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QUESTION: I am a 14-year-old.Why the pulling/pushing force of an object is equal to the friction acting on the object when it starts moving?According to Newton`s First law,if the friction=the pulling/pushing force,the net force should be zero and it cannot move!Could you tell me the reason?

ANSWER: Hello michael,

You are correct when you say that if the friction=the pulling/pushing force, the net force should be zero. However, if the pulling/pushing force increases by 0.000000001 N, then the net force will be non-zero and the object will move. And once it moves, the friction becomes another type of friction. We don't quibble about the 0.000000001 N. I agree that it would be more accurate to say it this way: the fiction is equal to the highest pulling/pushing force that will not move the object. But the 0.000000001 N additional that would break the object loose is probably such a small increment that your equipment can not measure it.

About the friction becoming another type of friction: The friction was static friction while the pulling/pushing force was smaller and the friction therefore held the object still. But once it is moving, the friction is then kinetic friction and kinetic friction is (almost always) smaller. (You could also call the friction sliding friction.) So then the friction is significantly less than the pulling/pushing force and the object will accelerate accordingly.

I hope this helps,
Steve

---------- FOLLOW-UP ----------

QUESTION: O I see,but I wonder why if the pushing/pulling force is smaller than the friction,the object will not move.It should`ve move to the direction opposite to the direction of the force!Also,what will happen if there is no force other than friction applied to the object?

Answer
Hello again michael,

Static friction is a potential to oppose an applied force. If someone gives you a slight push, you can easily oppose that push and stay in place. If that someone starts pushing harder, it will require more effort from you to oppose the push. There will be a maximum amount of opposition that you can generate. If 3 people start pushing you, the applied force will probably exceed what you can successfully oppose. But until the applied force reaches the amount of force that you can generate in opposition, you can remain in place. Important note: until that maximum point is reached, you only generate the exact amount of opposition that is required to yield a net force of zero.

Assume a 10 N object is sitting on a board that is tilted 5 degree from horizontal. There will be a fraction of the 10 N weight (which is a force) that points down the slope. Call that force Fds.
Fds = 10 N*sin1 = 10 N*0.087 = 0.87 N

Unless the board is coated with ice, the static friction probably can easily provide a force equal and opposite that 0.87 N force. If you slowly increase the angle of incline, the value of Fds will slowly increase. If you increase the angle of incline to 30 degrees, the value of Fds becomes
Fds = 10 N*sin30 = 10 N*0.5 = 5 N
At some point, Fds will have a value that exceeds the maximum amount that static friction can generate. So the object will have to slide.

The static friction "maximum amount" varies with changes in both surfaces. Friction is complicated. It depends on surface irregularities and also with chemical details in the atoms or molecules that make up both surfaces.

I hope this helps,
Steve

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Steve Johnson

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I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

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BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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