Guys, in the last couple of videos, we talked about kinetic friction. In this video, we're going to talk about the other type of friction, which is called static. They have some similarities, but static friction is a little bit more complicated. So let's check it out. So remember when we talked about kinetic friction, we said that this happens when the velocity is not equal to 0. You push a book and it's moving, and static, sorry. Kinetic friction tries to stop that object and bring it to a stop. Right? Static friction happens when the velocity is equal to 0. Imagine this book is really heavy and it's at rest on the table. What static friction tries to do is it tries to prevent an object from starting to move. So imagine this book was really, really heavy. You try to push it, and no matter how much you push, the book doesn't move. That's because of static friction. So the direction of kinetic friction, right, was always opposite to the direction of motion. Right? So you push this book across the table, it's going to the right. Kinetic friction opposes with to the left. Static friction's kind of similar except the direction is going to be opposite to where the object wants to move or would move without friction. So, this heavy book, right, you're pushing it without friction. It would move to the right. So static friction is going to oppose you by going to the left. So, this is
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Static Friction: Study with Video Lessons, Practice Problems & Examples
Static friction occurs when an object is at rest, opposing the force trying to initiate movement. It is defined by the equation
Static Friction & Equilibrium
Video transcript
Static & Kinetic Friction
Video transcript
So let's check out this problem here. I have a 5.1 kilogram block and what I want to do is I want to calculate the force that I need to get the block moving. And then I want to calculate the force needed to keep it moving at constant speed. So there are really two different situations here that I'm going to draw out the diagrams for. We've got a 5.1 kilogram block on the ground, and I'm going to be pushing it with some mysterious force here to get it moving. And then in the second case, I've got the block like this, except now it's moving with some constant speed. So
Preventing a Block From Moving
Video transcript
Guys, let's check this one out. We have a 15-kilogram block that's at rest. We're given the coefficient of static friction, and we want to figure out how hard you have to push down on this block to keep it from moving. So basically, let's sketch this out. We have a 15-kilogram block that's on a flat surface. Right? And basically, what we're going to be doing is pushing down. I'm going to call this force Fdown. That's what we're trying to find. And we want to push down hard enough so that we can generate enough friction so that a horizontal force, which we know is 300, cannot get it moving. Alright? So, what we want to do is first start off with a free body diagram. So let's go ahead and do that. So, we've got our box, we've got our mg that's downwards, and then we also have any applied forces. We know there are 2. Our Fdown is what we're trying to solve for. And we also have an applied force that acts to the right. This is F = 300. Now there is also going to be some normal force because you're on the floor. And then finally, what happens is without friction, the box would start moving to the right. But if we're going to keep this thing from moving, that's because there has to be some static friction that is opposing this. So there's some friction here that is opposing that. Right? So that actually goes so that actually brings us to the second step, which we've already just talked about here. We have to determine whether this friction is static or connected based on the problem text or by looking at all the forces involved. And what we've seen here is that we're pushing down on the block to keep this box from moving, basically. So what happens is we know that this friction is going to be static. Alright? So let's go ahead and now write our F = ma. So I'm going to write out my F = ma here. I'm going to just pick a direction of positive up and to the right. Now, usually, we would start with the x-axis, but because we're looking at a force that's in the y-axis, we're going to go and start with our y-axis first. So we've got our sum of all forces equals may. Now this box isn't going up or down in the vertical axis, so we know the acceleration is going to be equal to 0. So, therefore, when we expand our forces remember our normal is positive minus mg minus Fdown and this is equal to 0. Those forces have to cancel. So here's Fdown. And if I go ahead and just solve for this variable over here by bringing it to the other side and flipping the equation around Fdown is really just equal to the normal minus mg. Okay? So this Fdown, the force that I need to get this object to prevent this object from moving is going to be equal to the normal minus mg. But the problem is I actually don't know what this normal force is. So to figure this out, whenever I get stuck in one axis, I usually just go to the other axis to solve. So I'm going to go to the x-axis forces to now solve for this. So I've got F = ma in the x-axis. So what are our forces? We have our F, which is 300 minus your Fs. And what's the acceleration? Well, here's the kicker, guys. If we're trying to figure out how much we need to push on the block to prevent it from moving, we're basically trying to figure out what is the minimum force that we need so that the static friction exactly balances out the 300. And so what happens is this is going to be Fsmax. So the minimum force is going to be where the Fsmax is going to balance out the 300. If 300 were just a little bit more, then it would actually get the object to start moving. So this Fs here is actually maximum static friction. And so because of that, because the object doesn't move, then that means that the acceleration has to be equal to 0 and these forces have to balance. So that means that your F is equal to your Fsmax which is equal to, remember, μstatic times the normal. So remember we came to the x-axis because we were stuck, and we wanted to figure out what that normal force is, now we can figure it out. Our normal force is really just going to be equal to your F divided by μstatic. So your F is 300 Your μstatic is 0.7 You'll get a normal force of 429. So your normal force is 429. Basically, if the normal is 429 then your Fsmax is going to be 300 to balance out the 300 that you're pushing it with. So now we have our normal force here, which means that Fdown is just equal to 429 minus your mg, which is 15 times 9.8. So if you guys go ahead and plug this into your calculators, you're going to get 282 newtons. So if we look at our answer choices, that's answer choice C. Alright, guys. Let's hit this one.
A 36N force is needed to start a 7.0 kg box moving across the floor. If the 36.0 N force continues, the box accelerates at 0.70 m/s2. What are the coefficients of static and kinetic friction?
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More setsHere’s what students ask on this topic:
What is static friction and how does it differ from kinetic friction?
Static friction occurs when an object is at rest and opposes the force trying to initiate movement. It is defined by the equation
How do you calculate the maximum static friction force?
The maximum static friction force can be calculated using the equation
What happens when the applied force exceeds the maximum static friction?
When the applied force exceeds the maximum static friction, the object begins to move. At this point, the friction force transitions from static friction to kinetic friction. The kinetic friction force is then calculated using the equation
Why is the coefficient of static friction usually greater than the coefficient of kinetic friction?
The coefficient of static friction is usually greater than the coefficient of kinetic friction because it generally takes more force to overcome the initial resistance to motion (static friction) than to keep an object moving (kinetic friction). This is due to the microscopic interactions and interlocking between the surfaces in contact, which are more pronounced when the object is at rest.
How do you determine whether to use static or kinetic friction in a problem?
To determine whether to use static or kinetic friction in a problem, you need to compare the applied force to the maximum static friction force. If the applied force is less than or equal to the maximum static friction force, the object remains at rest, and you use static friction. If the applied force exceeds the maximum static friction force, the object starts to move, and you switch to using kinetic friction.
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