Chapter 7, Problem 49A

When you get up from a sitting position, do your feet push against the floor with a force equal to, more than, or less than your weight? Explain.

To determine

Whether the push exerted by the person’s feet against the floor is more than, less than, or equal to the person’s weight when he get up from a sitting position.

Answer to Problem 49A

More than the person’s weight.

Explanation of Solution

Introduction:

As per Newton’s third law of motion, for every force exerted there is an opposite and equal reaction force.

And, according to Newton’s second law of motion, if the force $F$ is applied to the mass $m$ , then the acceleration produced is given by,

  $a=\frac{F}{m}$

When the person is sitting, then the weight of the person acts in the downward direction. And, as per Newton’s third law of motion, the chair will exert an equal force on him in upward direction.

Now, if the person wants to get up, he needs to provide acceleration in upwards. Considering Newton’s second law of motion, then the net force has to be in upward direction. Thus, the push exerted by the person’s feet against the floor has to be greater than his weight.

Conclusion:

Hence, to move in further direction, the person has to push his feet against the floor with the force greater than his weight.