# A question about contact forces and friction

• devilish_wit
In summary, In the contact situation shown, the block of mass m1 experiences a force of magnitude F while the block of mass m2 experiences a force of magnitude P.
devilish_wit

## Homework Statement

Two blocks of masses m1 and m2 are placed on a table in contact with each other as shown in the figure below. The coefficient of kinetic friction between the block of mass m1 and the table is μ1, and that between the block of mass m2 and the table is μ2. A horizontal force of magnitude F is applied to the block of mass m1. We wish to find P, the magnitude of the contact force between the blocks.

## Homework Equations

F = ma
Kinetic force = μk x FN

## The Attempt at a Solution

I was pretty sure my answer for (d) was right. I'm not sure what I did wrong[/B]

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Draw a free-body diagram for ##m_2##. What forces are directly acting on ##m_2##?

devilish_wit said:
I was pretty sure my answer for (d) was right. I'm not sure what I did wrong
You would have to tell us what you did before we can tell you what you did wrong. Can you tell us what you were thinking when you were "pretty sure" you'd done it correctly?

Note, by the way, that ##P## is an internal force.

Mister T said:
Note, by the way, that ##P## is an internal force.
Whether a force is "internal", "external", or irrelevant depends on where you have drawn the boundaries around the system of interest. For question d, I would draw the boundaries such that P is an external force.

Mister T said:
You would have to tell us what you did before we can tell you what you did wrong. Can you tell us what you were thinking when you were "pretty sure" you'd done it correctly?

Note, by the way, that ##P## is an internal force.

That's my free body diagram

For m1:

Fnet = F - P - μ1(N) ---> F - (μ1m1)g - P

For m2: (I'm not sure if F is still included in this equation or not?)

Fnet = F + P - μ2(N) ---> F - (μ2m2)g + P (this answer was wrong)

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jbriggs444 said:
Whether a force is "internal", "external", or irrelevant depends on where you have drawn the boundaries around the system of interest. For question d, I would draw the boundaries such that P is an external force.

Is my free body diagram correct?

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devilish_wit said:
Is my free body diagram correct?
Yes, that looks correct.

## 1. What is a contact force?

A contact force is a force that occurs when two objects physically touch each other. It can be a push or a pull, and it requires direct contact between the two objects.

## 2. How is contact force different from non-contact force?

Contact forces require direct physical contact between objects, while non-contact forces, such as gravity and magnetic force, act at a distance without any physical contact between objects.

## 3. What is friction?

Friction is a type of contact force that opposes the motion of an object when it comes into contact with another surface. It is caused by the roughness of the surfaces and can slow down or stop the motion of an object.

## 4. How does friction affect motion?

Friction can either increase or decrease the speed of an object depending on the direction of the force. For example, when a car's brakes are applied, friction between the brake pads and the wheels slows down the car's motion. On the other hand, friction between the wheels and the road allows the car to move forward.

## 5. How can we reduce friction?

Friction can be reduced by using lubricants, such as oil or grease, between two surfaces to make them smoother. Additionally, using wheels or rollers can also reduce friction by allowing objects to slide or roll instead of rubbing against each other.

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