Force

[Joules23]

Two boxes (24 kg and 62 kg) are being pushed across a horizontal frictionless surface, as the drawing shows. The pushing force of 43 N is horizontal and is applied to the 24-kg box, which in turn pushes against the 62-kg box. Find the magnitude of the force F that the 24-kg box applies to the 62-kg box.

http://i2.photobucket.com/albums/y46/jumpman6235/sat_c04_q3_06_1.gif
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is this where i use the F=G(m1m2/r^2) ??

or i use F=ma, and solve for a, which for the 24kg box is 1.8m/s, then..
for the 2nd box of 62kg, i use do:
F=(62)(1.8)
F=111.6N

that doesnt look right..?

[Joules23]

anyone help me out?

[FunkyDwarf]

F=G \frac{m1m2}{R^2} is for acceleration due to gravity, but here you can assume its 9.8ms-2 as its on the surface of the earth (i think its a fair assumption anyway)

This might sound stupid but i think its 43N. I mean if you think about it, you can just as easily say its one large mass with 43 newtons pushing on it, the gap, as far as i can see, would make no difference.

better wait for more qualified help tho :P

[Warrzie]

You need to find the acceleration of the system using a=F/(m1+m2).

Then you have to take the formula for contact force of block 1 on block 2, which is P12=m2*ax and substitute the value of a in, which, with a little manipulation, gives (m2/(m1+m2))*F

That should work.

[FunkyDwarf]

ok so i was sort of right :P

tbh ive never actually seen the contact forumula thing, but im not doing mech eng or anything :S

[jlayedra]

the answer is 31 N.
1. draw free body diagrams for all forces
2. m1 has two forces acting on it 43 N directed to right and -P(the force resulting from mass 2 on mass 1) directed to the negative x axis
3. the sum of forces in x-direction of mass 1 is
sum of forces=43-P=mass1 x acceleration
4. the sume of forces in x-direction of mass 2 is:
sum of forces =+P=mass 2 x acceleration
5. note -P=P, as newton's third law
6. setting up your equations you have
-P=mass one x acceleration -43 and
p= mass two x acceleration
7. since you have two equations with two unknowns (namely P and acceleration, you need something else)
8. since you are working in a horizontally frictionless environment and the two masses are accelerating together as a unit, you can find the acceleration by:
force = mass x acceleration
43/ (24 + 62) =0.5 m/s^2
9. now plug in a=0.5 in any of the two equations to find P
10. P=0.5 x (62 for mass 2) = 31 N or
11. -P=0.5 x (24 for mass 1) - 43 = 31 N