# Problem Set V Solutions

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1 Problem Set V Solutions. Consider masses m, m 2, m 3 at x, x 2, x 3. Find X, the C coordinate by finding X 2, the C of mass of and 2, and combining it with m 3. Show this is gives the same result as 3 i= X = m ix i 3 i= m. i The center of mass for masses and 2 is given by The center of mass for all three masses is given by as we would expect. X 2 = m x + m 2 x 2 m + m 2. X = (m + m 2 )X 2 + m 3 x 3 m + m 2 + m 3 = (m + m 2 ) mx+m2x2 m +m 2 + m 3 x 3 m + m 2 + m 3 = m x + m 2 x 2 + m 3 x 3 m + m 2 + m 3 2. Consider a square of mass 4 kg, side 2m, negligible thickness, with its sides oriented along the usual axes with its center at (0, 0). (i) Determine its C using symmetry arguments. (ii) Imagine that the m m part of it in the fourth quadrant is chopped off. Where is the new C? Do this using the extension of result in previous problem. Repeat using the following trick: view the chopped off shape as the full square plus a m m square of negative mass kg in the fourth quadrant. (iii) A disk of radius R centered at the origin has a circular hole of radius R/2 centered at (x = R/2, y = 0). Where is its C? (i) The center of mass is located in the middle of the square. This must be the case because flipping the square around the x or y axis does not alter the square, so the center of mass cannot move during these rotations. (ii) Using the method from problem, we can consider this shape as three kg squares, one in each quadrent. Taking m i to be the square in quadrant i, X2 will be (x = 0, y = 0.5 m). Then, the x coordinate for the center of mass will be and the y coordinate will be X = Y = 2 kg(0) + kg( 0.5 m) 3 kg 2 kg(0.5 m) + kg( 0.5 m) 3 kg = 6 m = 6 m. So, the center of mass is at X = ( /6 m, /6 m). For the second method, we can think of the square without the piece in the fourth quadrant as a mass of = 4kg at (0, 0) plus a mass of kg = /4 at (0.5 m, -0.5 m). The x coordinate of the center of mass is ()(0) + ( /4)(0.5m) x cm = /4 ( ) ( ) ( ) 4 = m = 6 m.

2 2 Similarly, the y coordinate is y cm = ()(0) + ( /4)( 0.5m) /4 = 6 m. As above, the center of mass of the square missing the piece in the fourth quadrant is ( /6 m, /6 m). (iii) If the mass of the full disk is, the mass of the missing piece will just be times the ratio of the missing piece to the full piece. missing = π(r/2)2 πr 2 = 4 Following the argument for the square, the center of mass of a circle must be in the middle of the circle. Therefore, we have a mass of at (0,0) and a mass of /4 at (-R/2,0). Since both masses are centered at y = 0, y cm must equal 0. Solving for x cm x cm = (0) + ( /4)( R/2) /4 The center of mass for the circle with a hole in it is at (R/6, 0). 3. Ideal Zorro (mass, no height) swings down on a vine of length L from a height H and grabs a kid of mass m (zero height, standing on the ground) and together they barely reach safety at a height h. Relate H to the other parameters. Give H in meters if L = 40m, = 00kg, h = 6m, m = 30kg. To solve this problem we first need to use conservation of energy, then conservation of momentum, and then conservation of energy again. Ideal Zorro starts at at heigh H. By conservation of energy, when he reaches be kid on the ground, his velocity v will be 2 v2 = gh = v = 2gH. On the ground, Zorro has an inelastic colision with the child, so momentum is conserved. Their combined final velocity v 2 just as they are leaving the ground is = R 6 v = ( + m)v 2 = v 2 = + m v. () Ideal Zorro and the child just reach a final heigh of h, so conserving energy, Using equations and 2 to solve for v, 2 ( + m)v2 2 = ( + m)gh = v 2 = 2gh. (2) v = + m 2gh From above, we also have v = 2gH. Setting these two equations equal and solving for H we find that ( ) 2 + m H = h = 0. m. 4. Two identical stars of mass orbit around their C. Show that T 2 = 2π2 R 3 G where R is the distance between planets. (Draw a figure and keep track of the gravitational force on either star as well as its centripetal acceleration.) (ii) Repeat for two unequal masses m and and show where R is the separation between stars. T 2 = 4π2 R 3 G( + m)

