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Newton's Third Law: A Car Pushes A Truck 
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#1
Oct1507, 04:47 PM

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1. The problem statement, all variables and given/known data
A 1000 kg car pushes a 2000 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 4500 N. Rolling friction can be neglected. 2. Relevant equations F=ma acceleration of car = acceleration of truck 3. The attempt at a solution I have determined that both the car and the truck must move at the same acceleration. The car pushes off the ground with 4500N, so that: F = ma 4500 = (1000)a a = 4.5 m/s^2 I attempted to solve for F on the truck: F = ma F = (2000)(4.5) F = 9000 But this is incorrect. Should I be combining the masses so that F = (3000)(4.5)? Or am I just way off here? I am struggling to put together the proper freebody diagram and I think its messing up my understanding of what force is on what object. A push in the right direction is appreciated! Pun intended. 


#2
Oct1507, 05:12 PM

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You might want to draw a FBD of the enture truckcar system. What is the net force acting on the system? What is the mass of the system? Your FBD's are not correct since you have not properly identified all forces acting individually on the isolated FBD of the car or truck.



#3
Oct1507, 05:22 PM

P: 30

Hmmm, not exactly sure what you mean by a FBD of the entire system. I assume it would be a particle with a normal force, a weight force (consisting of the 1000 kg car + the 2000 kg truck), and the force of the car's tires on the ground (pointing to the right). That should be it since friction is neglected.
I haven't seen anything that combines the system into one FBD in my book... not sure how it helps me isolate the force on the truck? 


#4
Oct1507, 05:29 PM

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Newton's Third Law: A Car Pushes A Truck



#5
Oct1507, 05:49 PM

P: 30

Ok, I think I figured it out:
Fnet Car: 4500  Ftruck = (mass of car)(acceleration) Fnet Truck: Fcar = (mass of truck)(acceleration) Solve both for acceleration and set them equal to each other, remembering that Fcar = Ftruck from Newton's 3rd Law. (4500Ftruck)/(mass of car) = (Fcar)/(mass of truck) Ftruck = Fcar = 3000N 


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