Discovering Horizontal Acceleration: Newton's Law Question Solved

In summary: It's the ratio of the force of friction to the normal force. So if you have a force of 9.8 Newtons applied to the block, the coefficient of friction is 0.4. Because the normal force is always perpendicular to the direction of motion, the coefficient of friction is always 1.0.
  • #1
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I'm really stumped with this question.

A block is held against the front of a vertical wall of a railroud car. The coeffiecient of friction between the block and the wall is 0.4. When the train betgins to accelerate the block is released and begins to slide down the wall with an acceleration of 9.0m/s^2. What is the horizontal acceleration of the train?

I can't really show you much work that I've done because I can't really tell if i need to find the mass of the block..

I know that friction = 0.4*9.8*mass.

since the block has a normal force of mass*9.8 I don't knwo the normal force and I have no idea how to find it. someone give me a clue!
 
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  • #2
If the only force on the block was gravity, it should accelerate at g=9.8 m/s2.

But the friction force is opposing it.

Ffrict = mu * mblock * a, where a is the acceleration of the train.

The friction force must be = m(g-av), where av the block's vertical acceleration 9.0 m/s2.
 
  • #3
how do you know that the friction is that?
 
  • #4
also i don't know know the mass of the block
 
  • #5
aslo the train is a force on it and i have ot find the acceleration of the train that would give that particular force so it slides down at 9.0m/s^2
 
  • #6
Do it step by step. First assume that the applied force is just enough to hold the block against the wall if the train is not accelerating. What does that tell you about the applied force?

Now introduce acceleration. Write Newton's 2nd law for both the horizontal and vertical motion. You'll be able to solve for the horizontal acceleration.
 
  • #7
Doc Al said:
Do it step by step. First assume that the applied force is just enough to hold the block against the wall if the train is not accelerating. What does that tell you about the applied force?

Now introduce acceleration. Write Newton's 2nd law for both the horizontal and vertical motion. You'll be able to solve for the horizontal acceleration.


i'm sorry but i don't get what you mean
 
  • #8
Try writing a vector diagram of the forces on the block.

Gravity is down, and the force of friction is opposite of gravity for a block sliding down, right? Friction opposes the direction of motion.

The friction force is related to the force imposed by the train - or rather it is related to the same mass (that of the block) and the acceleration of the train (which is what you are trying to find).

Think about the definition of the coefficient of friction.
 

1. What are Newton's laws of motion?

Newton's laws of motion are three physical laws that describe the behavior of objects in motion. The first law states that an object at rest will remain at rest and an object in motion will continue in motion with a constant velocity unless acted upon by a net external force. The second law states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. The third law states that for every action, there is an equal and opposite reaction.

2. How did Newton come up with his laws of motion?

Newton's laws of motion were published in his book "Philosophiæ Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy) in 1687. These laws were based on his observations and experiments on the motion of objects and the force of gravity.

3. Are Newton's laws of motion still applicable today?

Yes, Newton's laws of motion are still applicable today and are considered as the foundation of classical mechanics. They are used to explain the motion of objects and predict their behavior in everyday situations as well as in advanced fields such as aerospace engineering and astrophysics.

4. Can Newton's laws of motion be broken?

No, Newton's laws of motion cannot be broken. They are fundamental laws that govern the behavior of objects in motion and have been extensively tested and proven to be true. However, there may be situations where these laws may seem to be violated, but this is due to external factors or the limitations of our understanding and measurement techniques.

5. Are there any limitations to Newton's laws of motion?

Yes, Newton's laws of motion have limitations. They are only applicable to objects moving at speeds much slower than the speed of light and in situations where there is no significant influence of external factors such as air resistance or friction. These laws also do not apply to objects on a microscopic scale, where quantum mechanics must be used instead.

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