# Simple projectile motion - problem with units

• Jack_Straw
In summary: I'm pretty sure you can't have a time in newtons.)In summary, the conversation is about a problem involving a ball being kicked towards a fence with given initial conditions and the goal of finding the time and height at certain points. The units mentioned in the questions, N and s, seem to be typos. The poster has tried to input their answers using different units but is still getting an incorrect message.
Jack_Straw

## Homework Statement

(a) A ball of mass 0.4 kg, initially at rest, is kicked directly toward a fence from a point 20 m away, as shown below.
The velocity of the ball as it leaves the kicker’s foot is 17 m/s at angle of 42◦ above the horizontal. The top of the fence is 5 m high. The ball hits nothing while in flight and air resistance is negligible.
The acceleration due to gravity is 9.8 m/s2.
Determine the time it takes for the ball to reach the plane of the fence.

(b) How far above the top of fence will the ball pass? Consider the diameter of the ball to be
negligible. Answer in units of s.

## Homework Equations

irrelevant for question

## The Attempt at a Solution

This problem seems quite easy. The problem is that its asking for the answer in units of N. How can I express time in Newtons?

The same problem occurs in question b. I know how to calculate the answer, but how can I express height in seconds?

I have tried inputting the answers as if they are supposed to be in seconds for number one and feet for number two (assuming the "answer in units of N" and "answer in units of s" are typos) but I'm greeted with the message, "sorry that answer is incorrect, try again."

What am I missing here?

Last edited:
Welcome to Physics Forums.

Jack_Straw said:

## The Attempt at a Solution

This problem seems quite easy. The problem is that its asking for the answer in units of N. How can I express time in Newtons?

The same problem occurs in question b. I know how to calculate the answer, but how can I express height in seconds?

I have tried inputting the answers as if they are supposed to be in seconds for number one and feet for number two (assuming the "answer in units of N" and "answer in units of s" are typos) but I'm greeted with the message, "sorry that answer is incorrect, try again."

What am I missing here?

It really does appear that those units are typos, in both (a) and (b). (And hopefully your answer for (b) was in meters, not feet?)

## 1. What is simple projectile motion?

Simple projectile motion is the motion of an object through the air or other medium under the force of gravity. It follows a parabolic path and only experiences the force of gravity.

## 2. What are the basic equations for simple projectile motion?

The basic equations for simple projectile motion are:
- Horizontal distance: x = v0 * t
- Vertical distance: y = v0 * t - 1/2 * g * t2
- Velocity in the horizontal direction: vx = v0
- Velocity in the vertical direction: vy = v0 - g * t

## 3. How do units affect simple projectile motion problems?

Units are important in simple projectile motion problems because they determine the scale of the problem and can affect the outcome of the calculations. It is important to use consistent units for all variables in the equations.

## 4. What are common units used in simple projectile motion problems?

The most common units used in simple projectile motion problems are meters (m) for distance, meters per second (m/s) for velocity, and seconds (s) for time. However, other units such as feet (ft), miles per hour (mph), and milliseconds (ms) can also be used.

## 5. How do you solve a simple projectile motion problem with units?

To solve a simple projectile motion problem with units, follow these steps:
1. Identify the given variables and their units.
2. Convert all units to a consistent system if necessary.
3. Plug the values into the appropriate equations for horizontal and vertical motion.
4. Solve for the unknown variable.
5. Check your answer by plugging it back into the equations and ensuring that the units are consistent.

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