Tagged Questions
0
votes
1answer
25 views
Kinetic energy when $F(t)$ graph is given
The very basic problem here is that the point where this graph ends is ambiguous. I mean, what is the corresponding $y$-value for the point whose $x$-valu is $4.5s$? Then I think we should find the ...
0
votes
2answers
51 views
Momentum, Impulse and Newton's Second Law of Motion
Newton formulated his Second Law as such:
$$\sum{\vec{F}} = \frac{\delta \vec{p}}{\delta t}$$
and of course, $\vec{p} = m \vec{v}$.
Why is it that if the net force $\sum \vec{F}$ is constant (which ...
1
vote
2answers
48 views
effect of vertical collision on kinetic friction and subsequent change in horizontal velocity
Suppose somehow a block of mass $m$ is moving on ground, and the coefficient of kinetic friction between the block and the block is $\mu_k$. If I drop a tennis ball(of same mass) on it from a ...
2
votes
2answers
114 views
Movement of man and ladder and their center of mass
Suppose there is a massless frictionless pulley.
A rope over it carries a mass $M$ and on other side carries a ladder of mass $(M-m)$ and a man on that ladder, of mass $m$. Now the man starts ...
0
votes
2answers
66 views
Conservation of Momentum from Recoil Speed
A gun has a recoil speed of 2 m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g (0.01 kg) what speed does the bullet come out at?
The gun has zero total momentum before ...
1
vote
4answers
202 views
Bat hitting a ball
When a bat hits a ball, consider two cases:
1) The batsman goes for a defense, and stonewalls it, to reduce its speed.
2) the batsman goes for a shot, e.g. a home-run, etc.
in which case will the ...
0
votes
2answers
442 views
Calculating force of impact
Since $\text{force = mass}\times\text{acceleration}$,
is it right to say that an object traveling at a high
constant velocity (zero acceleration), exerts zero
force upon impact with a stationary ...
1
vote
1answer
55 views
Mean value of the force in a collision problem
I have the following problem:
A point with mass $m$ and speed $v$ collides with a fixed obstacle and penetrates it, stopping in a space $\Delta x$. Calculate $\Delta t $.
I understand the way my ...
2
votes
2answers
229 views
Small car colliding with large truck
A small car collides with a large truck. Why do both vehicles experience the same magnitude of force? Wouldn't the large vehicle experience less force than the small one?
0
votes
2answers
104 views
Is air drag equation in term of momentum still valid?
This is the known equation of air drag:
$$m{\bf a}=mg-\mathcal D=mg-b{\bf v}.$$
Considering this, is air drag equation in term of momentum still valid?
$$m{\bf v}=mv_g-b{\bf r}.$$
3
votes
4answers
748 views
Is it possible to lift yourself off from the ground?
Say for instance a person who was strong enough to lift double his body weight. If he placed his hands under his bottom and tried to lift$^1$ himself$^2$ off the ground, could he?
--
$^1$In a ...
9
votes
1answer
209 views
Can a fly pierce itself onto a cactus needle?
Somebody on reddit posted a ridiculous picture today of a fly pierced onto a needle of a cactus: http://www.reddit.com/r/pics/comments/xarue/what_are_the_odds_of_this_accident/
Whilst the OP claims ...
1
vote
2answers
53 views
Can you launch an item farther by slamming into it or accelerating it along a length
I am trying to desgin a little penny balista toy (built with a 3d printer).
The short description is that there is a track with a mass ("hammer") that slides along it. There is a stack of pennies in ...
4
votes
3answers
499 views
Why do we need the quantity momentum?
Why do we need the quantity Momentum in physics when we have the quantities like Force and Energy? Isn't it possible to substitute the usage of Momentum with equivalent of Force and Energy?
0
votes
3answers
939 views
Why is force described as rate of change of momentum?
momentum = mass * velocity
Differentiating both sides leads to
force = mass * acceleration
since the mass doesn't participate in the differentiation as it is constant.
Is this a sound ...
0
votes
1answer
166 views
Momentum question?
The problem:
In a car crash, a passenger with a mass of 82kg is not wearing a
seatbelt. The car is travelling at 45km per hour. What impulse must
the car's airbag provide in order to stop the ...
6
votes
3answers
420 views
How does $F = \frac{ \Delta (mv)}{ \Delta t}$ equal $( m \frac { \Delta v}{ \Delta t} ) + ( v \frac { \Delta m}{ \Delta t} )$?
That's how it's framed in my Physics school-book.
The question (or rather, the explanation) is that of the thrust of rockets and how the impulse is equal (with opposite signs) on the thrust-gases and ...
3
votes
2answers
154 views
Does constraint for speed of Electric and magnetic fields violates Conservation of momentum or Newton's third law?
I'm just a beginner so bear with me. Consider two frames at rest wrt to each other separated by distance enough for light to take a minute or so. At a given instant we create two large dipoles by some ...
0
votes
1answer
160 views
Deceleration of Vehicle off pavement
How would I calculate the deceleration of a vehicle (ATV), wheels locked, on dirt? How long would it take to halt? How fast is it decelerating?
I used this calculator, but it seems incorrect for a ...
1
vote
2answers
341 views
When should we use the concept of Impulse/Momentum instead of Force?
In my notes it says
"The ideas of impulse and momentum is useful in solving problems where:-
a) the force F is not easily calculable (e.g. sudden impact or blow)
b) the impulse force is ...
0
votes
1answer
275 views
Force and Laws of Motion [closed]
While catching a fast moving cricket ball, why do fielders gradually pull back their hands backwards with the moving ball? Please advice which among the following is the correct answer. If both are ...
0
votes
0answers
82 views
Physics behind 2 different wall kicks
I'm looking at a situation where a sprinter starts at some distance x from a wall. The goal is to sprint to the wall, hit it, and get back to the start as quickly as possible. There are two ways ...
4
votes
5answers
4k views
Newton's second law of motion in terms of momentum
I am reading a document and in answer to the question State Newton’s second law of motion the candidate answers that The force acting on an object equals the rate of change of momentum of the object. ...
4
votes
3answers
1k views
Which will stop first a heavier car or a lighter car?
If the friction from brakes, wind resistance and all such factors remain constat, which will stop first? A heavier car or a lighter car? How will the momentum of the car and graviational pull on a ...
7
votes
3answers
5k views
How does the 'water jet pack' work?
So I was cruising around at YouTube and saw this sweet video, and as I was watching started to wonder: "How is this possible?".
For a little bit of background, in case you decide to not watch the ...
5
votes
2answers
482 views
Tricky spring on a surface question
I have this relative simple-looking question that I haven't been able to solve for hours now, it's one of those questions that just drive you nuts if you don't know how to do it.
This is the scenario:
...
5
votes
4answers
6k views
Is pushing actually easier than pulling?
It is generally assumed that pushing a cart is more easier than pulling one. But why?
Is there any difference in terms of force required to achieve the same amount of displacement?
Or is it just a ...
