Newton's THIRD

[anonuser0511]

IF any of you kids are still on and not doing your superstitious ritual crap i'd like to know...

A dog (mass 14kg) runs in a northerly direction across a beach with a speed of 3.0m/s and turns to run to the west at 3.0m/s

What is the magnitude and the direction of the impulse on the dog due to the sand?

Directions is what i'm stumped about

i thought since dog exerts it SW should the sand not be NE by Newton's 3rd?

[/0]

The change in the velocity of the dog is SW, therefore the impulse on the dog from the sand should be SW. The dog exerts an impulse on the sand in the direction NE, but that's not what we want to find.

[anonuser0511]

is this because the sand is what moves the dog therefore it determines the direction of the velocity?

[/0]

Yep, the sand's frictional force is what pushes the dog in the direction of its change in velocity.

[anonuser0511]

thanks bro

[Flaming_Arrow]

is impulse always in the opposite direction to motion?

[Damo17]

is impulse always in the opposite direction to motion?

No, say if a car traveling at 10m/s hits another car that is stationary the impulse= change in momentum. The change momentum of the car (initially traveling at 10m/s) will be negative so the impulse on that car will be opposite to the direction of motion. But for the stationary car, the impulse will be in the direction of motion.

[TonyHem]

Hey for Q14, How do you get South-West?
I thought he goes up north, goes west, so his direction would be NW <-[wrong]

[TrueTears]

Hey for Q14, How do you get South-West?
I thought he goes up north, goes west, so he's direction would be NW

By vector subtraction.

[/0]

Just imagine the dog going north, now what direction would you need to shove it to get it travelling west? South-west. So your impulse is SW and since impulse is in the same direction as change in velocity, that's the change in velocity too.