A block is initially moving right at 7 m/s on frictionless surface. After colliding with a wall on the right, it bounces back and moves left at 4 m/s. The mass of the block is 3 kg. Calculate the impulse applied to the block.

I know that Impulse is equal to the change in linear momentum, identified by the vector P, which would be P_final minus P_initial. I know P is defined as mass times velocity. Given the values above, I calculate an Impulse of 33 kgm/s; however, the answer depicts a negative value, why?

Question

A block is initially moving right at 7 m/s on frictionless surface. After colliding with a wall on the right, it bounces back and moves left at 4 m/s. The mass of the block is 3 kg. Calculate the impulse applied to the block.

I know that Impulse is equal to the change in linear momentum, identified by the vector P, which would be P_final minus P_initial. I know P is defined as mass times velocity. Given the values above, I calculate an Impulse of 33 kgm/s; however, the answer depicts a negative value, why?

gollywolly · Answer

Force is a vector. Because the original direction is the positive x direction. To reverse the direction the force must be in the opposite direction which is negative in this case.