One Dimensional Motion Question

In summary, the pelican must be spotted at a minimum height of 4.6m for the fish to escape, assuming the fish is at the surface of the water and the pelican starts its dive from a height of 18.0m. This calculation is based on the equation Δy=VoyT+0.5gT^2, where Δy is the change in height, Voy is the initial velocity (which is assumed to be 0m/s for the pelican), g is the acceleration due to gravity (9.81m/s^2), and T is the time it takes for the pelican to reach the surface of the water.
  • #1

Homework Statement

Pelicans tuck their wings and free-fall straight down when diving for fish. Suppose a pelican starts its dive from a height of 18.0m and cannot change its path once committed. If it takes a fish 0.25s to perform evasive action, at what minimum height must it spot the pelican to escape? Assume the fish is at the surface of the water. (Answer must be in 2 significant figures.)

Homework Equations


The Attempt at a Solution

I tried to go about solving this but it seems I am still getting the wrong answer.


Physics news on
  • #2
It looks ok to me except for significant figures.
  • #3
Tried 4.0 tells me I got the wrong answer.
  • #4
I'll try giving this problem a shot.

We know that the initial height is 18m, and the final height for when the pelican swoops down is 0m.
From the equation Δ y= VoyT + .5gT2, we have to plug Yf as 0 m, Yi as 18m,and Voy as 0m/s.

-18m= .5gT2


(pelican to catch fish)-(time for fish to react)
Yf-Yi= .5gT2

Yf-18= .5gT2

So,it needs to see the pelican at 4.6m.

Sorry if I didn't write my English well enough.
Last edited:
  • #5
One of my previous answers I entered was 4.7m. A slight discrepancy of about .1m and it still tells me I'm wrong so not exactly sure why it's telling me that. The concept and mathematics is suppose to work.

1. What is one dimensional motion?

One dimensional motion refers to the motion of an object along a straight line, in which the object's position can be described by a single coordinate (usually distance) with respect to a reference point.

2. What are the key factors that affect one dimensional motion?

The key factors that affect one dimensional motion are the object's initial position, velocity, and acceleration, as well as any external forces acting on the object.

3. What is the difference between speed and velocity in one dimensional motion?

Speed is the rate at which an object is moving, while velocity is the rate at which an object is changing its position. Velocity takes into account the direction of motion, whereas speed does not.

4. How is distance related to displacement in one dimensional motion?

Distance refers to the total length traveled by an object, while displacement refers to the change in an object's position. In one dimensional motion, distance and displacement will be equal only if the object moves in a straight line without changing direction.

5. Can an object's velocity and acceleration be in opposite directions in one dimensional motion?

Yes, an object's velocity and acceleration can be in opposite directions in one dimensional motion. This can occur when the object is slowing down or changing direction, resulting in a negative acceleration while still moving in a positive direction.

Suggested for: One Dimensional Motion Question