B What's the Difference Between Contact Force and the Normal Force

Shardul Khare

Can somebody pls explain this to me?

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ShayanJ

Gold Member
Contact force is a general term for any force that two objects exert on each other because they're in contact. The component of the contact force that is normal to the contact surface is called normal force and the parallel component is called friction.

• Dale

Shardul Khare

I didnt get it :(

ShayanJ

Gold Member
When you shoot a ball, you're exerting force on it. But if your foot didn't touch the ball, there would be no force. So when you shoot a ball, you're using a contact force. But the force your foot exerts on the ball doesn't have to be perpendicular to the surface of the ball. If it were, then the ball wouldn't rotate at all. But you know that when you shoot a ball, the ball may start rotating in addition to going forward. That rotation is because of the component of the contact force that is not perpendicular to the surface of the ball.

• Shardul Khare

emdezla

I have another different question regarding this same topic.

As far as I've understood from my physics lectures, the normal force is a contact force which arises as a reaction of another force (third Newton's law)
For example, in the case of weight, the earth exerts a force (mg) on us and so we exert a force on the Earth (i.e. the normal force).

However, given that the normal force is a contact force, what would be the force opossed to weight in the case of free fall?

Thank you very much and sorry for my English. It's not my native language

Mentor

pixel

However, given that the normal force is a contact force, what would be the force opposed to weight in the case of free fall?
The earth is exerting a force on the object in free fall. The object is exerting an equal and opposite force on the earth. As there is only the one force on the object, it is accelerating as it falls. The earth is also accelerating, but with a much smaller magnitude.

Saw

Gold Member
As far as I've understood from my physics lectures, the normal force is a contact force which arises as a reaction of another force (third Newton's law)
That is true, as with any force.

For example, in the case of weight, the earth exerts a force (mg) on us and so we exert a force on the Earth (i.e. the normal force).
No, that is wrong. Well, ultimately, it is true but you are seeing those two forces as a Newton's third law pair and that is wrong. As Pixel said, the Newton's 3rd law pair of gravity exerted by the earth on an object (1) is gravity exerted by the object on the earth (2). That is so when the object is in free fall and also when it is at rest on the ground. But in the latter case (object resting on the ground), there is also another pair of forces involved: contact force exerted by the object on the earth (3) and contact force exerted by the earth on the object (4).

A different thing is that the two forces acting on the object (1 and 4) usually balance and cancel out, because the earth resists "penetration" by cancelling "any" force that attempts to produce such effect (not only gravity: for example, if you push a block by jumping on it, the earth also succeeds in avoiding penetration). But this total "cancelling" is not a must. It depends on the firmness of the ground and the angle of the ground. For example, if you remove the rock and the sand and put water instead, the fluid has not enough cohesion to stand the weight of the object and it gives way, it flows away admitting its own displacement (not without opposing a force equal to the weight of the displaced fluid, i.e. buoyancy). And if the object lies on an inclined plane, the earth still avoids penetration through its normal force (which is always "normal" or perpendicular to the ground), but only penetration: it does not prevent the object from sliding down the plane (except as through friction...).

• vela

A normal force is one component of a contact force. Any contact force can be resolved into two components. The force perpendicular to the plane of contact is the normal force. The force in the plane of contact is friction.

You can find more information on :
https://en.wikipedia.org/wiki/Contact_force

Mentor

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