Frictional force- sign convention

In summary, in a horizontal spring mass system, the frictional force is considered negative (-μmg) as it always acts in the opposite direction of motion. This is represented by a minus sign to indicate that it impedes the motion of the block. However, when applying the work energy theorem, the frictional force is taken as negative for calculations. This is because work is considered positive when the force is in the direction of motion and negative when the force is opposite the direction of motion. Therefore, the frictional force is always considered negative in a horizontal system.
  • #1
sphyics
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Frictional force-- sign convention

For a horizontal spring mass system is the frictional force considered negative (-μmg)? i guess it will be negative; the minus sign used to convey the force is trying to impede the motion of block then with that guessing work; i think friction force will be always negative for a horizontal spring mass system. If I'm wrong please correct me :)
 
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  • #2


IN a horizontal system, "+" usually means "to the right", and "-" means "to the left." Is your frictional force to the right or to the left?
 
  • #3


jtbell said:
IN a horizontal system, "+" usually means "to the right", and "-" means "to the left." Is your frictional force to the right or to the left?

Appreciate your reply :)

The frictional force is to the right; the case under consideration is: when the mass is stretched to the extreme right and let it go, as the mass moves towards the left under spring action, the friction force will try to impede the motion hence to the right.. But while deriving the work energy theorem for the particular case the friction force is taken as negative.. hence I'm confused!
 
  • #4


Work is positive when the force is in the direction of motion (both + or both - in the case of one-dimensional motion) and negative when the force is opposite the direction of motion (one + and the other -).
 
  • #5


GOT IT! i made a mistake while calculation; i mistook force as +ve and negative whereas i was supposed to take the work due to frictional force negative :)

Thanks for your time jtbell. :) Really appreciate your help. :)
 

1. What is frictional force and how does it affect motion?

Frictional force is a resistive force that opposes the motion or attempted motion of an object. It is caused by the interaction between the surfaces of two objects that are in contact. Frictional force can either slow down or prevent motion from occurring.

2. What is the sign convention for frictional force?

The sign convention for frictional force is based on the direction of the force and the direction of motion. If the force and motion are in the same direction, the frictional force is considered positive. If the force and motion are in opposite directions, the frictional force is considered negative.

3. How does the surface area and roughness of objects affect frictional force?

The surface area and roughness of objects can affect frictional force. Generally, a larger surface area and rougher surfaces result in a greater frictional force. This is because there is more surface contact between the objects, which increases the amount of resistance to motion.

4. Can frictional force be both kinetic and static?

Yes, frictional force can be both kinetic and static. Kinetic frictional force occurs when two surfaces are in motion relative to each other, while static frictional force occurs when two surfaces are at rest relative to each other. The magnitude of static frictional force is typically greater than kinetic frictional force.

5. How can frictional force be reduced?

Frictional force can be reduced by using lubricants, such as oil or grease, between two surfaces. Lubricants create a smooth layer between the surfaces, reducing the amount of friction. Additionally, using smoother and less rough surfaces can also help reduce frictional force.

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