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Why is the coefficient of friction multiplied by a number that is not the force of gravity to find the force of friction? I know that you are supposed to multiply by the normal force, but I thought they were equal when the ground was flat.
The normal force and gravitational force are not always equal, in any scenario. The normal force is simply a force that balances out the force along a certain axis. On inclines, the perpendicular direction to the incline will have no net force, since the normal force balances it out. Here, the upward force is the vertical component of the applied force plus the normal force, which equals the gravitational force. Therefore, the normal force is less, so the force of friction is less.
I’m confused about how static friction works. Isn’t it the coefficient of friction times the normal force? So wouldn’t the object slide backwards if that was the only force or if it was greater than the forward force? I know that it doesn’t actually slide, but why not?
The force of static friction is calculated as an INEQUALITY, not an equation. So, the force of static friction is less than or equal to the coefficient of static friction times the normal force. As the applied force increases, the static frictional force will balance it out, until the applied force gets too high.
Kinetic friction is an equation; it is the normal force times the coefficient of kinetic friction (a different coefficient), so it is always constant as long as the normal force is constant.
In some of the force diagrams, why are the normal and gravitational forces stemming from the center, but the frictional force is on one of the edges of the object. Since the object is represented as a point, shouldn't they all start from the same place?
If the object was being represented as a point, then yes, we would draw all forces from the same point, since there would only be one point to draw from. However, very few things are actually just points, so we have to draw forces from where they are act. The gravitational force acts on the center of mass, so in the center. The force of friction acts between the ground and the object, so we draw it on an edge. The only other force that we draw not from the center is torque, since it acts rotationally and not transitionally.
Usually, you can assume something is a point for the sake of calculations. When drawing a free body diagram, the question will provide you with a shape to draw off of, and it will either be a point or a shape.
If you struggle with a specific topic or question, please feel free to post that in the comments below for solutions and clarifications. If you enjoyed this video or learned something from it, please like and subscribe! Thank you for watching!
Why is the coefficient of friction multiplied by a number that is not the force of gravity to find the force of friction? I know that you are supposed to multiply by the normal force, but I thought they were equal when the ground was flat.
The normal force and gravitational force are not always equal, in any scenario. The normal force is simply a force that balances out the force along a certain axis. On inclines, the perpendicular direction to the incline will have no net force, since the normal force balances it out. Here, the upward force is the vertical component of the applied force plus the normal force, which equals the gravitational force. Therefore, the normal force is less, so the force of friction is less.
I’m confused about how static friction works. Isn’t it the coefficient of friction times the normal force? So wouldn’t the object slide backwards if that was the only force or if it was greater than the forward force? I know that it doesn’t actually slide, but why not?
The force of static friction is calculated as an INEQUALITY, not an equation. So, the force of static friction is less than or equal to the coefficient of static friction times the normal force. As the applied force increases, the static frictional force will balance it out, until the applied force gets too high.
Kinetic friction is an equation; it is the normal force times the coefficient of kinetic friction (a different coefficient), so it is always constant as long as the normal force is constant.
For the first one, since sin is O/H. Wouldn't it be 20/sin30?
20 is the value of the hypotenuse. So it’s sin30 = O/20, which is algebraically rearranged to 20sin30=O
In some of the force diagrams, why are the normal and gravitational forces stemming from the center, but the frictional force is on one of the edges of the object. Since the object is represented as a point, shouldn't they all start from the same place?
If the object was being represented as a point, then yes, we would draw all forces from the same point, since there would only be one point to draw from. However, very few things are actually just points, so we have to draw forces from where they are act. The gravitational force acts on the center of mass, so in the center. The force of friction acts between the ground and the object, so we draw it on an edge. The only other force that we draw not from the center is torque, since it acts rotationally and not transitionally.
How do we know if something is represented as a point? Will the question just tell us to assume it is a point?
Usually, you can assume something is a point for the sake of calculations. When drawing a free body diagram, the question will provide you with a shape to draw off of, and it will either be a point or a shape.