The description of the movement of bodies by their position, velocity, acceleration (and possibly higher time derivatives, such as, jerk) without concern for the underlying dynamics/forces/causes.
37
votes
14answers
7k views
Why does kinetic energy increase quadratically, not linearly, with speed?
As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $mv^2/2$.
Why does this not increase linearly with speed? Why does it take so much ...
15
votes
5answers
274 views
Does the mass point move?
There is a question regarding basic physical understanding. Assume you have a mass point (or just a ball if you like) that is constrained on a line. You know that at $t=0$ its position is $0$, i.e., ...
14
votes
6answers
3k views
Is two cars colliding at 50mph the same as one car colliding into a wall at 100 mph?
I was watching a youtube video the other day where an economist said that he challenged his physics professor on this question back when he was in school. His professor said each scenario is the same, ...
14
votes
10answers
12k views
What is the difference between “kinematics” and “dynamics”?
I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics".
I first encountered this in Jackson's E&M book, where, in ...
13
votes
1answer
199 views
Modelling the movement and jumps of a chalk while drawing a dashed line on a blackboard
You probably know that if you try to draw a line using a piece of chalk on a blackboard , under some conditions (for example, $\alpha<\frac{\pi}{2}$ in the picture below) you will have a dashed ...
8
votes
3answers
1k views
Can superconducting magnets fly (or repel the earth's core)?
If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the ...
7
votes
1answer
432 views
I reached a result concerning displacement with quantized time intervals. Am I on to something?
A few days ago, I realized a similarity between distance with constant acceleration, $d = v_i t + 1/2 a t^2$, and the sum of integers up to n, $(n^2 + n)/2$. This came up again today when I decided to ...
7
votes
5answers
535 views
Should you run when under rain?
When it's raining, would you get less wet if you run or more wet?
I think you will get less wet, because rain is coming down in a constant volume and if you run you will have a shorter amount of time ...
7
votes
1answer
255 views
Is acceleration an average?
Background
I'm new to physics and math. I stopped studying both of them in high-school, and I wish I hadn't. I'm pursuing study in both topics for personal interest. Today, I'm learning about ...
7
votes
4answers
400 views
Why does a ping pong ball change direction when I spin it on a table?
When I spin a ping pong ball on the table, it rolls forward in the opposite direction of the spin, and then eventually changes direction and rolls backward.
Here's a video demonstrating the effect.
...
7
votes
5answers
440 views
How can kinetic energy be proportional to the square of velocity, when velocity is relative?
Let's start with kinetic energy (from los Wikipedias)
The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a ...
7
votes
3answers
403 views
What's my weight when jumping on a bicycle from a higher to a lower ground?
Before I start defining the situation and asking a question, I'd like to make a few things clear:
this is not a homework, merely a matter of personal interest and enthusiasm
I am neither a ...
7
votes
2answers
798 views
Should acceleration be included in state vector of a Kalman filter?
I'm developing (actually adopting existing solution) a Kalman filter to model motion of a vehicle (UAV or automobile). The state vector will include position, velocity, and, possibly, acceleration.
...
6
votes
5answers
8k views
How to calculate average speed
I recently encountered a puzzle where a person drove 120 miles at 40mph, then drove back the same 120 miles at 60mph. The average of the speeds is (40mph+60mph)/2 = 50mph, so the total trip time ...
6
votes
4answers
465 views
How to brake 'beautifully'?
Sometimes when I'm driving my car, I play a "game" against myself in which I try to minimize the deceleration felt by passengers (including myself) while still braking in a reasonable short space. I ...
6
votes
4answers
132 views
Integrating radial free fall in Newtonian gravity
I thought this would be a simple question, but I'm having trouble figuring it out. Not a homework assignment btw. I am a physics student and am just genuinely interested in physics problems involving ...
6
votes
2answers
788 views
Physics of simple collisions
I'm building a physics simulator for a graphics course, and so far I have it implementing gravitational and Coulomb forces. I want to add collisions next, but I'm not exactly sure how to go about ...
5
votes
3answers
316 views
Why isn't average speed equal to displacement over time?
