# I dont understand this question about total combined momentum..help appreciated?

#### kiwisurf

What is the size and direction of the total combined momentum of the two trucks before they collide? Use conservation of momentum.

*The thing I dont understand is that I thought to get the size and direction of the total COMBINED momentum the two trucks have to collide first...otherwise how can i give 1 size and direction for two trucks...and combine it???

#### arze

What is the size and direction of the total combined momentum of the two trucks before they collide? Use conservation of momentum.

*The thing I dont understand is that I thought to get the size and direction of the total COMBINED momentum the two trucks have to collide first...otherwise how can i give 1 size and direction for two trucks...and combine it???
Conservation of momentum means that the total momentum before and after the collision are equal. So when it says total momentum before collision, it would just be the total momentum of the two trucks. If they are traveling in opposite directions, one momentum is positive and the other is negative because momentum is a vector quantity.

#### kiwisurf

thanks, so if one truck is coming from the left and the other is coming from the right (driving towards each other), how do I know what the final direction is?

#### arze

thanks, so if one truck is coming from the left and the other is coming from the right (driving towards each other), how do I know what the final direction is?
in physics we take the frame of reference that is most convenient as the one we use. usually in such cases, the truck with greater momentum is used as the frame of reference, that is, its momentum would be positive and the momentum of the other truck becomes negative since it is traveling in the opposite direction to the first truck. the final direction after the collision would be in the direction the truck with the greater momentum was traveling.

#### Pmb

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What is the size and direction of the total combined momentum of the two trucks before they collide? Use conservation of momentum.

*The thing I dont understand is that I thought to get the size and direction of the total COMBINED momentum the two trucks have to collide first...otherwise how can i give 1 size and direction for two trucks...and combine it???
Calculate the momentum for each truck then add those two results. The resulting value is the total combined momentum. The direction of the final momentum is the direction of the resulting vector.

Suppose that the masses of the trucks are the same and the truck comming from the left has the greater speed. The truck comming from the left will have the greater momentum. The total momentum vector will then point to the right.

The important thing to keep in mind that the velocity is a vector quantity. If you're working in one dimension then you can work with the component of the momentum only. The velocity will then have a signed value according to the direction the object is moving. If it's moving to the right then the velocity will have a negative sign. If the object is moving to the right then it will have a positive sign.

Example: Let each truck have a mass of m = 10,000 kg. Let the speed of the truck coming from the left be 10 m/s and the speed of the truck comming from the right be 11 m/s. The velocity of the left truck is V_L = 10 m/s. The velocity of the right truck is V_R = -11 m/s.

The momentum of the left truck is

P_L = mV_L = (10,000 kg) (10 m/s) = 100,000 kg m/s

The momentum of the right truck is

P_R = mV_L = (10,000 kg) (-11 m/s) = -110,000 kg m/s

The total momentum is P = P_L + P_R or

100,000 kg m/s -110,000 kg = -10,000 kg m/s

The total momentum is to the left and has a magnitude of 10,000 kg m/s.

Last edited:
• arze

#### arze

Thanks Pmb that is a really good explanation!

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