Studio Session 4

Motion under the influence of a constant force

Newton's second law, F = ma, predicts the future motion of an object if we know its initial motion and the net force that is acting on the object.  Today you will explore this motion when the net force is approximately constant.

Equipment needed:

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


Free-fall - The acceleration of gravity

Gravity is the force of nature we are most aware of.  One can argue that other forces, such as the electromagnetic force, which holds molecules together in solid objects, or nuclear forces, which determine the structure of atoms, are more important, but these forces are less obvious to us.  Near the surface of Earth the force of gravity on an object of mass m equals Fg = mg.  It is constant and points straight down.  If we can neglect other forces and the net force is approximately equal to Fg, then we have motion with constant acceleration g.

Observation

Hold a tennis ball at about your height and then let go.  Observe the motion of the ball.

Describe the motion as the ball is falling.

Experiment 1

It does not take the ball a long time to reach the floor.  It is hard to get detailed information about its motion without using external measuring instruments.  In this experiment the instrument is a video camera.  You will analyze a video clip.  The clip shows a person dropping a ball.  You will determine the position of the freely-falling ball as a function of time by stepping through the video clip frame-by-frame and by reading the time and the position coordinates of the ball off each frame.  You will construct a spreadsheet with columns for time and position and use this spreadsheet to find the velocity as a function of time.  The slope of a velocity versus time graph yields the acceleration of the ball.

Procedure:

To play the video clip or to step through it frame-by-frame click the "Begin" button.  The "Video Analysis" web page will open.  Choose one of the ball_x.mp4 video clips.

Begin

Produce a graph of position versus time.  Label the axes.

Let us find the velocity of the ball as a function of time.  We find vy =  ∆y/∆t by dividing the difference in successive position by the difference in the times the ball was at those positions.

Produce a graph of velocity versus time. Label the axes.

For an object accelerating at a constant rate g we have y = y0 + v0t + ˝gt2, so y as a function of t is a polynomial of order 2 (a section of a parabola).  We can reduce numerical errors in finding the acceleration of the ball by fitting our position versus time data directly with a polynomial of order2.


Projectile motion

Experiment 2

Projectile motion is motion in two dimensions under the influence of a constant force.  It is hard to get detailed information about its motion without using external measuring instruments.  In this experiment the instrument is a video camera.  You will analyze a video clip.  The clip shows a person throwing a ball.  You will determine the position of the ball in two dimensions as a function of time by stepping through the video clip frame-by-frame and by reading the time and the position coordinates of the ball off each frame.  You will construct a spreadsheet with columns for time and position and use this spreadsheet to find the x and y component of the velocity as a function of time.

Procedure:

To play the video clip or to step through it frame-by-frame click the "Begin" button.  The "Video Analysis" web page will open.  Choose one of the throw_x.avi video clips.

Begin

Produce graphs of the x and y components of position versus time.  Label the axes.

Paste your graphs with trendlines into your log and record your answers and questions you have in your log.


Activity

With the help of your instructors you will practice solving physics problems concerning the topics listed below.


Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.