To help reduce the severity of accidents [Average net force]

[LionLieOn]

Homework Statement

To help reduce the severity of accidents, an engineering company designs large plastic barrels filled with antifreeze that can be placed in front of bridge supports. In a simple test, a 1200 kg car moving at 20 m/s [W] crashes into several barrels. The car slows down to 8.0 m/s [W] in 0.40 s.

a) Find the average net force acting on the car during the collision.

I was comparing answers with a friend of mine and his final answer was 36000 N [W] and I got 36000 N [E]

I think I'm right, since 36000N [W] doesn't make any sense, the car would of had to be driving E for that to happen. Then again, I'm unsure so I would like to have a second opinion/correction.

Homework Equations

[/B]

The Attempt at a Solution

Check the attachment.

 

[andrevdh]

Newton's 2nd law tells us that the force and the acceleration will be in the same direction.

 

[LionLieOn]

Newton's 2nd law tells us that the force and the acceleration will be in the same direction.

Ahh! Ok I forgot about that thank you!

Other then the direction, is my work correct? Anything I should correct or look over?

 

[andrevdh]

No, you seem to be on the right track :)

 

[andrevdh]

Looking at the provided solution the velocities before and after were inserted as positive values because + was chosen W.
The fact that impulse came out negative tells us that it is in the opposite or easterly direction ,which is also the direction of the force, since the time interval is not involved in the direction since it is a scalar quantity.

 

[LionLieOn]

Looking at the provided solution the velocities before and after were inserted as positive values because + was chosen W.
The fact that impulse came out negative tells us that it is in the opposite or easterly direction ,which is also the direction of the force, since the time interval is not involved in the direction since it is a scalar quantity.

So to correct my mistake in the attached file above, I should be putting 36000 N [W] instead of 36000 N [E] ?

 

[andrevdh]

No, the attached file is correct.
F_net comes out negative so it is in the opposite direction of the chosen + west.

 

[LionLieOn]

No, the attached file is correct.
F_net comes out negative so it is in the opposite direction of the chosen + west.

Ahh ok!

Thank you so much for your help and for checking over my work. Much appreciated!