# Homebrew Physics Problem: Projectile Motion with Camera Observation

• mettw
In summary, the conversation discusses the best way to learn a subject, creating physics problems, and the connection between mathematics and physics. The first problem involves a camera and a gun, while the second problem involves a skateboarder on a hill. Both problems neglect air resistance and friction. The conversation also touches on the relationship between mathematicians and physics.
mettw
I'm not sure if this is really the right forum for this, but the other forums insisted that all homework like posts should go here.

In his book "How to think like a Mathematician" Kevin Houston says that the best way to learn a subject is to create your own problems, since doing this requires far more understanding than simply selecting and applying the correct algorithm. So I thought I'd try my hand at creating some physics problems as I go through my old textbooks and post them online for anyone who is interested. Again, I apologise if this is the wrong forum for this.

Problem mettw-1

A camera at a proving ground has a field of view of 18$$^{o}$$ and is tilted at an angle of 31$$^{o}$$ to the horizontal. 9.9km distant a gun fires a shell directly up. The shell enters the field of view of the camera and then leaves it 15.6s later. The shell then explodes when it reaches its maximum possible height. Neglecting air resistance:

• At what height does the projectile explode?
• How long does it take to get there?
• What is the muzzle velocity of the gun?

Is the camera at the same elevation as the gun?

Many mathematicians I know don't particularly like physics.

SammyS said:
Is the camera at the same elevation as the gun?

Yes.

Many mathematicians I know don't particularly like physics.

I don't know if that would be a general feeling. After all, two of the millennium problems are physics problems.

Motion in 1 dimension - Problem 2

A skateboarder finds a long straight stretch of road of constant gradient. The road is 150m long and 100m down the hill is a speed camera. The skateboarder starts from rest at the top of the hill and as he passes the speed camera it goes off. Later in the local newspaper he sees the speed camera photo of himself with an article saying that he was photographed doing 65Km/h.

Neglecting air resistance and friction:

• What is the gradient of the hill as a percentage?
• How fast was he going when he reached the bottom of the hill?
• Are these results physically plausible? What does this say about the assumptions?

I appreciate your approach of creating your own problems to deepen your understanding of a subject. However, I would like to remind you that it is important to also have a solid foundation of knowledge and understanding of the principles and equations involved in projectile motion before creating your own problems. This ensures that the problems are accurate and relevant to real-world scenarios.

To answer your questions, we can use the equations of motion for projectile motion:

1) The height at which the projectile explodes can be calculated using the equation h = h0 + v0t - (1/2)gt^2, where h0 is the initial height (which we can assume to be 0), v0 is the initial velocity, t is the time taken, and g is the acceleration due to gravity. We can rearrange this equation to solve for h: h = v0t - (1/2)gt^2. Plugging in the given values, we get h = 0 + v0(15.6) - (1/2)(9.8)(15.6)^2 = 1191.6 meters. Therefore, the projectile explodes at a height of 1191.6 meters.

2) The time taken for the projectile to reach its maximum height can be calculated using the equation v = v0 - gt, where v is the final velocity (which we can assume to be 0 at the maximum height), v0 is the initial velocity, and g is the acceleration due to gravity. We can rearrange this equation to solve for t: t = v0/g. Plugging in the given values, we get t = v0/9.8. To find v0, we can use the equation v^2 = v0^2 - 2gh, where v is the final velocity (which we can assume to be 0 at the maximum height), v0 is the initial velocity, g is the acceleration due to gravity, and h is the height. We can rearrange this equation to solve for v0: v0 = √(2gh). Plugging in the given values, we get v0 = √(2(9.8)(1191.6)) = 109.4 m/s. Therefore, the time taken for the projectile to reach its maximum height is t = 109.4/9.8 = 11.2 seconds.

3) The muzzle velocity

## 1. What are "Homebrew physics problems"?

"Homebrew physics problems" are physics questions or challenges that are created and shared by individuals or groups outside of a formal academic or research setting. They are often created for the purpose of learning and exploring physics concepts in a hands-on and interactive way.

## 2. How are "Homebrew physics problems" different from traditional physics problems?

While traditional physics problems are typically presented in textbooks or assigned by teachers for students to solve, "Homebrew physics problems" are often more open-ended and require creativity and experimentation to find solutions. They also often involve building or creating physical models or apparatus to test hypotheses.

## 3. Who can participate in "Homebrew physics problems"?

Anyone with an interest in physics and a willingness to engage in hands-on experimentation can participate in "Homebrew physics problems". They are often popular among hobbyists, educators, and students.

## 4. Are there any benefits to solving "Homebrew physics problems"?

Yes, solving "Homebrew physics problems" can have many benefits. They can help improve critical thinking and problem-solving skills, deepen understanding of physics concepts, and foster creativity and innovation. They can also be a fun and engaging way to learn about physics.

## 5. Where can I find "Homebrew physics problems" to solve?

"Homebrew physics problems" can be found online through various websites and forums dedicated to physics, as well as through social media groups and communities. They can also be created and shared among friends or classmates. Additionally, some educational institutions may have resources or workshops dedicated to "Homebrew physics problems".

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