# Projectile Motion gun

## Homework Statement

"A 45.0 kg projectile is fired from a gun. The elevation angle of the gun is 30.0°. the projectile is in the air for 48.0 s before it hits the deck at the same height as it was fired. Ignoring friction find: the projectile's initial velocity, the horizontal distance the projectile traveled, and the potential energy of the projectile at it's highest point in its trajectory.

Weight: 45 kg
Angle of launch: 30°
Total time in air: 48 s

## Homework Equations

$$D = VT$$

## The Attempt at a Solution

We haven't done projectile motion problems since the beginning of the year. It seems this problem is a random one on the homework assignments. I've completely forgotten how to calculate these. I drew out the motion of it but it's not ringing any bells.

## Answers and Replies

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cepheid
Staff Emeritus
Science Advisor
Gold Member
Welcome to PF!

By symmetry, you know that the time it takes to reach the max height is half of the total travel time (think about it if you're not sure).

Since you know the vertical acceleration, and the time taken to reach the max height (i.e. the time needed to reduce the vertical velocity to zero), you can determine the initial vertical velocity.

Since you know the angle of launch, you can use the above result to determine the initial horizontal velocity, and from that, the total initial velocity.

Once you know the horizontal speed, you can figure out the total horizontal distance travelled.

The max height comes from basic kinematics. You already know the time taken to reach it, and the acceleration.

I hope this helps.

Since you know the vertical acceleration, and the time taken to reach the max height (i.e. the time needed to reduce the vertical velocity to zero), you can determine the initial vertical velocity.
Hey, thanks for your response! I don't quite understand how I have the vertical acceleration. Do I need to find it from the weight/gravity/time?

cepheid
Staff Emeritus
Science Advisor
Gold Member
Hey, thanks for your response! I don't quite understand how I have the vertical acceleration. Do I need to find it from the weight/gravity/time?
What defines projectile motion is that the particle is solely under the influence of gravity. That's the only force that acts on it. So, the vertical acceleration is due to gravity.