Physics Laboratory 4

Newton's laws

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

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 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 Pasco wireless force and acceleration sensor is 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 force measured by the force sensor is approximately equal to the net force acting on the cart.

Open Microsoft Excel and record the average readings of the force (N) and acceleration (m/s2) sensors in a table. (The readings may vary a little from frame to frame. Find a reasonable average.))

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)".

Paste your graph into your log.  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, Set Intercept 0, Display equation on chart".  An equation y = ax will appear on your graph, where the number a is the slope.  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

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.

• 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 what these graphs tell you.  Explain in detail how to relate the information in the graphs to a ride in an elevator.