I'm in an introductory Physics course and I need help!
During a one-hour trip, a small boat travels 80.0km north and then travels 60.0km east. What is the boat's average speed during the one-hour ...
5
votes
2answers
5k views
What is the relationship between kinetic energy and momentum?
I can't seem to figure out the relationship between $E_k$ and $p$ or $F$.
I understand that the units are pretty different. But for example:
A bullet with a mass of 10.0g is moving at the speed of ...
5
votes
5answers
1k views
Why does work equal force times distance?
My 'government-issued' book literally says:
Energy is the capacity to do work and work is the product of net force and the 1-dimensional distance it made a body travel while constantly affecting ...
5
votes
2answers
228 views
what is the kinematics of a particle with complex mass?
particles with real-mass have time-like kinematics ($ds^2 > 0$).
particles with zero-mass have light-like kinematics ($ds^2 = 0$).
particles with imaginary-mass have space-like kinematics ($ds^2 < ...
4
votes
2answers
1k views
Help me get out of a speeding ticket
I was driving uphill from a complete stop for a distance of .4 miles estimated to take 1 minute in a navigation app. I was pulled over right after cresting the hill. The cop had me on radar going ...
4
votes
5answers
2k views
Why there is a 1/2 in kinetic energy formula? [duplicate]
Possible Duplicate:
Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?
Hèllo, I have a question about kinetic energy formula.
As you know, in kinetic energy formula, we have:
...
4
votes
5answers
4k views
Distance when acceleration not constant
I have a background in calculus but don't really know anything about physics. Forgive me if this is a really basic question.
The equation for distance of an accelerating object with constant ...
4
votes
2answers
106 views
Best shape to reduce the splash of a droplet?
Our coffee machine catches the last couple of droplets, after your cup is removed on a shape to reduce plash of the coffee droplets.
These shapes are placed inside the spill reservoir.
The shape ...
4
votes
1answer
45 views
How to track the visual path of a LEO satellite as seen from the ground
I have been struggling with this problem for a while so I decided to ask. I'm new here and I'm not sure where this type of question belongs, so forgive me if this isn't the right section.
I am ...
4
votes
2answers
798 views
Rigid body dynamics joints
I can't seem to find any info on connected rigid bodies by a joint. Can someone explain the basics to me? I'm trying to do a little research to find out how feasible it would be to implement 3d ...
3
votes
2answers
616 views
How can an object's instantaneous speed be zero and it's instantaneous acceleration be nonzero?
I'm studying for my upcoming physics course and ran across this concept - I'd love an explanation.
3
votes
1answer
215 views
Why is the velocity on the top of a wheel twice the velocity of its axle?
When a wheel is rolling, not skidding, and its axle moves at velocity $\vec{v}$, then a point on the top of its circumference will move at velocity $2\vec{v}$, shown below.
I really don't ...
3
votes
3answers
1k views
Maximum range of a projectile launched from elevation “dumbed down”
I am trying to conceptually understand why the angle which produces the greatest range for a projectile launched with an elevation is not 45 degrees. I have exausted all other options, and I hope that ...
3
votes
3answers
3k views
Ion Drive Propulsion Top Speed
I would like to know if there is some formula / graph which would provide / show the efficiency of a certain type of propeller in space. Specifically, I'm interested in the acceleration attainable at ...
3
votes
2answers
67 views
Sign of acceleration
I'm developing an application using accelerometer sensor. I'm not good at physics so forgive me if the question is trivial. If I have 3 values of acceleration: $x$, $y$, $z$, I find acceleration ...
3
votes
2answers
284 views
Galilean invariance of the Schrodinger equation
I am only asking this question so that I can write an answer myself with the content found here:
http://en.wikipedia.org/wiki/User:Likebox/Schrodinger#Galilean_invariance
and here:
...
3
votes
2answers
171 views
Why is average velocity the midpoint of initial and final velocity under constant acceleration?
Since average velocity is defined as$^1$
$$\vec{\mathbf v}_\mathrm{av}=\frac{\vec{\mathbf x}-\vec{\mathbf x}_0}{t-t_0},$$
where $\vec{\mathbf x}$ denotes position, why is this quantity equal to
...
