Help interpreting a force problem

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In summary, to make the top block slip on the bottom one while it is held fixed, a force of at least 12N must be applied to the top block. This is due to the static friction between the two surfaces. The maximum horizontal force that can be applied to the lower block in order for the blocks to move together is 12N. The resulting acceleration of the blocks will be 1.3m/s2.
  • #1
lizette
A 4kg black is put on top of a 5 kg block. To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 12N must be applied to the top block. The assembly of the blocks is now placed on a horizontal, frictionless table. Find the magnitudes of the a) max horizontal force that can be applied to the lower block so that the blocks will move together and b) the resulting acceleration of the blocks.

The figure looks like this:

A
-
B --> F
--------

where block A is on top of block B

My question is, what does this mean:
"To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 12N must be applied to the top block."

Slip on it from where? What does this matter? Does it mean that for A to fall off of B it needs to be pushed off with a force of 12N? Is this in correlation to fs <= UsN? (Us = coefficient of static friction) Or does it mean that if block B is pushed with a F > 12N then A will slip off it?

Thanks!
 
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  • #2
the bottom block is kept fixed and therefore is just like the surface of a floor on which block A could slide. however, there is obviously friction between the two surfaces (the one of block A and the one of block B) and in order to make A slide over B the force applied to A has to be greater than the static friction between the two blocks -(4)(9.8)( CSFcoefficient of static friction)
12 N now move the upper block and therefore
12>(4)(9.8)( CSF)
CSF = 0.306
(a)When a force is applied to block B, the reaction of this force will act on block A (Newton's third Law) in the opposite direction. we know that if this force is greater than 12N the upper block will start sliding. therefore in order to make the two blocks move together the force on B has to be less than or equal to 12N.
(b)If the force acting on B is 12 N the acceleration of the system will be
12= (4+5)a
a=1.3m/s2
 
  • #3


Based on the given information, it seems like the problem is asking for the maximum horizontal force that can be applied to the lower block (block B) in order for both blocks to move together, as well as the resulting acceleration of the blocks.

The statement "To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 12N must be applied to the top block" means that in order for block A to start moving horizontally and slip off of block B, a minimum force of 12N must be applied to block A in a horizontal direction. This is in relation to the concept of static friction, where an object will remain at rest unless a force greater than the maximum static friction force is applied to it.

To answer your question, the 12N force is the minimum force needed to cause block A to slip off of block B. It does not necessarily mean that block B will be pushed off with a force of 12N, as the force required to move block B may be different depending on the coefficient of friction between the two blocks.

In terms of correlation to the equation fs <= UsN, this equation represents the maximum static friction force (fs) that can be applied before an object starts sliding, where Us is the coefficient of static friction and N is the normal force. In this problem, the given information does not specify the coefficient of friction or the normal force, so it cannot be used to find the maximum static friction force.

I hope this helps clarify the problem and the given information. It is important to understand the concepts of static and kinetic friction in order to solve this problem accurately.
 

1. What is a force and how is it defined?

A force is a physical quantity that can cause an object to accelerate or change its motion. It is defined as a push or pull exerted on an object due to the interaction between two or more objects or systems.

2. What are the different types of forces?

There are four main types of forces: gravitational, electromagnetic, strong nuclear, and weak nuclear. Gravitational force is the force of attraction between two objects due to their masses. Electromagnetic force is the force of attraction or repulsion between electrically charged particles. Strong nuclear force holds the nucleus of an atom together. Weak nuclear force is responsible for radioactive decay.

3. How do I interpret a force problem?

To interpret a force problem, start by identifying the given information and the unknown quantity. Then, draw a free-body diagram that shows all the forces acting on the object. Next, use Newton's laws of motion to set up equations and solve for the unknown quantity.

4. Can forces be balanced or unbalanced?

Yes, forces can be either balanced or unbalanced. When the net force on an object is zero, the forces are balanced and the object remains at rest or moves with constant velocity. When the net force is non-zero, the forces are unbalanced and the object undergoes acceleration.

5. How can I improve my understanding of forces and their interactions?

To improve your understanding of forces and their interactions, it is helpful to review and practice solving problems using Newton's laws of motion. You can also conduct experiments to observe and measure how different forces affect the motion of objects. Additionally, studying the fundamental principles and concepts of physics can deepen your understanding of forces and their interactions.

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