# One brick on another (friction, laws of Newton)

• Atx
In summary: The force on block B is x: T-0.3*mg=m*a . The force on block A is x: T+0.3*mg=m*a . The acceleration of block A is a: a=mg*x=5.00*x=50.00 m/s²
Atx

## Homework Statement

[PLAIN]http://img693.imageshack.us/img693/8219/utennavnu.png

## Homework Equations

n= normal force
µ*n=friction
F=ma.
T = tension , mA = mass of A, mB mass of B

## The Attempt at a Solution

a) Static friction should be used. Forces on brick A : x: T - µ*mg=0 y dir: n-mg=0
B: x: (mA+MB)g*µ , tension and friction between A and B, y: mg=n , force from A.
b) Kinetic friction is to be used, have tried to find an expression of the acceleration , but so far hasnt come to the right answer.

Last edited by a moderator:
Could you elaborate on the question please? What exactly are they asking here?

Jared

A force F is pulling on B in the right horizontal direction. The mass of A is 5.00 kg, the mass of B is 3.00 kg. Static friction between all surfaces is 0.600 , kinetic friction is 0.400. Assuming the blocks are at rest, for what value of F will the blocks be put into movement?

I'd appreciate any hints, I've been stuck on this problem for a while.

Are you sure it's block B that is getting the force? The picture makes it look like A. Just making sure.

Try modeling the two blocks separately in a free-body diagram, labeling all forces acting on them. Don't forget the forces the blocks exert on each other.

Always trust picture. A is getting the force. I've modeled the forces as a free body diagram.
The forces excerted on B is x: T(tension)-0.3*mg = m*a (where a is the same accelleration as the one on block A). B in y direction is mg-mg=0 which means there is no accelleration in y-direction for either A or B. A is tricky though because its affected by a force of mg in y direction (from the other block) , Mg and a normal force of equal magnitude. In x direction A is affected by T(tension)-0.3*mg but there is also friction between A and B, so how can I set up an equation to find out when the blocks move?

## 1. What is the definition of friction?

Friction is the force that resists the motion of two objects that are in contact with each other.

## 2. How does friction affect the movement of one brick on another?

Friction between two bricks creates a resistance force that opposes the motion of the top brick, making it harder to move or slide.

## 3. What are the three laws of motion by Newton?

The first law states that an object will remain at rest or in motion at a constant speed in a straight line unless acted upon by an external force. The 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. The third law states that for every action, there is an equal and opposite reaction.

## 4. How do the laws of Newton apply to the movement of one brick on another?

The first law explains why the top brick remains at rest until an external force, such as pushing or pulling, is applied. The second law explains how the force applied to the top brick affects its acceleration, and thus the force required to overcome the friction between the two bricks. The third law explains that the force applied to the top brick will result in an equal and opposite reaction force from the bottom brick.

## 5. Can friction be reduced or eliminated between two bricks?

Friction can be reduced by decreasing the roughness of the surfaces in contact or by applying a lubricant, such as oil or grease, to the surfaces. However, it cannot be completely eliminated due to the properties of matter and the laws of physics.

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