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while sitting on a tree branch \(10.0 \mathrm{m}\) above the ground, you drop a chestnut. When the chestnut has fallen \(2.5 \mathrm{m}\), you throw a second chestnut straight down. What initial speed must you give the second chestnut if they are both to reach the ground at the same time?

Short Answer

Expert verified
The second chestnut must be thrown downward with an initial speed of approximately 16.26 m/s.

Step by step solution

01

Analyze the motion of the first chestnut

The first chestnut is dropped from a height of 10.0 m above the ground. It falls freely under the influence of gravity. To find the time it takes to fall 7.5 m to the ground, use the equation of motion:\[ s = ut + \frac{1}{2}gt^2 \]where \(s = 7.5 \text{ m}\), \(u = 0 \text{ m/s}\) (since it is dropped), and \(g = 9.8 \text{ m/s}^2\).
02

Calculate time for first chestnut to reach the ground

Substituting the known values into the equation:\[ 7.5 = 0 \cdot t + \frac{1}{2} \cdot 9.8 \cdot t^2 \]This simplifies to:\[ 7.5 = 4.9t^2 \]Solving for \(t\), we get:\[ t^2 = \frac{7.5}{4.9} \]\[ t = \sqrt{\frac{7.5}{4.9}} \approx 1.24 \text{ seconds}\]
03

Analyze the motion of the second chestnut

The second chestnut starts traveling after the first chestnut falls 2.5 m. This means it must fall the entire 10.0 m, but it has less time to reach the ground. The delay before it is thrown is the time it takes the first chestnut to fall 2.5 m. Use the same equation of motion for this calculation:\[ 2.5 = \frac{1}{2} \cdot 9.8 \cdot t^2 \]
04

Calculate the delay for second chestnut

Solving the previous equation for \(t\) gives the time delay:\[ t^2 = \frac{2.5}{4.9} \]\[ t = \sqrt{\frac{2.5}{4.9}} \approx 0.71 \text{ seconds}\]Thus, the second chestnut is thrown 0.71 seconds after the first begins its fall.
05

Determine remaining time for the second chestnut

The first chestnut takes approximately 1.24 seconds to reach the ground. The second chestnut has 1.24 seconds minus 0.71 seconds to fall the entire 10.0 m. This gives the second chestnut approximately:\[ t = 1.24 - 0.71 = 0.53 \text{ seconds}\] to reach the ground.
06

Calculate initial velocity for second chestnut

For the second chestnut starting with initial velocity \(v_0\) and falling under gravity, use the equation:\[ s = v_0 t + \frac{1}{2}gt^2 \]\[ 10 = v_0 \cdot 0.53 + \frac{1}{2} \cdot 9.8 \cdot (0.53)^2 \]Solving for \(v_0\), we rearrange and calculate:\[ 10 = v_0 \cdot 0.53 + 1.38 \]\[ v_0 \cdot 0.53 = 10 - 1.38 \]\[ v_0 = \frac{8.62}{0.53} \approx 16.26 \text{ m/s}\]
07

Conclusion: The required initial speed

The second chestnut needs an initial speed of approximately 16.26 m/s downwards to reach the ground at the same time as the first chestnut.

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Key Concepts

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

Equations of Motion
When dealing with projectile motion, the equations of motion help us analyze the path and timing of objects in motion under gravity. These equations relate initial velocity, time, acceleration, and displacement, making them essential tools for solving physics problems. Here is a breakdown of the key equation used:For any object moving in a straight line under uniform acceleration, such as gravity, the following equation applies:\[s = ut + \frac{1}{2}gt^2\]- **s** is the displacement- **u** is the initial velocity- **g** represents gravitational acceleration (approximately 9.8 m/s² on Earth's surface)- **t** stands for timeThis equation is crucial when analyzing motion since it allows us to calculate the time an object takes to reach a certain point if we know its initial velocity and how far it needs to travel. By manipulating this equation, we determined the time it takes for two chestnuts to reach the ground in our exercise.
Free Fall
Free fall is the motion of an object when it is falling solely under the influence of gravity, with no other forces acting on it (like air resistance). In free fall conditions: - Objects accelerate downwards due to gravity at a rate of 9.8 m/s². - The initial velocity is zero if an object is simply dropped, not thrown. In our exercise, the first chestnut perfectly illustrates free fall. It begins with no initial speed as it is merely dropped from a height of 10 meters, relying solely on gravity to pull it to the ground. By calculating the time it takes to fall 7.5 meters (since it already fell 2.5 meters initially), we understood how long it would take to reach the ground. Understanding free fall helped in determining when to release the second chestnut to ensure they both hit the ground simultaneously.
Initial Velocity
The initial velocity is the speed at which an object begins its motion. In physics problems, initial velocity can significantly influence how an object travels over time. It is important to:- Identify if an object starts from rest (initial velocity of 0).- Determine if an object is thrown or launched, affecting its starting speed.In our exercise, the second chestnut requires a calculated initial velocity to ensure it completes the 10-meter fall in the same time frame as the first chestnut. We used the equation of motion:\[s = v_0 t + \frac{1}{2}gt^2\]where **v_0** is the initial velocity that needs to be calculated. By establishing that the second chestnut must hit the ground in 0.53 seconds (after accounting for the initial delay), we solved this equation to find an initial downward speed of approximately 16.26 m/s. Understanding the concept of initial velocity allowed us to plan the required conditions for both chestnuts to coincide their landing.

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