Why does friction point the opposite as gravity?

In summary, when a 280-kg piano slides down a 30 degree incline and is kept from accelerating by a man pushing back on it, the force the man needs to exert can be found by subtracting the friction force from the component of gravity. Friction always acts against motion, in this case helping the man rather than opposing him. Friction also acts against relative motion of surfaces in contact, and therefore does not oppose the motion of a rolling ball on a level surface or the forward movement of a driven car.
  • #1
Shindo
12
0

Homework Statement


a 280-kg piano slides 4.3m down a 30 degree incline and is kept from accelerating by a man who is pushing back on it parallel to the incline.

I know how to find the solution, but there's something that I don't understand about why it's that way. In order to find the force the man needs to exert to keep it from accelerating you subtract the friction force by the component of gravity. But I don't get why you subtract it rather than adding it. If it is not accelerating how do you determine which direction the friction force is acting? At first I thought it would be along the same direction of the horizontal component of gravity because it slides down that way, but instead friction is helping the man in this case. Why does it point against gravity and how are you able to tell? Thanks!
 
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  • #2
Friction always acts against motion. Acceleration is irrelevant here.
For example, if you throw a ball up in the air, its acceleration will always be the same - g pointing down, but the direction of velocity will change from up to down, and similarly the air resistance (friction) will change direction as well.
 
  • #3
Bandersnatch said:
Friction always acts against motion.
To be precise, it acts against relative motion of surfaces in contact. Thus, friction does not act to oppose the motion of a rolling ball on a level surface, since even if friction were to suddenly vanish the surfaces in contact would not move relative to each other. Similarly, friction does not oppose the forward movement of a driven car. It opposes the tendency for the wheels to skid, so it acts forwards, propelling the car.
 

1. Why do objects slow down when they slide along a surface?

Objects slow down when they slide along a surface because of the force of friction. Friction is the force that opposes motion and acts in the opposite direction of an object's movement. It is caused by the microscopic roughness of two surfaces rubbing against each other.

2. How does the direction of friction relate to the direction of gravity?

The direction of friction is opposite to the direction of gravity. This is because the force of friction acts in the direction that opposes an object's movement, while gravity acts in the direction towards the center of the earth. Therefore, the two forces point in opposite directions.

3. Why is friction sometimes referred to as a "necessary evil"?

Friction is often referred to as a "necessary evil" because it can both help and hinder motion. On one hand, friction is necessary for us to move and walk without slipping. On the other hand, it can also slow down or prevent motion, which can be frustrating and undesirable in certain situations.

4. What factors affect the amount of friction between two surfaces?

The amount of friction between two surfaces is affected by several factors, including the roughness of the surfaces, the force pushing the two surfaces together, and the type of material the surfaces are made of. Rougher surfaces, greater force, and certain materials (such as rubber) tend to have higher levels of friction.

5. How does the force of friction change as an object moves faster?

The force of friction generally increases as an object moves faster. This is because the faster an object moves, the more it rubs against the surface it's moving on, creating more friction. However, there is a limit to this trend as well. At very high speeds, an object may start to experience less friction due to factors such as air resistance, which can counteract the force of friction.

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