#$&* course Phy 231 8/2/11 If your solution to stated problem does not match the given solution, you should self-critique per instructions at
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Given Solution: `a** For an elastic collision we have m1 v1 + m2 v2 = m1 v1' + m2 v2' and v2 - v1 = -( v2' - v1'). We substitute m1, v1, m2 and v2 to obtain 450 kg * 4.5 m/s + 550 kg * 3.7 m/s = 450 kg * v1 ' + 550 kg * v2 ', or 4060 kg m/s = 450 kg * v1 ' + 550 kg * v2 ' . Dividing by 10 and by kg we have 406 m/s = 45 v1 ' + 55 v2 '. We also obtain 3.7 m/s - 4.5 m/s = -(v2 ' - v1 ' ) or v1 ' = v2 ' - .8 m/s. Substituting this into 406 m/s = 45 v1 ' + 55 v2 ' we obtain 406 m/s = 45 ( v2 ' - .8 m/s) + 55 v2 ' . We easily solve for v2 ' obtaining v2 ' = 4.42 m/s. This gives us v1 ' = 4.42 m/s - .8 m/s = 3.62 m/s. Checking to be sure that momentum is conserved we see that the after-collision momentum is pAfter = 450 kg * 3.62 m/s + 550 kg * 4.42 m/s = 4060 m/s. The momentum change of the first car is m1 v1 ' - m1 v1 = 450 kg * 3.62 m/s - 450 kg * 4.5 m/s = - 396 kg m/s. The momentum change of the second car is m2 v2 ' - m2 v2 = 550 kg * 4.42 m/s - 550 kg * 3.7 m/s = + 396 kg m/s. Momentum changes are equal and opposite. NOTE ON SOLVING 406 m/s = 45 ( v2 ' - .8 m/s) + 55 v2 ' FOR v2 ': Starting with 406 m/s = 45 ( v2 ' - .8 m/s) + 55 v2 ' use the Distributive Law to get 406 m/s = 45 v2 ' - 36 m/s + 55 v2 ' then collect the v2 ' terms to get 406 m/s = -36 m/s + 100 v2 '. Add 36 m/s to both sides: 442 m/s = 100 v2 ' so that v2 ' = 442 m/s / 100 = 4.42 m/s. * STUDENT NOTE AND INSTRUCTOR RESPONSE: Ok i wasnt sure about the formulas which are: m1 + v1 + m2 + v2 = m1 + v1' + m2 + v2' and v2 -v1 = -(v2' - v1') INSTRUCTOR RESPONSE: Those are the formulas, except that you don't have any * in the first (* should replace two of your + signs; should read m1 * v1 + m2 * v2 = m1 * v1' + m2 * v2' or just m1 v1 + m2 v2 = m1 v1' + m2 v2'). Note that the units of m1 and v1, for example, make it impossible to add m1 and v1. The meaning of the formulas is simpler and easier to remember than the formulas. If you understand the meaning you can always reconstruct the formulas: For an elastic collision: Total momentum after = total momentum before. Relative velocity reverses (i.e., relative velocity after collision is equal and opposite to relative velocity before collision). Recall that an elastic collision is one in which both momentum and kinetic energy are conserved. The formulas are straightfoward: m1 v1 + m2 v2 = m1 v1' + m2 v2' and 1/2 m1^2+ 1/2 m2 v2^2 = 1/2 m1 v1'^2 1/2 m2 v2'^2. The second formula is more complicated than v2 - v1 = - (v2' - v1'), which combined with the first formula gives us the same results, so we use this formula instead of the second. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): OK ------------------------------------------------ Self-critique Rating: 3 ********************************************* Question: `qUniv. 3.48. (not in 11th edition) A ball is thrown at an unknown initial speed at angle of inclination 60 deg. It strikes a wall 18 m away at a point which is 8 m higher than thrown. What are the initial speed of the ball and the magnitude and angle of the velocity at impact? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: horizontal vf = v0 v0x = v0 * cos(theta) v0x = v0(cos(60)) v0x = 0.5v0 x-x0 = 0.5v0 * t + 0.5 * a * t^2 18 m = 0.5v0 * t + 0.5 * -9.8 m/sec^2 * t^2
