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Q. 48

Physics for Scientists and Engineers: A Strategic Approach with Modern Physics
Found in: Page 130

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Short Answer

Problems 42 through 52 describe a situation. For each, draw a motion diagram, a force-identification diagram, and a free-body diagram.

You’ve jumped down from a platform. Your feet are touching

the ground and your knees are flexing as you stop.

The motion diagram states the velocity of the person jumping down from a platform also the direction of acceleration.

The free-body diagram and the force-identification diagram represent the clear idea on the object along with the forces acting on it.

See the step by step solution

Step by Step Solution

Step 1: Motion diagram

  • To create a motion diagram of a person jumping down from a platform, we have to consider the particle model for the diagram of the motion.
  • Model the person who is jumped down from a platform as a dot and assume the mass of the person is concentrated into a single point considering the particle model.
  • The person is jumping down so the velocity vector is directed along downward.
  • The frame between two consecutive dots is decreasing as the speed of the person will decrease as the persons feet are touching the ground and the person tends to stop after a while.
  • The acceleration vector is directed upward from the ground as the knees of the person are flexing as he/she tends to stop.

Therefore, the motion diagram of the person should be as follows:

Step 2: Force-identification diagram

To identify the forces acted on the rock, we have to follow the given steps:

  • We have to consider the grocery bag in which the force is acting upon.
  • Now we have to draw the image of the object and encircle it. Here the object is the grocery bag that is kept on the car seat.
  • Following forces are acting on the bag:
  1. A long-range gravitational force is acted straight downward while the person is jumping down and his/her feet are touching the ground.
  2. A normal force that is greater than the force of gravity is acting on the person vertically upward as the knees of the person are flexing upward and his/her feet are touching the ground which exert an opposite normal force so that the person can jump as shown below:

Step 3: Free-body diagram

The free body diagram is a pictorial representation of an object under some forces where the objects act as the particle and the forces acting on the object.

To draw a free-body diagram, we have to follow some steps as given below:

  • Draw a coordinate system.
  • Identify all the forces acting on the object and draw the vector of the forces.
  • Represent the object as a dot at the origin of the coordinate axes considering the particle model.
  • A long range gravitational force is acting on the person while he/she is jumping down and his/her feet touch the ground.
  • A normal force opposite to the force of gravity acts on the person along vertically upward direction.
  • Thus, the net force acts downward as the feet of the person touches the ground and he/she tends to stop in a while. The free-body diagram of the person is given below:

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