When a person jumps up from the ground, which way does the does Earth-person sys

Question

When a person jumps up from the ground, which way does the does Earth-person system center of mass move? a) Down b) Up c) Up then down. d) It does not move e) Impossible to determine Which one of the following statements concerning the momentum of a system when the net force acting on the system is equal to zero newtons is true? a) The momentum of the system is increasing b) The momentum of the system is decreasing, c The momentum of the system is equal to zero kg middot m/s. d) The momentum of the system has a constant value. e) The momentum of the system has a negative value. Skater A of mass 75 kg and skater B of mass 50 kg are initially at rest on frictionless ice some distance apart Each skater holds tightly onto a rope of negligible mass. Skater A only pulls on the rope with constant force so that the skaters approach each other and meet. Which statement is not TRUE? a) The center of mass of the skaters does not move. b) The magnitude of the acceleration of skater A is less than B c) Just before they meet, the speed of skater A is less than the speed of B. d) The work done by the tension on skater A is equal to the work done by the tension on B. e) While the skaters are moving, their momentum vectors have equal magnitudes. A boy of mass m runs with a speed v and jumps onto a sled on an icy pond. The sled was at rest before the boy jumped onto it. After the jump, the sled and boy move at a speed v/2. What is the mass of the sled? a) m/2 b) m c) 2m d) 3m e) 4m A collision between two objects is elastic. Which one of the following statements concerning this situation is true? a) The total kinetic energy of the objects is the same before and after the collision. b) The total momentum of the objects is zero kg m/s after the collision. c) The objects stick together and move as one object after the collision. d) The kinetic energy of the objects is zero joules after the collision. e) The vector sum of the velocities of the two objects is equal to zero m/s after the collision.

Explanation / Answer

1) D As the earth-person system doesnot have any external force on it, the C.O.M will not move at all.
2) D When there is no force on the system, the momentum will not change as force is the rate of change of momentum and it is not changing and hence the momentum remains same.
3) D Tension on both the person will be same, and acceleration is dependent on mass, so larger mass will have less acceleration. Now work done by tension will be converted to the kinetic energy which is given by (P^2/2*m), where P is momentum. Now the momentum magnitude will be same as no external force is applied, so larger mass will have less K.E and smaller mass will have larger K.E.
4) B Momentum remains conserved. Now m*v=(m+M)v/2. implies 2mv=mv+Mv
   so, mv=Mv. Hence, M=m. So, both of them has the same mass m.
5) A In an elastic collision, the K.E and momentum remains conserved.

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