[alsoccer213]

Two crates, of mass 74 kg and 108 kg, are in contact and at rest on a horizontal surface. A 731 N force is exerted on the 74 kg crate. The kinetic friction is .15

How do I find the acceleration?

[jiten55]

Total Normal Reaction = (74 + 108) g = 182 g

Total Friction Force = .15 * 182 * g

Net Force = 731 - .15 * 182 * g

Acceleration = Force/Mass

= (731 - .15 * 182 * g)/182

= (731 - .15 *182* 9.8)/182

= 2.546

[itsme_vipsdude]

The bodies are not lying on each oter but they ar just in contact... so the above work out is wrong according to me

[tjsail]

I agree with jiten55. It turns out that the force of friction is the same whether one is sitting on the other or if they are both sitting on the surface. You might quibble and say the way the equation is written it implies one is sitting on the other (contrary to the wording of the problem), but it comes out the same.

Total Friction Force = .15*(74 kg + 108 kg) g = .15*74 kg*g + .15*108 kg*g

There's some law from first year of algebra that says that's true.