N1L and N3L SDI Pt 2

With the glider initially at rest in the lab frame, give the glider a tap (an "impulsive force," i.e., a force acting only over a very small time interval ∆t) with your hand in a horizontal direction such that the glider slides over the air track. Sketch the glider over the air track at 3 positions while in motion after the glider has left your hand: near the start, middle, and end of its horizontal motion. Show all the F and v vectors in the snapshot sketches below.

FORCES ON AND VELOCITIES OF A GLIDER TAPPED AFTER INITIALLY BEING AT REST

Suppose you were to give the glider a push so that it moves along a straight line with a nearly constant velocity v . Can you propose a good method to continuously and uniformly slow the glider down to nearly zero velocity? ("Continuously and uniformly slow the glider down" means that the velocity of the glider decreases continuously by equal amounts in equal intervals of time, i.e., the acceleration is constant) {Y, N, U, NOT} The snapshot sketches below show the glider while in motion after the initial push: at times near the start, middle, and end of its uniformly decreasing-velocity motion. Show all the F and v vectors in the snapshot sketches below. Justify your above answer by indicating your method in the sketches.

PROPOSED METHOD FOR CONTINUOUSLY AND UNIFORMLY SLOWING THE GLIDER DOWN

Suppose the glider is initially stationary. Can you propose a good method to continuously and uniformly speed the glider up to some relatively high velocity? ("Continuously and uniformly speed the glider up" means that the velocity of the glider increases continuously by equal amounts in equal intervals of time, i.e., the acceleration is constant) {Y, N, U, NOT} (You may or may not wish to reorient the V-H axes shown in the drawing.) Show all the F and v vectors in the snapshot sketches below. Justify your above answer by indicating your method in the sketches.

PROPOSED METHOD FOR CONTINUOUSLY AND UNIFORMLY SPEEDING THE GLIDER UP

1. Suppose that you were to hitch a flea to a glider which is initially at rest. Could the flea continuously increase the velocity of the glider up to some value easily visible to the unaided eye? {Y, N, U, NOT} (Your lab instructor will take bets.)

a. In considering this problem would you be justified in ignoring the small frictional force f _{on glider by gas}? {Y, N, U, NOT}. [HINT: Pretend you are a flea trying to accelerate a glider. Would the frictional force be of any concern to you? {Y, N, U, NOT}]

b. Assume the glider is initially at rest. Can you figure out what physical parameters would determine whether or not the flea could accelerate the glider? {Y, N, U, NOT}

HINT #1: Sketch a thought experiment in which the flea is on the verge of accelerating the glider. HINT #2: Ignore m_harness, since m_harness<< m_glider. Then the force F_{on harness by flea} is just equal to the tension T in the harness and F_{on glider by harness} = T = F_{on harness by flea}.

FLEA IS ON THE VERGE OF ACCELERATING THE GLIDER

2. Try the above experiment and use force-motion-vector snapshot sketches to indicate the results in the space on the next page. Show the harness and the flea. Do your results agree with your prediction? {Y, N, U, NOT} (If you can’t find a flea or have trouble making a harness, then conduct and sketch a "thought experiment.")

CHECK ONE:

A FLEA DOES ______ , DOES NOT____ ACCELERATE AN INITIALLY STATIONARY GLIDER

1. Can a truck driver ‘pin’ (stuck) kids to their seats by driving his truck at very high constant velocity v as suggested in the cartoon above? {Y, N, U, NOT} [HINT: It may help to consider your earlier experience, Sec. III-D, in carrying a disk across a room at constant speed in a straight line.]

2. The schematic diagram below shows a kid in her seat in a truck. Three successive positions of the kid and seat are shown in the Earth’s frame of reference, as the truck moves forward at very high constant horizontal velocity v . Draw ALL the force vectors acting on the kid at these 3 positions. Show the velocity vectors in the 3 positions if they exist.

FORCES ON A KID SITTING IN A TRUCK MOVING AT A HIGH CONSTANT VELOCITY

A. HOLD TWO MAGNETS.

Hold two permanent magnets, one in each hand and as far apart as possible. Slowly bring the two magnets together. Play around so as to determine magnet orientations for maximum attraction and magnet positions so close (a millimeter or so) that you can barely keep them from touching.

In order to understand the physics of this situation, it is essential that you draw a LARGE LABELED DIAGRAM. In the space below draw a "FRONT VIEW" ( i.e., the view of an observer whose line of sight is horizontal) of both magnets [labeled "#1"(solid lines) and " #2"(dashed lines)] in this maximum attraction position. For simplicity orient the magnets so that the magnetic force (call it M) between them is horizontal. Draw the fingers of the hand holding magnet #1.

According to N1 and N2, the motion of magnet #1 depends on ALL the forces acting ONLY on magnet #1. Therefore, show ALL the force vectors acting ONLY on this magnet #1 (don’t forget the subscript designation "on #1 by________").

Hint #1: To assist your concentration in applying N1 to magnet #1, color ONLY that body yellow.

Hint #2: Don’t forget the reference axis V-H!

Hint #3: It’s useful to consider separately the vertical and horizontal components of the forces.

Hint #4: Although your fingers may pinch the magnet with relatively large opposing forces, it’s simplest to only show the unbalanced forces F _{net vertical on magnet #1 by hand} (call this simply N _{on #1 by hand}) and F _{net horizontal on magnet #1 by hand} (call this simply f _{on #1 by hand}) .

FORCES ON MAGNET #1 HELD VERY CLOSE TO ATTRACTING MAGNET #2