Force when 2 blocks pushing against each other

[adrian783]

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.
What is the magnitude of the force of the car on the truck?
What is the magnitude of the force of the truck on the car?

for the first one
i did 4500/1000 = car's acc = 4.5
4.5 *2000 (trucks mass) = 9000 = wrong

can some1 tell me what i did wrong??

 

[ktd]

I have the same problem - I'm confused, too!

 

[adrian783]

k ktd
here's how you do it lol
see 2 blocks as one 4500 / 3000 = 1.5 (both's acc)
f = ma, so force of car on truck = 1.5 * 1000 (car's mass) = 1500
now, the car's wheel produces 4500 force, we said 1500's car pushing truck, so where did the other 3000 go??
its truck pushing back
<----- 3000 (truck on car) CAR ---(car on truck)1500---->

 

[ktd]

Ok, are you using mastering physics? when I put in those answers (which now make sense), I'm still being told they're wrong!

 

[Doc Al]

see 2 blocks as one 4500 / 3000 = 1.5 (both's acc)

Right.

f = ma, so force of car on truck = 1.5 * 1000 (car's mass) = 1500

That's the total force acting on the car (you applied F = ma to the car). To find the force on the truck, apply F = ma to the truck.

now, the car's wheel produces 4500 force, we said 1500's car pushing truck, so where did the other 3000 go??
its truck pushing back
<----- 3000 (truck on car) CAR ---(car on truck)1500---->

I hope you realize that this answer contradicts Newton's 3rd law! The force that the car exerts on the truck must be equal (and opposite) to the force that the truck exerts on the car.

Find the force acting on the truck. (Assume that the only force acting on the truck is what the car exerts.)