* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Name(s) Hr. ____ Investigating Newton`s Second Law by Pulling a
Survey
Document related concepts
Classical mechanics wikipedia , lookup
Fictitious force wikipedia , lookup
Equations of motion wikipedia , lookup
Jerk (physics) wikipedia , lookup
Rigid body dynamics wikipedia , lookup
Newton's theorem of revolving orbits wikipedia , lookup
Work (physics) wikipedia , lookup
Mass versus weight wikipedia , lookup
Centripetal force wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Transcript
Name(s) ________________________________________________ Hr. ____ Investigating Newton's Second Law by Pulling a Wagon Background: Isaac Newton comes up with three laws (or guidelines) of motion. These laws integrate (or bring together) the concept of the force (push or pull) with our concepts of motion. The first law states that to change the state of motion of an object, we need to place an unbalanced force on that object. To get an object to move, we need to apply a force, etc. Newton's second law states that if we have a set of forces on an object that is unbalanced, the object will accelerate [Fnet = ma (not your dad, but your...)]. Newton's third law is action/reaction. In this activity, we will try to test Newton's second law by pulling a wagon. I've created a movie and would like you to do the same. My movie is located on the web here: However, to make things work out right we have to make a careful video. Here are important things to keep in mind when making this video: film from left to right attach a rope or string tightly to the handle of the wagon to this handle attach the hook of the spring scale (luggage scale). You will pull on the handle of the luggage scale. start pulling the wagon (and person) and keep the pull constant. This will be difficult but try your best. By keeping the pull constant, you should feel like the wagon is getting faster and faster. Watch your dial and TRY TO KEEP the reading constant. MAKE SURE THAT YOU REMEMBER WHAT YOUR DIAL said as you pulled. move your camera back so you can film about 5 meters and keep the camera very still as you film scale the video using a meterstick (place it on the wagon with the holder) or scale from the wagon itself. VERY important: keep the handle and string as horizontal as possible! Investigating Newton's Second Law by Pulling a Wagon, p. 1 Force Analysis (Force diagram): We have four basic forces acting on the wagon and rider. I pull on the string which is causing tension in the rope. This tension is the forward force. However, friction is acting against my tension. The wagon and rider have weight (downward) and the floor supports the wagon and rider (normal force): FRICTION TENSION Normal Force Weight If you pull hard and consistently, the TENSION that pulls the rider and wagon forward should be a bit larger than the FRICTION. We have an UNBALANCED set of forces horizontally. Since our horizontal forces are unbalanced, we should see the wagon and rider accelerate or get faster. Vertically, the wagon is not moving. Its acceleration is zero and thus the WEIGHT and NORMAL force are equal in value but opposite in direction. These two forces are balanced. KEY IDEAS integrating forces and motion: Unbalanced forces mean the object will accelerate. This is what is happening horizontally to the wagon. When forces are balanced, we have no acceleration (v = 0 or v = constant). This is what is happening vertically to the wagon. Investigating Newton's Second Law by Pulling a Wagon, p. 2 PROCEDURE: 1. BEFORE you create your video, we need to determine the friction that will be present when you pull the wagon and occupant. a) Make sure you have the wagon wheels straight. b) Attach your rope or string to the handle and then the hook of the luggage spring scale to the string. Make sure you can see the dial of the luggage spring scale. c) Make sure the black needle of the luggage spring scale is reset back to zero. d) Slowly pull the wagon using the handle of the luggage spring scale attached to the string. PULL horizontally (parallel to floor) until the wagon starts to move. e) The black needle will move to a reading that indicates the pull force needed to start motion. This will represent the friction that you need to overcome to activate motion. f) Write down the number that the black needle is showing. Use the outside numbers on the dial. This will be in pounds. Friction = ______________________ lbs g) convert this to Newtons by multiplying by 4.45 Newtons (4.45 Newtons per lb) by your previous number Friction = ______________________ Newtons Investigating Newton's Second Law by Pulling a Wagon, p. 3 2. Create your video but read the following steps first. a) make sure your wheels are straight before you begin. b) You need to pull HARDER than the number you pulled on earlier (step 1, letter f). c) Keep this pull constant to the best of your ability. The puller needs to remember what the dial said when you pulled (numbers on the outside of the dial). d) Keep the pull horizontal (very important!) e) make sure you film left to right f) make sure the camera is far enough back to get 5 meters or so of motion. g) make sure the wagon is NOT moving when you start filming. h) the wagon and rider should get faster and faster if you are pulling hard. i) hold the camera really still 3. Before you analyze your video, let’s write down your numbers and make some calculations. 