will use a spring scale to measure the size of forces. Before you get your license to operate a spring scale, you need to know how to calibrate it. Hold the scale horizontally or vertically, just as you will use it when measuring, but without pulling on the hook. Adjust the scale (a sliding cover or nut at the top) so it reads zero. The scale reads in units called newtons.
1. Why is it important to hold it with the same orientation as you would when making your measurement?
2. Create your standard unit of force but replace one finger with a spring scale. What is your unit of force equivalent to in newtons?
3. Make a prediction: When you replace the second finger with a second spring scale (and hold the elastic to its standard length), what will each scale read?
4. Now actually replace the second finger with a second spring scale and note the readings. Offer an explanation for any differences with your prediction and try to explain why the readings are they way they are. (This is related to how strings and ropes work.)
5. In question B#4 you used two elastics and described how the feeling of the force changed. Test this situation with one spring scale. Does the result agree or disagree with the sensation you experienced? Justify your answer.
6. Predict the size of the force three elastics stretched to your standard length would exert. Explain.
7. Measure the force exerted by three elastics. Explain any discrepancies between your prediction and measurement.
8. Describe how you could create your own (and maybe different) markings on the spring scale if all the numbers and lines rubbed off!
9. In this activity we combined two and then three equivalent forces from the elastics and measured the results with the spring scale. Describe a general rule for determining the total effect of a number of equivalent forces acting together. (We will develop this into a very powerful rule!)
SPH3U: What is the Effect of a Force? – Day 66
What happens when a single force acts on an object? This is a tricky question that took very clever people about 2000 years to answer. Now it’s your job to figure it out! (Don’t worry - it won’t take as long this time around.) In all the examples that follow, we will be examining the effect of a single, constant force.
A: The Steady Pull
1. Prediction: Describe how the dynamics cart would move if it experiences a constant horizontal force (a steady push or pull).
2. Get a dynamics cart, some masses, an elastic and ruler. Determine a technique that will allow you to exert a constant force on the dynamics cart using these materials. You should be able to do this for an interval of about four seconds (this is why the masses may be necessary). Take your time doing this. (Hint: Think back to the activity from last class.)
3. Describe what a skeptical person would observe that should convince them the cart is experiencing a steady force.
* Practice your technique until you are good at it! Demonstrate this for your teacher before going on.
4. Prediction: Revise your prediction from question A#1, if necessary, based on what you have observed. Predict what the v-t graph and tickertape dot pattern for the dynamics cart might look like.
Get a tickertape timer and attach one metre of tickertape to the cart. Practice pulling your cart with a steady force through the timer a few times before turning the timer on!
5. Examine the pattern of dots on the ticker-tape. Why might the dots near the very beginning and end be less reliable?
6. Draw a sample of the ticker-tape dot pattern below, only while the cart experienced a constant force. Describe the motion. Explain how this data supports or contradicts your prediction from question A#4.
B: Release the Cart!
Pull the cart along the floor, allow it to speed up and then release it. All of the questions in part B will be about the cart’s motion after it is released.
1. Describe the motion of the cart after it has been released.
2. A student says, “Once the cart is released, it comes to rest on its own. There are no longer any forces acting on it – it just slows down by itself because we’re not pushing on it anymore.” Agree or disagree with the student. Explain your reasoning.
3. Describe how the motion of the cart would be different if we could smooth out the floor to reduce friction a bit.
4. Imagine we very carefully remove all sources of friction. What would we observe in this very special situation after the cart is released? In this situation what horizontal forces are acting on the cart?
C: Conclusions – Forces and Motion
Complete the chart below based on today’s observations and your conclusions. This will become an important rule about the effects of forces!
Situation / Resulting MotionOne single, constant force
No forces at all
(two possibilities!) / 1)
2)
1. Two students discuss today’s activities. Do you agree with either student? Explain.
Student 1: “The cart experiences friction all the time, even during Part A. But we said there was only one force, the one from our elastic. This means our conclusions might be wrong.”
Student 2: “I agree there was friction. We also saw in Part B that it took friction a long time to stop the cart. That means it’s a much smaller force than the pulling force. So maybe we can say that there was only one important force acting on the cart in the first activity.”
2. The two students are still discussing. Do you agree with either student? Explain.
Student 2: “Wait a minute. I bet there were other forces, vertical ones like gravity. Gravity is pretty strong so maybe that will ruin our conclusions. Still no single force!”
Student 1: “Well, we didn’t see the cart move downwards at all, so it looks like gravity is not having an effect on the cart’s motion. So there was only one horizontal force. I think our conclusions are still fine.”
SPH3U: What are the Effects of Many Forces? – Day 67
A car driving down the road experiences many forces at the same time. What happens in such a case? In this activity you will find out. You will need: a dynamics cart, a heavy mass, and three identical spring scales (5 or 10 N). Throughout this activity friction is very small compared to the other forces involved and therefore its effects can be ignored.
A: One Single Force
Place the mass on the dynamics cart and exert a steady 2 N horizontal force using a spring scale.
1. Describe the motion of the cart.
To help us understand the effects of forces, we introduce a new tool called the free-body diagram (FBD). We simplify the object in question by modeling it as a point. Next, we draw a vector arrow, starting at that point, for each force the object experiences. The length of the arrow is not important, but it is helpful if their relative size is roughly correct.
2. Draw a FBD for the cart showing the horizontal forces. Label your force using the vector symbol .
B: Two Forces
Exert two equal forces on the cart, but in opposite directions.
1. Describe the motion of the cart. Record the size of the forces.
2. Draw a FBD showing the horizontal forces. Label the forces and .
3. Describe the total effect of the two forces on the cart.
We can mathematically represent the total effect of the two forces by defining a new quantity called the net force, which is the sum of the forces acting on the object.
4. Fill in the vector equation and find the net force. Be sure to indicate the direction of each force.
______+ ______= ______
An alternative and convenient way (and the one we will use) to find the net force is by changing the vector equation into a scalar equation. Use the sign convention indicated in the diagram above. Forces acting in the positive x-direction are labelled positive and forces acting in the negative x-direction are labelled negative. The values of the force symbols are all positive.
5. Complete the scalar equation below and find the net force.
= ______N - ______N = ______N
When the net force equals zero we say that the forces acting on the object are balanced. In part B the horizontal forces acting on the object are balanced.
* Check your results with your teacher before continuing.
C: Net Force Zero
The example in Part B demonstrated that an object initially at rest that experiences a net force of zero will remain at rest. What will happen to an object that is already moving which experiences a zero net force? This is a class demonstration your teacher will lead.
1. Draw a FBD for the cart your teacher has set up. Note that the strings attached to the weights are pulling on the cart horizontally. Use the force values your teacher gives you.
2. Write an expression for the net force and calculate the result.
Your teacher will start the cart moving and then let go. Once released, the only horizontal forces acting on the cart will be those provided by the strings.