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Given Solution: `a** We know the following: For y motion `dsy = + 8 m, ay = -g = - 9.8 m/s^2 and v0y = v0 sin(60 deg) = .87 v0. For x motion `dsx = 18 m, ax = 0 and v0x = v0 cos(60 deg) = .5. Assuming a coordinate system where motion starts at the origin: The equation of motion in the x direction is thus x = .5 v0 * t and the equation of y motion is y = .87 v0 t - .5 g t^2. We know x and y at impact and we know g so we could solve these two equations simultaneously for v0 and t. We begin by eliminating t from the two equations: x = .5 v0 * t so t = 2 x / v0. Substituting this expression for t in the second equation we obtain y = .87 v0 * (2 x / v0) - .5 g ( 2 x / v0) ^ 2. Multiplying both sides by v0^2 we obtain v0^2 y = .87 v0^2 ( 2 x) - .5 g * 4 x^2. Bringing all the v0 terms to the left-hand side we have v0^2 y - 1.73 v0^2 x = -2 g * x^2. Factoring v0 we have v0^2 ( y - 1.73 x) = -2 g x^2 so that v0 = +-sqrt(-2 g x^2 / ( y - 1.73 x) ) ) . Since we know that y = 8 m when x = 18 m we obtain = +- sqrt( -9.8 m/s^2 * (18 m)^2 / ( 8 m - 1.73 * 18 m) ) = +-sqrt(277 m^2 / s^2) = +-16.7 m/s, approx.. We choose the positive value of v0, since the negative value would have the projectile moving 'backward' from its starting point. Substituting this value into t = 2 x / v0 and recalling that our solution applies to the instant of impact when x = 16 m and y = 8 m we obtain t = 2 * 18 m / (16.7 m/s) = 2.16 s. Alternatively we can solve the system for v0 less symbolically and perhaps gain different insight into the meaning of the solution. Starting with the equations x = .5 v0 * t and y = .87 v0 t - .5 g t^2 we see that impact occurs when x = .5 v0 t = 18 m so that t = 18 m / (.5 v0) = 36 m / v0. At this instant of impact y = 8 m so substituting this and the t just obtained into the equation of motion for y we get y = .87 v0 (36 m / v0 ) - .5 g (36 m / v0 )^2 = 8 m. The equation .87 v0 (36 m / v0 ) - .5 g (36 m / v0 )^2 = 8 m is easily solved for v0, obtaining v0 = 16.7 m/s. With this initial velocity we again confirm that t = 2.16 sec at impact. Note that at t = 2.16 sec we get y = 14.4 m/s * 2.16 s - 4.9 m/s^2 * (2.16 s)^2 = 8 m, within roundoff error, confirming this solution. We need the magnitude and direction of the velocity at impact. We therefore need the x and y components of the velocity at the t = 2.16 sec instant. At this instant we have x and y velocities vx = dx/dt = .5 v0 = 8.35 m/s and vy = dy/dt = 14.4 m/s - 9.8 m/s^2 * t = 14.4 m/s - 9.8 m/s^2 * 2.16 s = -5.6 m/s, approx. The velocity at impact therefore has magnitude sqrt( (8.25 m/s)^2 + (-5.6 m/s)^2 ) = 10 m/s, approx. and the angle is arctan(vy/vx) = arctan(-5.6 / 8.25) = -34 deg, approx. At impact the object is moving at 10 m/s and at 34 deg below horizontal (i.e., it's on its way back down). ** STUDENT COMMENT I think I have the right understanding of how to do this but I keep getting different answers than the given ones. They seem to use many rounding errors. For example, from the given solution above I found this vy = dy/dt = 14.4 m/s - 9.8 m/s^2 * t = 14.4 m/s - 9.8 m/s^2 * 2.16 s = -5.6 m/s, approx. When doing this in my calculator, I get -6.7689, so I don't see how they think -5.6 is approximate. INSTRUCTOR RESPONSE looks like I approximated very roughly, using 9.8 * 2.16 = 20. Clearly 9.8 * 2.16 is closer to 21. Should have been more careful since the result was then used in a subtraction. Remember, though, that my numbers are mostly the result of mental approximation. That is by design; they aren't intended to be all that accurate, though in most cases they should be reasonably close. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): I think I did this correctly. Would you let me know if these answers are wrong. ------------------------------------------------ Self-critique Rating: 3