Answer:
If I had to guess I would say this is what the question fully looks like:
A block having mass m travels along a horizontal frictionless surface with speed vx. What impulse must be delivered to the mass to reverse its direction and continue to travel with a speed of vx?
A) -mvx
B) -2mvx
C) 0
D) 2mvx
E) mvx
F) none of the above
G) cannot be determined
Explanation: The answer is
The initial momentum is m vx. So what change do you need to have a momentum -m vx afterwards? -2 m vx, because m vx + (-2m vx) = - m vx
So the correct answer is b.
