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### Course: Physics archive>Unit 3

Lesson 2: Normal force and contact force

# More on Normal force (shoe on floor)

David explains how to determine the normal force for a variety of scenarios (extra forces, diagonal forces, acceleration) involving a shoe on the floor. Created by David SantoPietro.

## Want to join the conversation?

• What about the instant when a falling shoe hits the ground? What would be the normal force then?
• the normal force would be the reaction from the ground 'hitting' back on the shoe
• I saw a question: a boy of mass M_ holding a box of mass _m above his head jumps from a building. What is the force exterted by the box on his head during the freefall? Does it increase during the period he balances himself after hitting the ground?

Is there a normal force when two bodies in contact are in freefall? Will they stay in contact?
• Great question! i don't want to give a straight up answer because you'd appreciate it more when you'll come to conclusion yourself. Let's think about what Galileo told us about object in a free fall. A stone and a feather, ignoring any air resistance, will fall from a building at the same rate! Of course, you knew that already. But it's the key to your answer. Imagine yourself holding a ball in your palm and jumping from a high diving board. Both you and ball are falling at the same rate. Which means if you removed your palm from underneath the ball, it would still keep falling the same way. Think another case: Even if you didn't have the ball on your palm and both you and the ball had been dropped side by side from the diving board, the picture would still be the same.

So is there a normal force on the ball in free fall?

If you don't get the answer, ask again. I promise to tell the answer but you have to give the problem your best effort. Read the chapter again.
• what about the force on the horizontal direction? what happens to it?
• Because all the other forces were Vertical we only use the vertical force on F3.
• Why is sin phi used instead of sin theta?
• You get to pick the greek letter you use. As long as it is consistent with the diagram of the situation you create, you can use whatever you want. Personal preference really.
• In the 'shoe falling through the air' case, what if we take in consideration the air resistance? Could that be viewed as a normal force? (it still is a contact force, since we're talking about the air molecules reacting to the shoe in the opposite direction when they come in contact).
Of course, in this case the normal force (air resistance) would be a lot smaller than the force that caused it (mg), but then again that's why it's falling through the air, not levitating.
• Seeing as how this was made 7 years ago, you might not care anymore but I would like to still give my answer.

Air resistance like normal force is a kind of contact force, forces acting upon objects in contact. Air resistance is really a special kind of frictional force(also a contact force). The difference between frictional and normal force that you will see in diagrams is that the normal force is perpendicular to the place of contact while friction is along it or parallel.

Normal forces are also described as the reactionary force of the compounds or atoms of a material to not allow another object to pass through it. This is an important distinction in comparison to frictional forces which does "let an object pass through it" or does not completely stop an object despite being in contact with it. For example gases(air resistance) and liquids(water resistance) and solids whose point of contact is parallel to the force acting upon it.

However, as you've mentioned isn't this kind of the same fundamental force. And you are right! fundamentally, they are really the same forces, intermolecular ones at points of contact. However, because of the fact that macroscopically normal force, air resistance, water resistance, and the usual kind of friction are so different looking and whose emergent properties differ, it is much easier to deal with them as different kinds of forces.
• Hi, If acceleration = force/mass doesn't that mean that the object with more mass falls slower than object with less mass? now here doesn't mass means mass of the object as f=ma so we're taking force on the object. But that doesn't happen according to experiments. Where am I wrong?
• Both objects will undergo same acceleration. So F=ma. Hence more force is exerted on object with greater mass. So. acceleration is constant, not the force.
• In the above video what will happen to the horizontal force F3x ?
• As Sal mentioned in the previous video, to calculate the force, we will only have to consider the vertical component
• When he gives the example of Fsub2 being applied in the upward direction,the equation will become
Fn=mg-F2
or
mg=Fn+F2
Does this means that the weight of shoe will be increased? (I'm referring this concept from the weightlessness in an elevator)
Please clearify this to me.I'm finding this very confusing.In case of an elevator it was obvious but here it is not making sense..
• No, the weight remains constant, remember weight cannot change, as long as you are near the surface of the Earth. Because remember, weight is just the measure of the force of gravity on an object. No, the weight of the shoe will not increase, the normal force will decrease ( assuming the acceleration is 0 ), because the F2 relieved some of the force that was needed to be applied by the normal force.
I hope this helps.
Comment if you still have some doubts.
-ƙαɾƚιƙҽყҽ ʂԋɾιʋαʂƚαʋα™
• At , why doesn't the net force on the RHS change if the total acceleration of the shoe changes? Won't there be an extra force due to this acceleration?