Calculating work done on a truck by friction when a truck is pushing a car

AI Thread Summary
A truck exerts a horizontal force of 500 N to push a car, while a frictional force of 300 N opposes its motion over a distance of 4 m. The work done by friction is calculated as 1200 J, but this does not represent the work done on the truck. To find the work done on the truck, the net force must be considered, which is the difference between the pushing force and the frictional force. Thus, the correct calculation for the work done on the truck is 800 J, derived from the equation W = (500 N - 300 N) x 4 m. Understanding the distinction between the forces acting on the truck and the car is crucial for accurate calculations.
kristy hardy
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Homework Statement



A truck pushes a car by exerting a horizontal force of 500 N on it. A frictional force of 300N opposes the cars motion as it moves 4m. Calculate the work done on the truck.


Homework Equations



work = force x distance


The Attempt at a Solution



work done = 300N x 4 m = 1200J work right?
 
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kristy hardy said:

Homework Statement



A truck pushes a car by exerting a horizontal force of 500 N on it. A frictional force of 300N opposes the cars motion as it moves 4m. Calculate the work done on the truck.


Homework Equations



work = force x distance

For constant forces, the work would be:

<br /> W=F d\cos\theta<br />

where \theta is the angle between the force diretion and the displacement direction.


The Attempt at a Solution



work done = 300N x 4 m = 1200J work right?

No, I believe that would be the magnitude of the work done by friction. Remember that if we want to calculate the work done on the truck, you have to use the forces that act on the truck.
 
My contribution: since the work done by resulting force on the body is the path integral along the path and this is a scalar (inner product from two vectors), is straightforward that the work of resulting force shall be the algebraic sum of work from each force acting on the considered body; in this way:

W = 500N x4 m - 300N x 4m = 800J

Danpos.
 
kristy hardy said:

Homework Statement



A truck pushes a car by exerting a horizontal force of 500 N on it. A frictional force of 300N opposes the cars motion as it moves 4m. Calculate the work done on the truck.


Homework Equations



work = force x distance


The Attempt at a Solution



work done = 300N x 4 m = 1200J work right?

Not correct: the 300N force is NOT the force acting on the truck.
 
The title of your post asks a different question that your post itself: which one is the actual question? And, don't you have more info?
 
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