# Homework Help: Propulsive force and force at interface

1. Oct 16, 2014

### cuj93

1. The problem statement, all variables and given/known data
Two blocks, one of mass 6 kg and the other of mass 2 kg, are placed in contact on a horizontal table, and a constant horizontal force is applied to the 6 kg mass. There is a constant frictional force of 18 N between the 6 kg mass and the table, but no frictional force between the 2 kg mass and the table. the blocks accelerate at 3 m/s2.
a. what is the propulsive force applied to the 6 kg mass?
b. what is the force at the interface between the two blocks?

2. Relevant equations
F=ma
3. The attempt at a solution
I honestly have no idea where to start I have read the book and looked at my notes and I am not sure where to start.
Fnet=ma
fnet= F-18

2. Oct 16, 2014

### Staff: Mentor

Start by drawing a large diagram on which you mark everything relevant. Examine it closely, looking for inspiration.

Post it here.

3. Oct 16, 2014

### cuj93

4. Oct 16, 2014

### Staff: Mentor

Now you can see what is going on!

Apply your Fnet = ma formula to the situation.

5. Oct 16, 2014

### cuj93

Fnet=mneta
F-18=mneta
F-18=(6+2)(3)
F-18=24
F=42N

6. Oct 16, 2014

### cuj93

is this correct?

7. Oct 16, 2014

### Staff: Mentor

That looks right.

Next, what force must be being applied to the 2kg body to cause it to be accelerating at this rate? (Remember, if you don't apply sufficient force to it it won't accelerate at this rate, and if you were to apply more than the required force it will accelerate faster.)

8. Oct 16, 2014

### cuj93

Will that force be the force at the interface. Since 2 kg mass experiences no friction
Fnet=ma
Fnet=2(3)
F=6N

9. Oct 16, 2014

### Staff: Mentor

Yes that force can only come from the bigger mass pushing on that leading body.

10. Oct 16, 2014

### cuj93

So a) what is the propulsive force applied to the 6kg mass? 42N
and b) what is the force at the interface between the two blocks? 6N

11. Oct 16, 2014

### Staff: Mentor

It's always a good idea to associate a direction with the description of vector quantities (even when the direction may not be explicitly sought).

So, for each answer decide whether the horzontal force is directed to the right or the left.