Newton's laws

In this laboratory, you will perform a series of experiments that demonstrate Newton's laws of motion.

• Tennis ball
• Track
• Two wireless smart carts
• Electronic balance

Open a Microsoft Word document to keep a log of your procedures and your results.  This log will form the basis of your lab report.  Address the points highlighted in blue.  Answer all questions.

Newton's 1^st Law

"When viewed in an inertial reference frame, an object at rest remains at rest and an object in motion continues in motion with constant velocity unless it is acted on by an external net force."

Assume that you sitting in your stopped car with your seatbelt fastened waiting for a green light.  Another car suddenly hits your car from behind.  After recovering from the surprise, you notice a pain in your head and neck.

• Discuss with your partners what you think happens to the head of a buckled-up driver when the car is hit from behind.

Now assume you are a passenger in a moving car and this car hits the back of a stopped car.

• Discuss what you think happens to the head of a buckled-up passenger in a moving car when the car hits a stopped car.

Experiment 1

Place a ball on a book that you hold out in front of you like a tray with one hand.  Record what happens to the ball when you conduct the following three experiments.

• From rest, walk quickly forward.
• From rest, walk quickly backwards.
• Walk forward at a steady pace, keeping the ball on the book with your other hand.  Let go of the ball while walking steadily.  Then stop suddenly.

Are your observations consistent with Newton's first law?  Discuss!

Reconsider the situation where a stopped car is hit from behind by a moving car. 

• Using Newton's First Law, predict what should happen to the head of the buckled-up driver in the stopped car.  Where should the brain trauma occur in this type of accident?
• Using Newton's First Law, predict what should happen to the head of the buckled-up passenger in the moving car.  Where should the brain trauma occur in this type of accident?

Newton's 2nd law

Experiment 2

You will use the Pasco Wireless Smart Cart to make measurements.  The Smart Cart is a cart equipped with a position decoder, a force sensor, an accelerometer and a gyroscope.  The force sensor measures the force along the rolling direction that is applied to the place where the hook attaches.  The 3-axis accelerometer measures x, y, z components of acceleration of the cart.

You will produce a graph of the horizontal force applied to the cart versus the horizontal acceleration of the cart.  For this experiment, you will use a red Smart Cart with a hook attached.  Close the Hardware Setup window.

• Press the power button on the side of the Smart Cart to turn it on.
• In Capstone, click Hardware Setup.  Under available devices, you should see your Smart Cart.  If you see more than one cart, refer to the label your cart to add the correct one.  Choose to measure Force (N) and  Acceleration - x, (m/s²).
• Create a Graph Display by dragging the Graph icon onto the main display.  Select Force, (N) for the y-axis and Acceleration - x, (m/s²) for the x-axis.
• Place the cart in the middle of your track.  Make sure the track is level and the cart stays at rest.
• In the sampling control panel, press the "Zero Sensor" button for each sensor.  (Always zero a the sensors with the cart at rest before a measurement.)
• Before you collect data, practice rolling the cart in a forwards and backwards motion by only holding on to the hook. You want to apply a force along the cart's x-axis, and have the cart roll only along this direction.  Try not to wiggle or knock the Smart Cart hook as this will result in extraneous data points.
• Press the record data button.
• Holding only the hook, roll the Smart Cart forwards and backwards in the x-direction.
• Repeat this motion at least 4 times to generate enough data points.
• Stop data collection.
• Turn on the "Linear Fit" tool.  The software will calculate the equation for the best-fitting straight line, F = ma_x + b, to the data points and display the fitting parameters on the screen.

• Copy your graph and paste it into your log.
• Refer to your graph and describe the relationship between force and acceleration using words.
• What value did you obtain for the mass of the cart from this measurement?

• Add two metal blocks to your cart and repeat data collection and analysis for this new system.
  Paste your new graph into your log.  What value did you obtain from this measurement for the mass of this system?

Use a digital balance and weigh the cart with and without the metal blocks.  The scale is calibrated in kg, so it displays the mass using weight= mg.

• Compare the masses obtained from your force and acceleration measurements with those obtained using the scale. 
• Comment on your results.  Are they reasonable?  What are possible sources of error?  What do you think?

Activity

An elevator ride

The acceleration of the old elevator in the Nielsen Physics Building was measured as it traveled from the second to the sixth floor, starting from rest.  The data were taken using the acceleration sensor in a cell phone.

Open the linked Excel Spreadsheet.

• Produce a Graph of acceleration versus time.  The phone recorded a data point every 0.15 s.
  Paste this graph into your log.
• Use ∆v = a*∆t to find the velocity of the elevator as a function of time. 
  Into cell C3 type "=C2+B2*0.15".  Copy the formula into the other cells of column C. 
• Use ∆y = v*∆t to find the position of the elevator as a function of time. 
  Into cell D3 type "=D2+C2*0.15".  Copy the formula into the other cells of column D. 
• Produce a graph of velocity versus time and a graph of position versus time.  Paste these graphs into your log.

Discuss with your partners what these graphs tell you.  Explain in detail how to relate the information in the graphs to a ride in an elevator.

Newton's 3rd Law

Experiment 3

You will use two carts with force sensors. 

• Delete all your data runs.
• Turn on the blue Smart Cart.  Add it to your devices under Hardware Setup.  Choose to measure Force (N).
• Add a plot to your graph.  For the x axis of both plots choose time.  For the y axis of one plot choose Force, Red (N) and for the other plot choose Force, Blue (N).
• Connect the force sensor hooks with two rubber bands.
  
• Zero both force sensors.
• Start taking data.
• Pull the carts apart with varying amounts of force.  You can hold one cart fixed and pull on the other cart, or you can pull on both carts.  Do not pull too hard to avoid breaking something.  The force sensors read a positive force when pulled and a negative force when pushed.  The magnitude of the maximum force they can tolerate is 100 N.
• Paste your graph into your log.  Compare the two plots.  Discuss your observations with your partner.  Interpret your results in terms of Newton's third law.
• Start a new data run.  Push the carts together with varying amounts of force, hook against hook..
• Paste your graph into your log.  Compare the two plots.  Discuss your observations with your partner.  Interpret your results in terms of Newton's third law.

Before you leave, please plug the carts into USB ports of a computer to keep them fully charged for the next group of students.

Convert your log into a lab report.

Name:
E-mail address:

Laboratory 2 Report

• In one or two sentences, state the goal of this lab.
• Make sure you completed the entire lab and answered all parts.  Make sure you show your work and inserted and properly labeled relevant tables and plots.
• Add a reflection at the end of your report in a short essay format.

Save your Word document (your name_lab2.docx), go to Canvas, Assignments, Lab 2, and submit your document.