4. Write down your pull force that you pulled on the video. This will be equal to the tension created in the string. It will be in lbs. a) Tension (pull force) = ______________________ lbs Now convert to Newtons: (x 4.45 N/lb to get Newtons): b) Tension (pull force) = ______________________ Newtons 5. Get the weight of the rider in lbs (step on our bathroom scale) and add 20 lbs for the wagon: a) Weight of rider AND wagon = ________________________ lbs x 4.45 Newtons/lb to get Newtons. b) Weight of rider AND wagon = ________________________ Newtons Now get the mass of the rider by dividing the previous answer by 9.8 (Weight/g = m): c) Mass of the rider AND wagon = ________________________ kg Investigating Newton's Second Law by Pulling a Wagon, p. 4 6. Newton's Second Law states that [Fnet = ma (not your dad, but your...)]. FRICTION TENSION Our Fnet would be our (Tension – Friction). Friction is negative as it acts to oppose the tension force. So our equation would be: Fnet = ma OR Tension – Friction = m(a) where m is the mass (in kilograms) and acceleration is a. We are going to solve this for the acceleration. Our Tension and Friction must be given in Newtons. The acceleration will be in m/s/s. The tension is what we you found in step 4b The friction is what you found in step 1g The mass is what you found in step 5c ________________Newtons - ________________ Newtons = ___________ kg ∙(a) FIRST, subtract the left side and then divide by your mass to get your acceleration. Acceleration = _________________ m/s/s (Newtons divided by kg = m/s/s) Now let’s find the acceleration using Logger Pro. We will compare the acceleration we get on Logger Pro with the acceleration determined on the previous line with Newton’s 2nd Law. 7. Open up logger pro and insert your movie. 8. When the movie opens, right click, and activate “First VA point…” I also advanced the movie ’10’ frames. Investigating Newton's Second Law by Pulling a Wagon, p. 5 9. Scale the movie. I didn’t have a meterstick that day so I used the length of the sitting part of the wagon (0.95 meters): 10. Now fast forward to the part of the movie where the wagon is just about ready to move. Start putting in your points. I got about 15 points (1 point per 10 frames). The spacing of the points should grow, meaning that the wagon is getting faster! 11. You will see x and y data on your graph. Choose the x data. Notice we get a parabola. This means that the wagon is accelerating! Investigating Newton's Second Law by Pulling a Wagon, p. 6 12. Now choose the X-velocity data. This should look linear, at least part of it should be nicely linear: Remember to get the acceleration, we need to get the slope. Highlight the linear part and do a linear curve fit. Write down the slope below Acceleration = _____________________________________ m/s/s ……………………………………………………………………………………………….. Go to the next page…please Investigating Newton's Second Law by Pulling a Wagon, p. 7 13. So, we first got our acceleration using Newton’s second Law (force analysis): FRICTION TENSION Fnet = ma Tension – Friction = m(a) Your acceleration found this way is on PAGE 5, step 6. Rewrite it here: Acceleration = _________________ m/s/s (Newtons divided by kg = m/s/s) The second way to get the acceleration is through logger pro (video analysis). We did that on the previous page. Rewrite it here: Acceleration = _____________________________________ m/s/s Both of these numbers should be close as you are finding the same concept (acceleration) but rely on different methods. Were your two accelerations close? ______________________ …………………………………………………………………….. Summary, PLEASE READ: Newton’s second law deals with forces. When we pull the wagon, we have two horizontal forces: your pull force (creating a tension in the string) and friction. Friction works against your pull force. We can subtract these two forces to get the net force. However, Newton’s second law is expressed: Fnet = ma So, if we know our net force, we can then divide by the mass of what we were pulling (wagon and rider) to determine the acceleration. KEY IDEAS integrating forces and motion: Unbalanced forces (Fnet ≠ 0) mean the object will accelerate, either getting faster or getting slower. When forces are balanced (Fnet = 0), we have no acceleration (v = 0 or v = constant). Investigating Newton's Second Law by Pulling a Wagon, p. 8