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Lesson: Newton’s Third Law of Motion

Sample Question Videos

Worksheet • 25 Questions • 1 Video

Q1:

An elevator is accelerating vertically upwards at 2 . 6 / m s 2 . A man of mass 1 2 4 k g is standing inside. Determine the reaction force of the floor on the man.

  • A 1 537.6 N
  • B 1 215.2 N
  • C 892.8 N
  • D 322.4 N

Q2:

A body of mass 75 kg was attached to a string fixed to the ceiling of an elevator. Starting from rest, the elevator began accelerating vertically upwards at 44 cm/s2. During this time the tension in the string was 𝑇 1 . After accelerating, the elevator continued moving at the speed it had gained, at which point the tension in the string was 𝑇 2 . Finally, the elevator decelerated at 24 cm/s2, at which point the tension in the string was 𝑇 3 . Determine the magnitudes of 𝑇 1 , 𝑇 2 , and 𝑇 3 . Consider the acceleration due to gravity to be 9.8 m/s2.

  • A 𝑇 = 7 6 8 1 N , 𝑇 = 7 3 5 2 N , 𝑇 = 7 1 7 3 N
  • B 𝑇 = 7 1 7 1 N , 𝑇 = 7 3 5 2 N , 𝑇 = 7 6 8 3 N
  • C 𝑇 = 7 3 1 N , 𝑇 = 7 5 2 N , 𝑇 = 7 8 3 N
  • D 𝑇 = 7 8 1 N , 𝑇 = 7 5 2 N , 𝑇 = 7 3 3 N

Q3:

A body of mass 130 kg was placed on the floor of an elevator of mass 761 kg. The elevator ascended from rest and it covered 15 m while accelerating uniformly. Following this, it continued moving at the speed it had gained for a certain distance. Finally, it decelerated uniformly for 5 seconds over a distance of 10 m until its speed became 1 m/s. Find the force of the elevator’s motor 𝐹 when the elevator was accelerating upwards and the magnitude of the reaction 𝑅 exerted on the body by the floor of the elevator during the third stage of the motion. Take 𝑔 = 9 . 8 / m s 2 .

  • A 𝐹 = 8 9 9 9 . 1 N , 𝑅 = 1 2 2 2 N
  • B 𝐹 = 7 6 8 6 . 1 N , 𝑅 = 1 2 7 4 N
  • C 𝐹 = 7 9 1 4 . 4 N , 𝑅 = 1 3 2 6 N
  • D 𝐹 = 9 2 6 6 . 4 N , 𝑅 = 1 2 7 4 N
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