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### Course: Class 11 Physics (India)>Unit 9

Lesson 5: Normal force and contact force

# More on Normal force (shoe on wall)

David shows how to determine the normal force for a shoe shoved against a wall with a diagonal force. Created by David SantoPietro.

## Want to join the conversation?

• when i draw my free body diagram i am havng trouble with the trig of phi, i tend to always assume cos(phi) in the horizontal, is there a trick as to how you decided to use sin? I see how you would use sin in the diagram on the left but i cannot see where the corresponding phi angle would be on your free body diagram. thanx in advance
• Look at the trig from the perspective of the angle you are using. So, say you have a right triangle, and phi is on the top of that right triangle. The side opposite that angle is the sine, and the side of the triangle adjacent to that angle is cosine. I came up with word-play to remember. Cosine has "co" in front of sine, sort of like co-worker, so it is always going to be the side adjacent to the gang;e you are using in your trig calculations. Sine will always be the side opposite the angle, regardless is that's in the horizontal or vertical direction. A triangle is a triangle, it doesn't have direction itself. So the direction (vertical or horizontal) of the sides has no bearing on the trig calculations.
• I get this whole thing but i don't figure out why should we use F4Sin not F4cos
• Nope. Did you see his diagram? The angle that was given to him is opposite of the horizontal components. Soh Cah Toa Cosine is the side adjacent to the angle divided by the hypotenuse. Sine is the side opposite to the angle divide by the hypotenuse. Because the horizontal component is opposite of the given angle, you have to use sine. Hope this helps!
• then how are nails penetrated in walls or any solid
• The nail imparts more force than the strength of the cohesive forces holding the material together allowing the nail penetrate the material.
• Can there be more than one Normal Force on an object? (e.g. if the shoe is being pushed into the left bottom corner of the box)
• Just out of curiosity, when I'm pushing something like a shoe against a vertical wall extremely hard, the shoe doesn't fall to the ground although it seems like I only apply horizontal force to it. Is this some phenomenon that I don't know or just an illusion (maybe in reality I apply some vertical force to counteract gravity without realizing it)
• Because of the the force you are applying on the shoe it has a force between it and the wall. Because of the force between the wall and the shoe there is a static friction that is proportional to the force and the coefficient of static friction. This static friction is what resists the downward for frm gravity.
• I think I understand this, but what happens to the normal force if you break through a surface? For example, when you fall through a table.
• Why didn't he take the gravitational force into consideration while using Newton's second law?
(1 vote)
• Because he is only taking into consideration the horizontal forces. ΣF = m⋅a is true for any direction. As the gravitational force occurs in the vertical direction, he just ommited it. That's why he only took into account the horizontal component of F4 (sin ϕ).
• If you're pushing against a surface like a sponge, what happens to the normal force?
• It's there, pushing back on you however hard you are pushing on the sponge, at all times.
(1 vote)
• I was wondering, why are you using phi instead of theta? I've never worked with phi before, only theta.