How Do You Calculate the Coefficient of Kinetic Friction for a Sled?

In summary, the conversation revolves around finding the coefficient of kinetic friction for a 20.0kg sled being pulled at a constant velocity by a 80.0N force at an angle of 30 degrees above the horizontal. The formula for calculating the coefficient is -(F cos theta) / -(mg), where F is the applied pulling force. However, the normal force is reduced by the vertical component of the applied force, so the equation should be adjusted to include -(F sin theta) in the denominator.
  • #1
PhunWithPhysics
24
0
Can some one help explain?
A 20.0kg sled is being pulled across a horizontal surface at a CONSTANT VELOCITY. The pulling force has a magnitude of 80.0 Newtons(N) and is directed at an angle of 30 degreesabove the horizontal. What is the coefficient of kinetic friction?
 
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  • #2
Do a FreeBody Diagram and apply Newton's 1st Law.
 
  • #3
what? can you explain?
 
  • #4
Do you know Newton's 1st Law?

Do you know what is a Freebody diagram?
 
  • #5
Since the object is dragged along the horizontal, the normal reaction will be equal and opposite to the object's weight (W = mg).

The force is applied at an angle to the horizintal, but the friction will only act across the horizontal, so you need to find the horizontal component of the force (Fx = F cos theta).

As the object moves at a constant velocity, there are no unbalanced forces acting on the object, so the friction force must be equal in magnitude to the pulling force.

The friction force of a sliding object is proportional to the coefficient of kinetic friction and the normal reaction, so:

coefficient = -(F cos theta) / -(mg),

F being the applied pulling force.
 
  • #6
still not working

It says that I should come up with an answer of 0.444 but I keep getting 0.357, What is going wrong?
 
  • #7
El Hombre Invisible said:
The friction force of a sliding object is proportional to the coefficient of kinetic friction and the normal reaction, so:

coefficient = -(F cos theta) / -(mg),

F being the applied pulling force.
Since the 80N force is applied at an angle, the normal force is not mg. The normal force is reduced by the vertical component of the applied force.
 
  • #8
just write it out for him

Just write the equation out i don't think he understands!
 
  • #9
Doc Al said:
Since the 80N force is applied at an angle, the normal force is not mg. The normal force is reduced by the vertical component of the applied force.
*^$*! Very true. Sorry, Phun, I forgot that part. You'll need to subtracted F sin theta from -mg. Thanks, Doc Al, for spotting my clumsiness.
 
  • #10
bloccubed said:
Just write the equation out i don't think he understands!
He does, I think, I just missed off part of the solution and confused him. Entirely my fault, not his.
 
  • #11
got it

Thats much better! Thanks guys!
 

1. What are Newton's three laws of motion?

Newton's first law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. Newton's second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Newton's third law states that for every action, there is an equal and opposite reaction.

2. How do forces affect motion?

Forces cause changes in an object's motion. If there is a net force acting on an object, it will accelerate in the direction of that force. The greater the force, the greater the acceleration. Forces can also cause objects to change direction or rotate.

3. How is force measured?

The unit of force is the Newton (N). One Newton is equal to the force needed to accelerate one kilogram of mass at a rate of one meter per second squared (1 N = 1 kg * m/s^2). Force can be measured using tools such as a spring scale or a force meter.

4. What is the difference between mass and weight?

Mass is a measure of the amount of matter in an object, while weight is a measure of the force of gravity acting on an object. Mass is constant and does not change with location, but weight can vary depending on the strength of gravity. Mass is measured in kilograms (kg) and weight is measured in Newtons (N).

5. How can you apply Newton's laws to everyday situations?

Newton's laws of motion can be applied to everyday situations, such as when driving a car or riding a bike. The first law explains why you feel a force pushing you back when the car suddenly accelerates. The second law can be seen when you push a shopping cart with a heavier load and it takes more force to get it moving. The third law can be seen when you jump off a boat and feel it move in the opposite direction.

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