Physics Question

[mastamark]

Hey guys,

Again I know this may seem like a dumb question to you all, but I am trying to piece together some small concepts.

On a level plane when pushing a box with Force(N) is the Force of friction just the same but opposite in direction? Will F(friction)= Force(N)?

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[BrownBradPitt]

Hey guys,

Again I know this may seem like a dumb question to you all, but I am trying to piece together some small concepts.

On a level plane when pushing a box with Force(N) is the Force of friction just the same but opposite in direction? Will F(friction)= Force(N)?

Wouldnt the force of friction be umg in the opp direction...u = mew

 

[EddieIndy]

The case you described is when the box isn't moving at all. When it's moving, F > f, since you had to overcome the force of friction to move the box. BradPitt was right, the force of friction is equal to the coef. of friction times the normal force.

 

[mintendo]

The case you described is when the box isn't moving at all. When it's moving, F > f, since you had to overcome the force of friction to move the box. BradPitt was right, the force of friction is equal to the coef. of friction times the normal force.

That's not entirely true. When the two forces are equal and opposite, there is no change in velocity, but there could still be velocity (i.e. moving at 2 m/s the whole way).

If F pull is > then F friction, then the object is accelerating because there is net force difference.

 

[mastamark]

Wouldnt the force of friction be umg in the opp direction...u = mew

Yeah but what if I don't have u. All I have is Force in the forward direction. Would that force just be opposite for Ff.

 

[mastamark]

That's not entirely true. When the two forces are equal and opposite, there is no change in velocity, but there could still be velocity (i.e. moving at 2 m/s the whole way).

If F pull is > then F friction, then the object is accelerating because there is net force difference.

Ok so if I have a constant velocity Ff equals Fp.

 

[mintendo]

Ok so if I have a constant velocity Ff equals Fp.

I THINK that's right. If you stick to the definition of net force, then that's the case. On the other hand, it would be hard to convince someone that when you pull something across the floor at 3 m/s and 200 m/s (both constant velocity), you're exerting the same amount of force.

Forces are weird.

 

[gridiron]

I THINK that's right. If you stick to the definition of net force, then that's the case. On the other hand, it would be hard to convince someone that when you pull something across the floor at 3 m/s and 200 m/s (both constant velocity), you're exerting the same amount of force.

Forces are weird.

You are right. Constant velocity means zero acceleration. If the box is moving at 3 m/s or 200 m/s (constant velocity) then the net force is equal to zero. This mean (F=ma) is equal to zero. You are not exerting the same force but the net force in each instance is zero.

To go back to the earlier point. On a level plane, a box will not move when force static friction is greater than force push. Force static friction ( mg)* coefficient of static friction, is the maximum force that must be overcome to produce movement. If the box is moving, that means force push is greater than force kinetic friction. Hope that helps!

 

[whatdapho]

Ok let me take a shot at this bad boy...
If you push the block on a leveled plane and it doesn't move, then the exerted force is equal to the static frictional force (but not static max which is the given mu x normal). When you push the same block with a greater force to move the box, you've exerted a force greater than the static frictional force and now it's kinetic frictional force going against the exerted force.
Now, the kinetic friction force going against this exerted force will not be the same as the exerted foce because it will depend on the mu (coefficient of kinetic friction) multiplied by the normal. I think that should answer your original question. Hope the long explanation helps.

 

[Tanner82]

Ok let me take a shot at this bad boy...
If you push the block on a leveled plane and it doesn't move, then the exerted force is equal to the static frictional force (but not static max which is the given mu x normal).



Correct Whatdapho, but in the first case, the static friction would be LESS THAN OR EQUAL TO the pushing force.

 

[whatdapho]

Ok let me take a shot at this bad boy...
If you push the block on a leveled plane and it doesn't move, then the exerted force is equal to the static frictional force (but not static max which is the given mu x normal).



Correct Whatdapho, but in the first case, the static friction would be LESS THAN OR EQUAL TO the pushing force.

if block is moving, static force is less than pushing force but if the block is not moving, static force is equal to but opposite to the pushing force.

 

[gridiron]

if block is moving, static force is less than pushing force but if the block is not moving, static force is equal to but opposite to the pushing force.

That is correct. One thing that is often misinterpreted is the static force. The equation to calculate it ( mu * normal force) gives the maximum static force. Once that force is overcome, the block can move. If forces smaller than the max static force are applied, the block will remain motionless because there is a equal but opposite static force.