Physics Help Forum Kinematics IB question, explanation needed

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Aug 9th 2019, 07:32 AM #1 Junior Member   Join Date: Aug 2019 Posts: 1 Kinematics IB question, explanation needed Could someone please explain (c) and (d) in the question below and attachment. When I tried, I got 735 N, but that isn't the right answer. Thanks in advance for the help. 51 The diagram shows a person in an elevator pulling on a rope that goes over a pulley and is attached to the top of the elevator. The mass of the elevator is 30.0 kg and that of the person is 70 kg. a On a copy of the diagram, draw the forces on the person. b Draw the forces on the elevator. c The elevator accelerates upwards at 0.50 m s−2. Find the reaction force on the person from the elevator fl oor. d The force the person exerts on the elevator fl oor is 300 N. Find the acceleration of the elevator (g = 10 m s−2 ). Attached Thumbnails
 Aug 10th 2019, 02:26 PM #2 Junior Member     Join Date: Jan 2019 Posts: 25 forces on the person; Tension, T, upward Normal force of the elevator floor on the person, N, upward person weight, Mg, downward forces on the elevator; Tension, T, upward Normal force of the person on the elevator floor, N, downward elevator weight, mg, downward (c) a = 0.5 m/s^2 upward T + N - Mg = Ma T - N - mg = ma ---------------------- 2T -(M+m)g = (M+m)a 2T = (M+m)(g+a) = (100 kg)(10.5 m/s^2) = 1050 N T = 525 N using the net force equation on the person N = M(g+a) - T = (70 kg)(10.5 m/s^2) - 525 N = 210 N (d) N = 300 N T + N - Mg = Ma -(T - N - mg = ma) ----------------------- 2N - Mg + mg = Ma - ma 2N - (M-m)g = a(M-m) [2N - (M-m)g]/(M-m) = a [600 N - 400 N]/(40 kg) = 5 m/s^2 topsquark likes this.

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