# Tag Info

### The trajectory of a projectile launched from a hilltop

The trajectory of a projectile is parabolic. Consider a simpler set-up, with ground-to-ground projection from the origin, the range is maximum when $\theta = 45^{\circ}$, as then the distance between ...
1 vote

### Is gravitational potential energy of body by $mgh$ negative?

The key here is that it is the difference in potential energy that is important and this yields the same result in magnitude and sign, using using either equation. The potential energy is obtained by ...
• 6,182

### Is gravitational potential energy of body by $mgh$ negative?

Gravitation potential energy can have any sign you want: $$U(h) = mgh + C$$ since you can add any constant $C$. Down is the direction of decreasing $U$, though, as the $m$ becomes more tightly bound ...
• 35.5k

### Is gravitational potential energy of body by $mgh$ negative?

The two expressions have different locations for their energy zero points, that is why they disagree. $mgh$ is zero at wherever you have set $h=0$ to be with your coordinate system. Typically that is &...
Accepted

### How accurate does the ISS's velocity and altitude need to be to maintain orbit?

For a particular orbital period and altitude (or altitude and eccentricity, or eccentricity and period), there is exactly one velocity that produces the correct orbit. Of course the ISS doesn't have ...
• 698

### What would happen to the moon's orbit if we reduce (instantaneously) its mass?

I will assume moon's orbit around the earth is modelled as a Kepler problem. The moon's mass $m_m$ is about 80 times smaller than earth's mass $M_e$. But if you don't neglect it completely, reducing ...
• 426

### What would happen to the moon's orbit if we reduce (instantaneously) its mass?

Mass is a conserved quantity. Having it simply disappear violates the known laws of physics. Once you have violated the laws of physics you can no longer ask what the laws of physics predict, since ...
• 103k

### When an object is thrown towards the sky it starts to gain potential energy, why?

The push force makes the object move towards the sky and the pull force of gravity makes the object come down towards the Earth's surface. You are mistaken about this part. If you throw an object ...
• 6,182
Accepted

### Doubt on conservation of angular momentum for Kepler's laws

The definition of a plane can be written as $\vec{a}\cdot \vec{r} = 0$, where $\vec{a}$ is any vector perpendicular to the plane. In this case, you have a vector quantity $\vec{L}$, which is from its ...
• 133k

### Doubt on conservation of angular momentum for Kepler's laws

The angular momentum ${\bf L}$ of a point-like particle is proportional to the vector product of position ${\bf r}$ and velocity ${\bf v}$. It is a vector orthogonal to the plane containing ${\bf r}$ ...

### When an object is thrown towards the sky it starts to gain potential energy, why?

Every body has total energy, which is the sum of its kinetic and potential energy .This total energy is always conserved for conservative forces. $U=V+K$ Here U denotes total energy, V denotes ...

### Why are fields described as force divided by mass or charge?

You are right in demanding a clear answer for this issue (in school, the teacher either can't answer or can't explain the thing). What is the difference between physics and philosophy if we are not ...

### When an object is thrown towards the sky it starts to gain potential energy, why?

I think what you're missing is that potential energy can be negative. If you hold a ball and drop it from infinity, at the start the ball has no kinetic and no potential energy. When you drop it the ...
1 vote

### When an object is thrown towards the sky it starts to gain potential energy, why?

There are two ways to view the exchange of kinetic energy and potential energy. But before I do, I'd like to point out that your learning of physics will be much much easier if you drop the concept ...
• 50.2k

### When an object is thrown towards the sky it starts to gain potential energy, why?

The object has energy, whether it is kinetic or potential. There is a point in its trajectory where the object is not moving, and this is at the highest point of its trajectory. At this point, it has ...

### Has our knowledge of astrophysics and gravity reached the point where we can accurately calculate Lagrange points?

There are no true Lagrange points in the Solar System. Lagrange points represent solutions of the restricted three body problem, but the Solar System is a many body system. There are orbital regions ...
• 21.8k

### Local vs distant gravity effects where is the boundary line of effective control?

A helpful way into understanding this: calculate the acceleration due to gravity for the Earth due to the Sun and for the Moon due to the Sun. They're roughly the same, because it doesn't depend on ...
• 698
1 vote

### Local vs distant gravity effects where is the boundary line of effective control?

Two objects fall equally fast in a vacuum . Even if they are of different sizes and masses. Armstrong famously dropped a hammer and a hawk feather on the moon and they fell and hit the ground ...
• 51.5k

### Local vs distant gravity effects where is the boundary line of effective control?

so why is it that the moon hasn't yet been drawn away from earth and to sun You might ask, why is it that the earth hasn't been drawn to the sun? In the absence of drag, objects can remain in ...
• 41.2k
Accepted

### Error concerning projectile motion in respected textbook?

I agree that the statement I made in the book is wrong: On a flat earth the time of flight is not affected by the initial horizontal velocity, only the horizontal distance is. I will post a correction ...
• 656

### Error concerning projectile motion in respected textbook?

As @RatulThakur points out, this is pretty clearly an error. ToF is not affected at all by horizontal velocity on a flat Earth, unless you're considering the aerodynamics of the object (there's a ...
• 698

### Why is pressure in the outermost layer of a star lower than at its center?

Imagine you are sandwiched between two moon like objects each with mass M. The forces acting on you is $$F_{centre} = \frac{2 \times G\times M\times M}{D^2}$$ where D is the distance from the centre ...
• 6,182