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Name: Brian S.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 2001-2002


Question:
Please email me a reply to this, I am stuck on helping a student.

Ball 1 has a mass of 2kg and is traveling east at 14 m/s on a flat surface. Ball 2 is 9 kg and is at rest. Ball 1 elastically collides with ball 2. After the collision, ball 2 rolls off to the east at 3 m/s.

****I can find the velocity of ball 1 after the collision using conservation of momentum assuming an elastic collision. V1 = 0.5 m/s after the collision

**********
MY QUESTION IS BELOW, CAN FRICTION STOP THIS ROLLING BALL? IF SO, HOW COULD I PROVE IT.

If the coefficient of kinetic friction (m) between ball 2 and the surface is 0.25, how long after the collision does it take ball 2 to roll to a stop. Hint: find "a" and apply a kinematic equation to find t.

I KNOW I AM IGNORING ROTATIONAL MOTION, BUT HOW WRONG IS THIS if we ignore slipping

I USED

DELTA KE = WORK FRICTION

1/2 M V^2 = (MU) M g D D = distance

THUS, D = (V^2)/ (2 mu g)

then use

Vf^2 = Vo^2 + 2 a D and solve for a -then use

Vf = Vo + at

to find time (t)


Replies:
Brian S.,

First, conservation of momentum applies regardless of whether the collision is elastic. "Elastic" refers to no loss of kinetic energy in the collision. If you want to PROVE that friction is the force causing the slowing to occur, you must define what friction is. After doing so, you must vary factors that contribute to the friction, showing that these factors have the greatest effect on distance travelled. Try rougher surfaces vs. smoother surfaces. If friction is the cause, the rougher surface should slow sooner. Try surfaces that are known to have large frictional coefficients vs surfaces with small coefficients. If air resistance is the major factor, such variations shouldn't matter much. If friction is the most significant force, these variations should make a big difference. You can check the importance of air resistance with a small desk fan. Place the fan such that it blows lightly against the ball. Does this greatly affect the time and distance over which the ball comes to a stop? Try air speeds comparable to the speed of the ball. Air speed can be estimated by dropping very light pieces of fluff into the air stream. Horizontal distance divided by time of fall is a good estimate of air speed.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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