Newton s first law states that if all the forces cancel each other out.
The force that the floor exerts on you is.
Equals your change in momentum the floor has to exert the same impulse on you no matter how you land.
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But since impulse equals force times time you can decrease the force that the floor exerts on you by increasing the stopping.
The force that your foot and the floor exert on each other is a friction force between the sole of your shoe and the floor.
The external force is the wire that pulls the elevator.
The actual normal force comes from the floor of the elevator exerting the same force as the baby s weight force which follows newton s third law.
Similarly a car accelerates because the ground pushes forward on the drive wheels in reaction to the drive wheels pushing backward on the ground.
In the vertical direction the net force is zero since the velocity component in that direction must remain zero.
Therefore the baby does not plummet down the earth.
We call this force the reaction force.
True when you jump you accelerate upward because the floor exerts an upward force on you.
Newton s third law states that every action has an equal and oposite reaction you may not think this but when you re standing ont he floor you are excerting a force against the floor.
This physics video tutorial explains how to find the normal force on a scale in a typical elevator problem.
And it goes same for the fourth case.
The floor exerts a reaction force forward on the professor that causes her to accelerate forward.
Your weight caused by gravity is pulling you down the floor is reacting by pushing back.