3 3 FIG. : Two stars of unequal mass orbiting about their center of mass. FIG. 2: A cone of heigh h and radius R. The angle a is the same no matter where you slice the cone, so this allows you to find an expression for r in terms of z. (i) For two stars of equal mass a distance R apart, the center of mass will be half way between the two stars. Therefore, the radius of their orbit will be R/2. To keep the stars in orbit, the gravitational acceleration must balance the centripetal acceleration. Also, remember that v = 2πrf = 2πr/T where f is the frequency of rotation and T is the period of rotation. G R F = G R 2 = 2(2πR/2)2 T 2 T 2 = 2π2 R 3 G = v2 R/2 = 2(πR)2 T 2 (ii) For two stars of unequal mass, their center of mass is no longer exactly between the two stars. If you take the star of mass m to be at x = 0 as shown in Figure, X cm is given by X cm = R m +. The radius of star m s orbit will be given by r = X cm, so balancing the gravitational force with the force necessary to maintain a circular orbit and solving for the period, as desired. Gm R 2 G R 2 = G R 2 = = mv2 R m+ ( 2π R m+ T ) 2 m + ( ) 2 2π R T m + T 2 = 4π2 R 3 G(m + ) 5. Consider a massless boat of length L on frictionless water. At the left end is a person P of mass m holding a writhing snake of mass m 3 (Treat snake as rigid point particle. Treat P who is clearly rigid at this point as a point.) At the right end is a person P 2 of mass m 2. At t = 0, P throws the snake towards P 2 at a speed v. (i) What is V, the magnitude of the velocity of the boat and passengers relative to water when snake is airborne? (ii) How long does snake take to reach P 2? (iii) During this time how much has the boat moved to the left? (iv) Locate the C at the end of all this snake throwing and show it is same as at beginning. The initial momentum of the snake, boat, person and person 2 system is 0, so the total momentum of this system will remain 0 throughout the problem. R

4 4 (i) The total momentum is zero, so m 3 v (m + m 2 )V = 0 = V = m 3 m + m 2 v. The boat is moving to the left with a speed V = m3 m +m 2 v relative to the water. (ii) The time it takes the snake to reach P 2 will be the time in which it takes the snake and the boat to go a combined distance of L with the snake moving to the right and the boat moving to the left. where = m + m 2 + m 3. (iii) During this time, the boat has moved a distance L = vt + V t ( ) m 3 L = + vt = t = L(m + m 2 ) m + m 2 v d = V t = m 3 v L(m + m 2 ) m + m 2 v = d = m 3 L or m3 L to the left. (iv) Taking P to be at x = 0 before the snake was thrown, at the beginning of the problem X cm = m 2 L. The simplest way to find the center of mass after the snake is thrown is to again put P at x = 0 and then shift the system by the amount found in (iii). X cm = m 2 + m 3 L m 3 L = m 2 L Since there were no external forces acting on the snake, people, boat system, the center of mass has stayed the same throughout the problem. 6. Find the C of a cone of radius R and height h. (Think in terms of slices of thickness dy at height y.) We know by symmetry that the center of mass of a cone must lie along its axis which runs perpendicular to the base that goes through the point. If we consider a slice of the cone perpendicular to this line, it is a circle with radius r and width dz. Using Figure 2 it is easy to find the radius r in terms in the height z of the slice. We see that h R = z r = r = R z h. The center of mass of each circle is along the axis of the cone, so we consider each circle as a point mass along the axis with mass dm = πr 2 dz/(v olume of Cone). The volume of a cone is πr 2 h/3, so the center of mass is given by Z cm = = = h 0 h 0 h 0 = 3 4 h zdm z π(r z h )2 πr 2 h/3 dz 3z z2 h 3 dz This says the center of mass is 3/4 of the way from the top to the base, or a distance h/4 from the base.

