# Force, motion and energy problems and answers

Recent questions in Force, Motion and Energy
Talon Mcbride 2022-07-16

### A force F directed upwards is applied on an object resting on a flat surface. The object then moved to the right. Which of the following is TRUE about the upward force?a. F does work on the objectb. F is a conservative force c. F is gravitational forced. F caused the displacement of the objecte. Only the 1st and 3rd optionsf. Only the 2nd and 4th optionsg. None of the above

on2t1inf8b 2022-07-16

### From classical mechanics, the force on a spring is given by the negative gradient of the potential energy with respect to position or displacement.Can we also say $F=-\mathrm{\partial }U/\mathrm{\partial }x$, where U is the "internal" energy of the spring?

Libby Owens 2022-07-16

### The total energy of an object comes from the time part of the four-momentum, and so isn't a Lorentz invariant. On the other hand, is the potential energy of a compressed spring a Lorentz invariant?

Elsa Brewer 2022-07-16

### Can the second law of motion for rotation, $\stackrel{\to }{\tau }=I\stackrel{\to }{\alpha }$, be used for any axis?Is there any case that acceleration $\stackrel{\to }{\alpha }$ is not in the direction of applied torque $\stackrel{\to }{\tau }$?

logiski9s 2022-07-16

### In my lecture today my professor briefly mentioned that force is the derivative of energy but I did not really get what he meant by that. I tried to express it mathematically:$\frac{d}{dt}{K}_{E}=\frac{d}{dt}\left(\frac{1}{2}m{v}^{2}\right)=mv\frac{dv}{dt}$This looks really close to Newton's second law $F=ma$ but there is an extra "v" in there. Am I missing something here?

kislotd 2022-07-15

### When an astronaut with a mass of 55 Kg goes from Earth to the Moon, how much does their weight change? Answer in N OR in Pounds. gmoon= 1.62$=9.8m/{s}^{2}$, gearth$=9.8m/{s}^{2}$

Lorena Lester 2022-07-15

### While working with problems on elastic collisions, I have come across this observation, that the elastic potential energy of a two-body system is the maximum when the relative velocity equals zero. In fact, the same applies when we are talking about a 2-block+spring system. Why is it so?

Jayvion Caldwell 2022-07-14

### We choose a system to be a spring + a connected body. If we stretch the body and leave it, it will make a simple harmonic motion. The center of mass of this system is approximately the center of mass of the body because the spring has a negligible mass. Newton's second law states that: ${F}_{external}=M{a}_{cm}$ However, it is apparent that the center of mass has an acceleration though the total external force on the system is zero. What is the explanation?

EnvivyEvoxys6 2022-07-14

### Non-Constant Acceleration due to GravityRecently, I had the first physics lab for my university physics course. This lab was fairly simple, as we were merely using a computer and a distance sensor to graph the position, velocity, and acceleration of a cart as it moved along a linear track.One of the situations we captured data for involved starting the cart at the bottom of an inclined ramp and giving it a push upwards. As expected, it rolled up, came to a stop, and then came back down the track to its starting position. The position-vs-time graph was essentially parabolic, the velocity-vs-time graph was essentially linear, and the acceleration-vs-time graph was essentially linear. So far, so good.At this point in the lab, the instructor pointed out that, if the data was examined closely, the acceleration of the cart was greater while the cart was traveling upwards than when the cart was traveling downwards (approximately $0.546\frac{m}{{s}^{2}}$ and $0.486\frac{m}{{s}^{2}}$, respectively), and asked us to determine why in our lab report.Now, gravity was the only force acting upon the cart, and thus it's acceleration should be a constant, at least at the scale our experiment was conducted at, so these results are completely baffling my lab group. So far, we have proposed the following ideas, but none of them seem very plausible.Doppler effect on the ultrasonic distance sensorFrictionAir resistanceHuman errorThe first seems highly improbable, and the last three are more obfuscation and hand waving than actual theories.Why does our experimental data show the acceleration due to gravity to change based on the direction the object is moving?

kwisangqaquqw3 2022-05-20

### Meaning of acceleration due to gravityI found article in which it is said that it would be incorrect to say that g is not acceleration due to gravity but local gravitational field as there is no acceleration on a block placed on a table. Please can you explain this as I am in a school and I have read only that g is acceleration due to gravity and textbooks say this too.

kromo8hdcd 2022-05-20

### Acceleration due to gravity during its journey up and downWhen we throw an object up into the air, ignoring air resistance, etc, we define acceleration to be -9.8 m/s^2. When it goes down after its journey up, like a parabola, do we define the acceleration as 9.8 m/S^2, OR still the same as -9.8m/s^2?I know that acceleration due to gravity is always constant, but would it be wrong to say it like the question i have above?

lasquiyas5loaa 2022-05-20

### The force that accelerates a rocket into outer space is exerted on a rocket by the exhaust gases. Which Newton’s Law of motion is illustrated in this situation?

Kiersten Hodge 2022-05-20

### How do you calculate the acceleration due to gravity of a massive object (black hole) Is there an online calculator to find the acceleration of gravity a massive object exerts from a specific distance? I know the effects of gravity kind of just fade out with distance but I'm asking for mass-specifics.For example, you plug in the mass of the object and the distance from the object to get the m/s^2 (and maybe even the escape velocity)???(I'm also thinking this would be some sort of exponential graph, like "acceleration" on the y axis and "distance" on the x axis)

Tristan Meyers 2022-05-19

### a. What is the gravitational force of the sun on the earth?b. What is the gravitational force of the moon on the earth?c. The moon’s force is what percent of the sun’s force?

Jayden Mckay 2022-05-19

### A body weighs 72 Non the surface of the earth. Calculate the gravitational force on it due to the earth at a height equal to half of radius of the earth.

Regina Ewing 2022-05-19

### I have this expression for the gravitational force between 2 masses when the gr term is added:$F=-\frac{GMm}{{r}^{2}}+\frac{4{G}^{2}mM\left(M+m\right)}{{r}^{4}{c}^{2}}$which I got from the internet since I haven't bee able to find a similar expression in any book. But i'm not sure if it's right or needs fixing.Is the second term in the force correct?

uto2rimxrs50 2022-05-19

### In an experiment, an object is released from rest near and above Earth's surface. A student must determine the relationship between the direction of the gravitational force exerted on the object and the change in momentum caused by that force. What data could the student collect to determine the magnitude and direction of the gravitational force and the change in momentum of the object? Justify your choices. Select two answers.

Jayden Mckay 2022-05-18

### Is the acceleration due to gravity the same no matter the mass of the object? The University of Illinois Department of Physics article Q & A: Heavy and Light - Both Fall the Same states that the acceleration due to gravity is the same no matter the mass of the object.However if an object, say for example the moon, were falling to the Earth, gravity would have to pull harder to speed it up the same. Would the acceleration of gravity be the same?My question is not the same as Don't heavier objects actually fall faster … because I'm asking about acceleration of gravity, I'm asking if heavier objects would fall slower, not faster. So, wouldn't they fall slower because gravity has to pull harder?Side note: Let's just say we also ignore the effects the other question mentions.

dresu9dnjn 2022-05-18