Question

A heavy bar falls straight down onto the bed of a horizontally rolling truck. What happens to the momentum of the truck at the instant the bar lands on it?

How many correct answers do you think are possible? Make sure you think of what "falls straight down" means.

When the bar falls straight down, it has zero velocity in the horizontal direction either with respect to the truck or with respect to Earth. However, the behavior of the truck would be the same in both cases, so the problem has one answer.

When the bar falls straight down, it has zero velocity in the horizontal direction either with respect to the truck or with respect to Earth. Thus, the problem can have two answers.

When the bar falls straight down, it has zero velocity in the horizontal direction either with respect to the truck or with respect to Earth, or it has the horizontal velocity with the same magnitude as the truck. Thus, the problem can have three answers.

Answer 1

Straight down may imply that the bar is falling vertically down with respect to earth or with respect to the truck.

1) If the bar was falling vertically with respect to the earth, then it has no horizantal momentum. So after falling the bar will share some of the truck's momentum. So truck's momentum is reduced.

2) If the bar was falling vertcally with respect to the truck, that implies the bar and truck will be moving at the same speed and after falling too they both move at the same speed horizantally. So the momentum of the truck will not change.

So, the answer is : Option (b) - there can be two answers.