The external force is the wire that pulls the elevator.
The force that the floor exerts on you is.
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.
True when you jump you accelerate upward because the floor exerts an upward force on you.
The floor exerts a reaction force forward on the professor that causes her to accelerate forward.
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.
Equals your change in momentum the floor has to exert the same impulse on you no matter how you land.
It discusses how to calculate the apparent weight.
Your weight caused by gravity is pulling you down the floor is reacting by pushing back.
We call this force the reaction force.
Therefore the baby does not plummet down the earth.
Similarly a car accelerates because the ground pushes forward on the drive wheels in reaction to the drive wheels pushing backward on the ground.
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.
It can t go through the floor the normal force fn exerted by the floor equals the weight w plus the downward component of the applied force 38 sin 41 24 9.
Newton s first law states that if all the forces cancel each other out.
Fn 150 24 9 174 9.
And it goes same for the fourth case.
This physics video tutorial explains how to find the normal force on a scale in a typical elevator problem.