Forces and Motion online

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University of Arkansas, Little Rock *

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5380

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Physics

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Apr 3, 2024

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Physics I Laboratory Force and Motion 1. Objective  Investigate the relationship between a Push (Force) and how an object changes its motion (acceleration), and  To use this knowledge to study a system. 2. Apparatus For parts I and II of this lab you will need a stopwatch (a smart phone works wonderfully) and the PhET simulation Forces and Motion (you may download it or run it online) : https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion- basics_en.html 3. Theory Newtons’ Laws describes the relationships between forces and objects. Newton’s 1 st Law of Motion: An object at rest will remain at rest or an object in motion will remain in constant motion (same speed and same direction) unless acted on by a net force. This describes what happens in the absence of forces. Motion AT constant velocity (even ⃗ v = 0 ) is the natural state of things. The property of an object that leads to this is called inertia . Mass is the measure of inertia. More mass means it is more difficult to change the motion of that object. This is why a dump truck is harder to make move than a toy car. Newton’s 2 nd Law of Motion: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. commonly written : ∑ ⃗ F = ¿ ⃗ F net ¿ m∙ ⃗ a This law describes what happens in the presence of forces. A net force causes objects to change the way they are moving, thus a change in velocity ( ⃗ a≡ ∆ ⃗ v ∆t ) requires a force. A change in velocity can be accomplished by either a change in speed or a change in direction. We will only investigate the change in speed in this lab. Newton’s 3 rd Law of Motion: For every action, there is an equal and opposite reaction.

If object 1 and object 2 interact, the force exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force exerted by object 2 on object 1. All Forces occur in pairs, thus a single isolated force can not exist. 4. Procedures I. Forces and the Resulting Motion Begin by launching the PhET Simulation Forces and Motion, https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion- basics_en.html . Once running you will land on the home screen with four selection possibilities, NET FORCE, MOTION, FRICTION, and ACCELERATION. Feel free to explore the NET FORCE portion to understand the results of net force on the motion of an object. We will not be using that portion for this lab. We will begin with the ACCERLERATION selection. In this selection, we will push a variety of objects to investigate the effects of our push and use the motion to determine properties of the system, namely acceleration and mass of the pushed object. Once opened you will see a box in the upper right side of the screen. For your convenience, select all 6 boxes. These will show values and directions of the speeds , forces, masses, and acceleration. Move the Friction slider to “None”. Just below this box is a pause button, hit it. This will prevent the simulation from running as you make your other selections. Next to this is the reset button. Be warned this resets everything. What begins in the simulation is your avatar standing next to a wooden crate. Below that you will see a selector for setting the force of your push. You will select a force, then hit play. At the same time you will begin a stopwatch and time how long it takes for the box to reach 40 m/s. As an example, I set the force to 250 N. Upon hitting play, I started a timer and the crate wen from 0 m/s to 40 m/s, 8.15 s later. Try this. This gives an acceleration of ⃗ a≡ ∆ ⃗ v ∆t = 40 8.15 = 4.91 m s 2 . Since we applied a force of 250 N to a 50 kg mass, we should obtain an acceleration of 5.0 m s 2 . This gives a 2% error for our acceleration. I could also use this acceleration and force to determine the mass that was pushed. In the same acceleration screen, reset your screen. Reset your selections, move friction to None, and pause the simulation. Remove the crate from the runway by dragging it down. Drag the refrigerator (200 kg) from the bottom left to the runway. Select an applied force to push on the appliance. Indicate the applied force below. applied Force = ____________ 200 N ______________________

Based on this applied force and the mass of the object, what should the acceleration be? Show your calculation. Note, when running your simulation the theoretical acceleration will be displayed on the screen. ⃗ a≡ ∆ ⃗ v ∆t = 40 40 = 1.00 m s 2 theoretical acceleration = ____________ 1.00 m/s^2 ________________ You will measure the time for your push to give the object a speed of 40 m/s. Start a stopwatch as you begin the simulation and stop when the object reaches 40 m/s. Record your time below. time to 40 m/s =___________ 39.9 ______________________ Using this time and the change in speed, determine the acceleration of your object. Show your calculation below. ⃗ a≡ ∆ ⃗ v ∆t = 40 39.9 = 1.00 m s 2 acceleration = ___________ 1.00 m/s^2 _____________________ Compare your measured acceleration to your theoretical acceleration using percent error. You may try various other settings and objects to try. You need not report any other settings you investigate here. Percent error – 0% 1.00 − 1.00 1.00 = 0 1.00 = 0% II. Determining Mass For this investigation we will use the MOTION screen. Hit the home selector near the bottom of the simulation screen and enter the MOTION screen. Select the boxes in the top right of the screen and hit pause. Drag the crate off the runway and put another object on the runway (except the mystery box). Select an applied force to apply to your object. Indicate the object and the applied force below. Object______ 80 kg man ______________, Applied Force_________ 150 n _______________

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Run the simulation, recording the time for your object to reach 40 m/s. Record this below Time = _________ 21.50 s __________________ Use this time and change of speed to calculate the objects acceleration. Show your work and record below. ⃗ a≡ ∆ ⃗ v ∆t = 40 21.50 = 1.86 m s 2 Acceleration of object_____________ 1.86 _______________________ Using the applied force and the calculated acceleration, determine the mass of your object. Record this below. Show your work here. 150 = m x 1.86 m = 80.6 Object Mass_________________ 80.6 ___________________________ Compare the calculated mass to the mass given for your object and calculate a percent error. Record this below. 80.6 − 80 80.6 x 100% = 0.007 percent error Next drag your object off the runway and put the mystery box on the runway. Select an applied force to push the box. Hit play and record the time to reach 40 m/s. From this change in speed determine the box’s acceleration. From this acceleration and the applied force, determine the

mass of the mystery box for this trial. For trial 2, select a different applied Force and repeat the measures and calculations. Do this for a total of 5 trials. Trial number Applied Force Time to reach 40 m/s Calculated acceleration Mystery mass 1 400 5.30 7.55 52.98 2 300 7.04 5.68 52.82 3 200 9.93 4.03 49.63 4 150 13.36 2.99 50.17 5 50 40.04 1.00 50 Determine the average Mass, standard deviation of the mass measures, and the error in the mean mass. Indicate those below. Average Mass = ______ 51.12 _____________ Standard Deviation = _______ 1.64 ____________ Error in the Mean = _______ 0.73 ______________ Average Mass (52.98+52.82+49.63+50.17+50) / 5 = 51.12 Standard Deviation (52.98-51.12)^2 + (50-51.12)^2 / 5-1 = (10.7266/4) = √ 2.68165 = 1.64 Error in the Mean 1.64 / √ 5 = 0.73

Forces and Motion online

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