### Newton's laws

In this laboratory you will perform a series of experiments that demonstrate Newton's laws of motion.   For your laboratory report, you should answer all questions (in blue).

Open a Microsoft Word document and keep a log.  This log will form the basis of your lab report.

Newton’s 1st 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.

 What do 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.

 What do 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:

Step through 4 video clips frame by frame.  The clips show a cart on a track.  An acceleration sensor and a force sensor are attached to the cart.  A force is applied to the force sensor by a falling weight, and the computer screen displays the output of the force and acceleration sensors as the cart accelerates.  The pulling fore measured by the force sensor is approximately equal to the net force acting friction cart.

Open Microsoft Excel and record the average readings of the force and acceleration sensors in a table.

 a (m/s2) F (N)

Produce a graph of force versus acceleration.

 Give the graph a title and label the axes. The label for the x-axis should be "a (m/s2)", and the label for the y-axis should be "F (N)".

Refer to your graph and describe the relationship between force and acceleration using words.

Right-click your data and choose "Add Trendline".  Choose "Type, Linear" and "Options, Display equation on chart".  An equation y = ax + b will appear on your graph, where the number a is the slope and the number b is the y-intercept.  Paste the graph with the trendline into your Word document.   What is the physical meaning of the slope?

Write down Newton’s 2nd law in the form of an equation.  Define any variables and/or constants.  What is your best estimate for the mass of the cart and the sensors in the video clips?

Experiment 3:

The old elevator in the Nielsen Physics Building traveled a vertical distance of 14.6 m from the second floor to the sixth floor.

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

 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 C3 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.  Give you graphs names and label the axes, with units.  Paste these graphs into your log.

Discuss what these graphs tell you.  Explain in detail how to relate the information in the graphs to your own experience riding elevators.