Question #43264
1. I put a downward force of 490N on a chair I am sitting on
2. I exert a backward force of 25N on the ground as I walk
3. Soccer player kicks a ball with 250N toward the south
4. Earth pulls the moon to its centre with a force of 2.0N
Expert's answer
Answer on Question #43264, Physics, Mechanics | Kinematics | Dynamics
For each of the following: Which object exerts the reaction force, on which is the reaction exerted, the magnitude of the reaction force and its direction:
1. I put a downward force of 490N on a chair I am sitting on
2. I exert a backward force of 25N on the ground as I walk
3. Soccer player kicks a ball with 250N toward the south
4. Earth pulls the moon to its centre with a force of 2.0N
Solution:
1. I put a downward force of 490N on a chair I am sitting on
As you sit in a chair, you experience two forces - the force of the Earth's gravitational field pulling you downward toward the Earth and the force of the chair pushing you upward.
The upward chair force is the reaction force and results from the contact between the chair top and your bottom end. Its magnitude is 490 N.
2. I exert a backward force of 25N on the ground as I walk
When you walk, you apply a force backwards on the earth. Likewise, the ground applies a force to you of equal magnitude (25 N) but in the opposite direction (forward direction). So, you move forwards.
3. Soccer player kicks a ball with 250N toward the south
The reaction force of 250 N of the ball acts on the foot toward the north.
4. Earth pulls the moon to its centre with a force of 2.0N
From Newton's third law, of course, the moon attracts the earth with a force of equal magnitude, which accelerates the earth. So the earth and the moon each trace (almost perfectly) circular orbits about their common centre of mass. The sketched below illustrates this, but is not to scale.
Thus, the Earth exerts a force on the Moon and the Moon in turn exerts an exactly equal force (2.0N) on the Earth directed to its centre.
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