Answer:
Explanation:
Given
mass of cannon and cart is 
Spring constant 
mass of Projectile 
Launch Velocity of cannon 
Launch angle 
As the External Force is zero therefore we can conserve momentum
Initially both Projectile and cannon is at rest
Conserving momentum in horizontal direction




i.e. cannon is moving in opposite direction of Projectile
Answer:last choice is correct. Work is equally to force times distance
Explanation:to generalize, W is equal to to force as a function of distance integrated over distance. Work is equivilent to energy. See work/force equivalency
Answer:

Explanation:
The center of mass of the system (two girls) is constant, as the velocity of the center of mass of the system is also constant.

<u>The initial velocity of the system is zero, since both girls are at rest.</u> So the velocity of the total system at any point should be zero as well.

This is true, because there is no friction between the girls and the ground. Otherwise, the velocity of the center of mass wouldn't be constant.
Heat!!!!!!!!!!!!!!!!!!! sorry about the exclamation marks but it wont let me post in less it has more than 20 characteristics