5 7. A person of mass = 32.5kg on ice disdainfully throws my quantum text book weighing m = 2.25kg at v b = 2m/s. The book is thrown from zero height and the total distance between the book and the offender is 5.2m when the book lands. At what angle was this excellent book thrown? How fast is the offender moving? Since there are no external forces in the horizontal direction on the person-book system, the x component of the center of mass does not move while the book is in the air. We can use this fact to find how far the book traveled before it hit the ground. X cm = 0 = x b m + (x b x) = x b = x m + where x = 5.2 m is the total final separation of the book and the person. Plugging in numbers, x b = 4.2 m. Take the book to be thrown at and angle α with respect to the horizontal. Then the book s initial velocity in the x direction is v b cos α and in the y direction is v b sin α. In the x direction we know that x b = v b t cos α = t = x b v b cos α. In the y direction, the book starts and ends and the same height so, Plugging in for t, we find 0 = v b t sin α 2 gt2 = v b sin α = 2 gt. 5 (3) sin 2α = gx b 2v 2 b = α = Taking the person s final velocity to be v, we know from conservation of momentum that v = mv b cos α = v = m v b cos α. Plugging in, we find v = 0.66m/s. So, the book is thrown at an angle, α = and the offender has a final velocity of 0.66 m/s. (Note: because of the double angle formula for α, there is a second solution when the angle is equal to 90 α.) 8. Block A of mass m is moving at velocity +v towards mass B of mass 2m which is at rest. To its right and at rest is mass C of mass m. Find the ultimate velocities of all three masses assuming all collisions are elastic. Since all the collisions are elastic, both energy and momentum are always conserved. First consider the collision of block A with block B. Conservation of energy and momentum tell us 2 mv2 = 2 mv2 A + 2 (2m)v 2 B = v 2 = va 2 + 2v B 2 mv = mv A + 2mv B = v = v A + 2v B Solving these equations we find that v A = v/3 and v B = 2v/3. Setting up similar equations for the collision of B with C, 2 (2m)v 2 B = 2 (2m)v2 B + 2 mv2 C = 2v B 2 = 2vB 2 + vc 2 2mv B = 2mv B + mv C = 2v B = 2v B + v C we find that v B = v B /3 and v C = 4v B /3. So, the final velocities for all the blocks are v A = v/3, v B = 2v/9, v C = 8v/9 9. Two identical frictionless billiard balls are symmetrically hit by a third identical ball with velocity iv 0. Find all subsequent velocities following this elastic collision. (Draw a picture at the moment of collision. What does no friction say about direction of forces?) In this problem, both energy and momentum are conserved. However, I will start out by first by using symmetry to simplify the problem. The two balls that are hit (m 2 and m 3, see figure 9) have the same mass. Therefore, the magnitudes of their final velocities must be equal and they move in a direction ±α relative to the horizontal.

6 6 FIG. 3: Problem 9 From figure 9, it is easy to see that the angle α must be equal to (The consequence of there being no friction is that the force between the billiard balls acts directly between the centers of mass. If there were friction acting at the contact point, the net force on the billiard ball would be in a different direction.) With this information, we will now conserve energy and momentum. Conservation of energy plus the fact that v 2 = v 3 tells us that 2 mv2 0 = 2 mv2 + 2 mv mv2 3 v 2 0 = v 2 + 2v 2 2. In the y direction, we know there is no initial momentum. From our symmetry arguments above, m 2 and m 3 will have equal and opposite components of momentum in the y direction. This sums to zero, so by conservation of momentum, m will still have zero momentum in y following the collision. Conserving momentum in the x direction, mv 0 = mv + mv 2 cos mv 3 cos 30 0 v 0 = v + 2v = v 2 = v v v 0 v 2 Eliminating v 2 between the energy and momentum conservation equations we find, v 2 = v 0. Plugging back into solve for v and realizing that m will move off in the minus x direction we find v = v 0 /5. The final velocities following the collision are, v = v 0 5 î ( ) v 2 = v 0 ( ) 2 î + 2 ĵ ( ) v 3 = v 0 ( ) 2 î 2 ĵ.

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