Why Are the Signs Reversed in the Baseball Momentum Problem?

[bona0002]

Hey guys,

I'm currently taking the first physics course right now (calculus-based physics), but it's not going as smoothly as I would've hoped. Anyways, here's one of the many questions I shall be posing on the forum: A baseball approaches home plate at a speed of 47.0 m/s, moving horizontally just before being hit by a bat. The batter hits a pop-up such that after hitting the bat, the baseball is moving at 50.0 m/s straight up. The ball has a mass of 145 g and is in contact with the bat for 1.80 ms. What is the average vector force the ball exerts on the bat during their interaction?

Now, I know how to draw the diagram to visualize the approach, I know that ρ = m * v, and I know that force = Δp/Δt. When I get my final answer, the components of the average force are (-3786i + 4028j)N. However, the answer has the signs reversed. Why is that the case?

Thanks in advance for the help!

 

[gneill]

Hey guys,

I'm currently taking the first physics course right now (calculus-based physics), but it's not going as smoothly as I would've hoped. Anyways, here's one of the many questions I shall be posing on the forum: A baseball approaches home plate at a speed of 47.0 m/s, moving horizontally just before being hit by a bat. The batter hits a pop-up such that after hitting the bat, the baseball is moving at 50.0 m/s straight up. The ball has a mass of 145 g and is in contact with the bat for 1.80 ms. What is the average vector force the ball exerts on the bat during their interaction?

Now, I know how to draw the diagram to visualize the approach, I know that ρ = m * v, and I know that force = Δp/Δt. When I get my final answer, the components of the average force are (-3786i + 4028j)N. However, the answer has the signs reversed. Why is that the case?

Thanks in advance for the help!

Hi bona0002, Welcome to Physics Forums.

The signs will depend upon your choice of coordinate axes. Be sure to apply them consistently for all vector quantities (velocities, momenta).

 

[rude man]

Hey guys,

I'm currently taking the first physics course right now (calculus-based physics), but it's not going as smoothly as I would've hoped. Anyways, here's one of the many questions I shall be posing on the forum: A baseball approaches home plate at a speed of 47.0 m/s, moving horizontally just before being hit by a bat. The batter hits a pop-up such that after hitting the bat, the baseball is moving at 50.0 m/s straight up. The ball has a mass of 145 g and is in contact with the bat for 1.80 ms. What is the average vector force the ball exerts on the bat during their interaction?

Now, I know how to draw the diagram to visualize the approach, I know that ρ = m * v, and I know that force = Δp/Δt. When I get my final answer, the components of the average force are (-3786i + 4028j)N. However, the answer has the signs reversed. Why is that the case?

Thanks in advance for the help!

If the i direction represents direction from home plate to center field then the i component of force should be +. But then the y-axis must point dow, a strange choice in my opinion but entirely permissible.

 

[bona0002]

That makes sense. Thanks guys!