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A ball rolls of the top of stair-way with a horizontal velocity of magnitude \(1.8 \mathrm{~ms}^{-1}\). The steps are \(0.20 \mathrm{~m}\) high and \(0.20 \mathrm{~m}\) wide. Which step will the ball hit first? (a) First (b) Second (c) Third (d) Fourth

Short Answer

Expert verified
The ball skips the first three steps; thus, it first hits the fourth step.

Step by step solution

01

Determine the time to reach each step

The ball falls with projectile motion due to gravity. Let's first calculate the time it takes to fall each vertical distance. The first step is to use the formula for vertical motion \[ y = \frac{1}{2} g t^2 \]where \(y\) is the vertical distance, and \(g = 9.8 \text{ m/s}^2\) is acceleration due to gravity. For each vertical step \(0.2 \text{ m}\), solve for \(t\).* Step 1 vertical calculation: Substitute \(y = 0.2\) into the equation: \[ 0.2 = \frac{1}{2} \times 9.8 \times t^2 \] \[ t^2 = \frac{0.2 \times 2}{9.8} \] \[ t^2 = \frac{0.4}{9.8} \approx 0.0408 \] \[ t \approx 0.202 \text{ s} \]
02

Calculate horizontal distance for each step

We calculate how far the ball travels horizontally in this time using the formula for horizontal distance because velocity in the horizontal direction is constant:\[ x = v_x t \]where \(v_x = 1.8 \text{ m/s}\).For step 1:\[ x_{1} = 1.8 \times 0.202 \approx 0.3636 \text{ m} \]So, for step 1, the ball travels \(0.3636\) meters horizontally.
03

Determine the width of steps

Each step has a width of \(0.2 \text{ m}\). We need to check if the horizontal distance covered for each corresponding time calculated in Step 1 exceeds the width of the step.
04

Compare horizontal distance to step widths

For the first step, the distance \(0.3636 \text{ m}\) exceeds the horizontal width \(0.2 \text{ m}\), indicating that the ball will skip over it since the ball will land further than the first step width.Perform similar calculations for the second step:1. Time to fall the height of two steps: \[ y = 0.4 \text{ m} \] \[ 0.4 = \frac{1}{2} \times 9.8 \times t^2 \] \[ t^2 = \frac{0.8}{9.8} \approx 0.0816 \] \[ t \approx 0.286 \text{ s} \]2. Horizontal distance: \[ x_{2} = 1.8 \times 0.286 \approx 0.5148 \text{ m} \] Since \(0.5148 \text{ m} > 0.4 \text{ m}\), it will skip the second step as well.Now for the third step:3. Time to fall the height of three steps: \[ y = 0.6 \text{ m} \] \[ 0.6 = \frac{1}{2} \times 9.8 \times t^2 \] \[ t^2 = \frac{1.2}{9.8} \approx 0.1224 \] \[ t \approx 0.350 \text{ s} \]4. Horizontal distance: \[ x_{3} = 1.8 \times 0.350 \approx 0.630 \text{ m} \] The distance \(0.630 \text{ m} > 0.6 \text{ m}\) exceeds the next step's foot, so it skips the third step.For the fourth step, let’s perform similar calculations, but since the previous results underestimate the specific step the ball will fall on after further horizontal consideration, the next detailed calculations involve analyzing exact positioning given previous approximations.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Vertical Motion
In projectile motion, vertical motion involves only the influence of gravity. This means that regardless of the object's horizontal speed, gravity pulls it down at a constant acceleration. This acceleration is known as "acceleration due to gravity" and is symbolized by the letter "g." For Earth, the value of "g" is approximately 9.8 meters per second squared (m/s²). When calculating how long it takes for an object to fall a certain vertical distance, we use the formula:
  • \[ y = \frac{1}{2} g t^2 \]
Here, "y" is the vertical distance, "g" is acceleration due to gravity, and "t" is the time taken. To find the time "t" for a ball to fall, you substitute the known values into the equation.
For instance, if the vertical distance is 0.20 meters, solve for "t" by replacing "y" with 0.20 in the formula: \[ 0.20 = \frac{1}{2} \times 9.8 \times t^2 \]. Solving this will provide the time taken for the ball to fall this vertical distance.
Horizontal Motion
Horizontal motion in projectile problems happens at a constant speed. This is because no forces act on the object horizontally, unless specified, so it keeps moving at the same speed it started with. In the context of our problem where a ball is rolling off a stairway, the initial horizontal velocity (\( v_x \)) remains unchanged as there is no horizontal acceleration.

The formula to calculate the horizontal distance covered over a time period is:
  • \[ x = v_x \times t \]

This means that you multiply the horizontal speed (\( v_x = 1.8 \, \text{m/s} \)) by the time it fell (\( t \)) to calculate how far it travels horizontally in that duration. In our case, to find how much horizontal distance is covered before hitting a step, you use the time calculated for falling vertically and plug it into the horizontal distance formula. If the calculated horizontal distance is greater than the width of a step (0.2 meters), it will skip that step.
Acceleration due to Gravity
The acceleration due to gravity is the constant rate at which objects accelerate towards the Earth. It is a critical factor in vertical motion calculations but does not affect the horizontal motion directly. This value is always used as 9.8 m/s² on Earth.
This properties of gravity are taken into account during vertical motion calculations, such as finding the time to fall a certain height. Acceleration due to gravity means that an object will increase its velocity by 9.8 m/s every second it is in free fall.

To see its effects, notice how the time required to fall increases as the height increases. By using the formula \( y = \frac{1}{2} g t^2\) and iteratively increasing the height "y", you can calculate how gravity influences the speed at which an object hits each subsequent step.

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