3
votes
2answers
218 views
The Double Integrator: Matching velocity and position as quickly as possible with only a limited amount of force available
If a body with mass $m$ begins at position $x_0$ with velocity $v_0$ and experiences a force that varies as a function of time $f(t)$ (and we ignore gravity, friction, and everything else that might ...
3
votes
3answers
119 views
When can one write $a=v \cdot dv/dx$?
Referring to unidimensional motion, it is obvious that it doesn't always make sense to write the speed as a function of position. Seems to me that this is a necessary condition to derive formulas ...
3
votes
1answer
87 views
How deep is the well, given this data?
A rock is dropped into a well. A timer starts when the rock is dropped and is stopped when the noise of the rock hitting the ground of the well is heard. How deep is the well?
Here's what I have so ...
3
votes
1answer
182 views
So what is happening in this angular kinematics equation?
The wheel of a car has a radius of 20 cm. It initially rotates at 120
rpm. In the next minute it makes 90 revolutions. What is the angular
acceleration?
So the answer is solved by using one ...
3
votes
1answer
1k views
How do kinetic energy and linear momentum relate?
It took me quite a long time to click my gears in place and even then I'm not sure it's completely correct.
The problem is that I need to understand these concepts (physics concepts; not just these ...
3
votes
3answers
361 views
A freefalling body problem, only partial distance and time known
Well, I've been trying to figure out a problem which I imposed on myself, so no literal values included. Unfortunately, my brain is not cooperating.
The problem states:
What is the height from ...
3
votes
1answer
141 views
A practical deceleration question
My friend is a U.S. Army paratrooper. Today, through an unfortunate series of events, he was jerked out of a C-17 traveling at 160 knots by his reserve parachute. First-hand accounts describe it as he ...
3
votes
1answer
305 views
How do I find the initial velocity in this problem?
An X-ray tube gives electrons constant acceleration over a distance of $20\text{ cm}$. If their final speed is $2.0\times 10^7\text{ m/s}$, what are the electrons' acceleration?
I know this ...
3
votes
2answers
173 views
Find distance between turtles
I've been thinking about the following problem for some time now and was wondering if anyone could shed any light on it.
At time $t = 0$, turtle A sits at ...
3
votes
1answer
163 views
Lab observation correct? As distance decreases, velocity increases, stderr decreases
The experiment goes like this:
Allow a moving cart to move from the top of an incline plane ($x_0$) downwards. The time taken will be recorded by the picket fence (those things you see wired up). ...
3
votes
2answers
302 views
Can these figures demonstrating the safety of “Archery Tag” arrows be correct?
There is a new sport called "Archery Tag" that involves shooting opponents with foam-tipped arrows fired out of a real bow. The official Archery Tag web site presents data that claims to show the ...
3
votes
2answers
961 views
Formulas for ball rolling in a bowl?
I'm developing a program where I've a ball/sphere rolling in a bowl from the side at top, till the center at bottom, and I'm trying to get the formulas for:
The rotation angle and the position of ...
3
votes
2answers
678 views
Is there a 2D generalization of the coefficient of restitution?
The coefficient of restitution characterizes a collision in one dimension by relating the initial and final speeds of the particles involved,
$$C_R = -\frac{v_{2f} - v_{1f}}{v_{2i} - v_{1i}}$$
In a ...
3
votes
2answers
106 views
Is a heavier skier faster? [duplicate]
Is it true that a heavier skier goes faster? If it is, why is that?
My intuition would be that the speed gained by a skier should be independent from its mass, since both its acceleration and the ...
3
votes
1answer
112 views
Is it possible to determine particles' velocities from their crossings on a ring?
2 particles are constrained to move in a ring. Both particles begin moving at $t=0$ from $p=0$, each particle moving in the opposite direction of the other. It is known that they are moving at ...
2
votes
12answers
3k views
List of good classical physics books
What are some good books for learning the concepts of Kinematics, Newton laws, 2D Motion of Object etc.?
