Question dealing with friction
A team of eight dogs pulls a sled with waxed wood runners on wet snow (mush!). The dogs have average masses of 19.0 kg, and the loaded sled with its rider has a mass of 210 kg. (a) Calculate the magnitude of the acceleration starting from rest if each dog exerts an average force of 185 N backward on the snow. (b) What is the magnitude of the acceleration once the sled starts to move? (c) For both situations, calculate the magnitude of the force in the coupling between the dogs and the sled.
So for A, I tried finding the static friction between only the sled and snow, the coefficient of kinetic friction is 0.1 and static friction is .14. I tried doing 0.1 times the normal force that the ground exerts on the sled (9.8*210), then subtracting the force that the dogs are exerting backwards on the snow and also subtracting friction since these forces are opposing motion. However, the answer I get for acceleration is too small compared to the correct answer, which I found is supposed to be 3.29m/s^2. If someone could help me understand where I'm going wrong, I'd really appreciate it!
Answer
It sounds like the issue might be in how you are adding and subtracting the forces, though your wording is a bit ambiguous as to your exact approach. Looking ahead in the problem, it seems that constructing a full force diagram of the system might be necessary. While this could be complicated and tedious for this particular problem, part c) will be easy to mess up without it.
For part a), the diagram should let you see the directions of the forces (that don't cancel out internally) and add them accordingly. If you want to avoid doing the diagram for now, I recommend thinking about what conceptually should be pushing the sled forward and what should be pulling it back and basing your math on that. Also, make sure you are using the correct coefficient of friction.
If none of this helps, upload a picture of your work so that I can better understand where the issue might lie.
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It sounds like the issue might be in how you are adding and subtracting the forces, though your wording is a bit ambiguous as to your exact approach. Looking ahead in the problem, it seems that constructing a full force diagram of the system might be necessary. While this could be complicated and tedious for this particular problem, part c) will be easy to mess up without it.
For part a), the diagram should let you see the directions of the forces (that don't cancel out internally) and add them accordingly. If you want to avoid doing the diagram for now, I recommend thinking about what conceptually should be pushing the sled forward and what should be pulling it back and basing your math on that. Also, make sure you are using the correct coefficient of friction.
If none of this helps, upload a picture of your work so that I can better understand where the issue might lie.