Homework Help - Physics
 

A semi truck barrels down the interstate at a constant speed of 80 mph. If the frictional force acting on it is 1406 N. How much force does the truck's engine supply in order to keep it moving at constant speed (in newtons)?
wouldn't the anser be 1406 newtons, seeing as how the speed is constant?

also

When swimming across a river you swim to the east at 1.1 mph while the river flows southward at 2.1 mph. Your velocity is a superposition of these two velocities. What is your speed (i.e. the magnitude of your velocity)?

I got 2.37 (ish) mph, but i don't think that is right.

2.37 is right, although because of significant digits, it should be rounded to 2.4.

thnx, any ideas on the first one?

1406 N sounds right to me, but I'm not sure, because friction is a downward force.

not necessarily, friction goes in opposite direction or whatever if moving.

(just checked, i am right on both, 1406 and 2.37, thnx.)

3 more

If you run 100m at a speed of 5m/sec and then walk 80m at a speed of 1m/sec, what is your average speed?

i got approx. 3 m/s
--------------------------
A parachutist of mass 70 kg jumps from an airplane. After 10 seconds her parachute opens and the force of air resistance is 83 N. What is the net force on the parachutist at that time (in newtons)?
----------------------
A mover pushes a piano up a ramp into the truck. If it requires a net force of 93 Newtons to push the piano without friction, with how much force must the mover push if there is a frictional force of 31 Newtons?
i got 93+31 = 124 N
----------------------------

100m at 5 m/s takes 20 seconds
80m at 1 m/s takes 80 seconds

So you've travelled 180m in 100s, assuming instant deceleration and no pausing to catch your breath.

Therefore your average speed is 180/100 = 1.8 m/s

dlink:

1. I got the average speed as 1.8 m/s (exactly). Check your answer again.

2. Net Force = 603 N.

3. 124 N. is correct.

thnx a lot! those are all right, i may have a few more in a bit, idk. :)

woo hoo! 100%. thnx :D

cheating on your homework? goddammit, why didnt i do that last year instead of dumping the worksheets and getting in trouble.. :igor:

im scared. i'm taking physics this year. Quote from my male physics teacher: "I'm seeing this guy this weekend, actually, i'm sleeping with him"

not cheating..per say....using my resources. just the same as using my calculator ;)

Re: Homework Help - Physics
 

1) Let the direction the truck travels be forward (+)
v = 80 mph = 128 km/h
Ffriction = (mu-kinetic)(Fnormal) = 1406 N [forward]

code:
FBD:

                          Fnormal

                             ^

    Ffriction = 1406 N <-- truck --> Fmotor = ?

                             \/

                Fgravity = (mass)(accel) = Fnormal

Fnetx = Ffriction + Fmotor
Fnetx = 0
Ffriction = Fmotor

Thus, the trucks motor outputs 1406 N of force forward to overcome the frictional force and keep the truck moving at a constant speed of 80 mph/128 km/h.

Note though: friction does not act in the opposite direction of motion, friction is a force which tries to oppose motion (in this quesiton friction was the same direction as the truck's travels). The reason for this is, the wheels which are spinning forward, are trying to propel you forward by pushing backwards on the concrete, so the friction, to try and prevent this motion, pushes forwards.

a ball thrown vertically upward returns to the hand of the thrower 6.00 seconds later. For how how many seconds did the ball fall after reaching its high point?

I don't understand how to take into account when the ball is going up.

I assume they are looking for 3 seconds as the answer, but I would imagine you would need to take into account vertical acceleration....

well the acceleration would be the same for upwards and downwards: 9.8m/s

oh look. i just answered my own question. damn im stupid

;)

Does this comunity support biology, I have a killer biology teacher, and i have an F in her class currently! AHHHHH-

Physics teachers are always some of the coolest people. I have it next semester. I'm wondering how that's going to turn out. x_X

Hehe, this isn't physics. It's mathematics. It's mechanics. Ok, so it's physics too but I did it as part of my A Level Maths course.

Except that on the way up, the acceleration is negative and on the way down positive. But yeah, kinda.

No... the acceleration is always 9.8 m/s in a downward direction...

taking an online test, want to (honestly) check my answers

Combining Newton's second law and his law of gravitation allows you to

a) derive Kepler's second law
b) derive Galileo's law of parabolic projectile motion
c) derive Kepler's first law
d) derive Kepler's third law
e) calculate the value of g, the acceleration of gravity near Earth's surface

------------------------

The planet Venus has a mass of 4.88 x 1024 kg and radius 6.06 x 106 m. What would the value of g(the acceleration of gravity) be on the surface of Venus?

also, skinme, can you please remove the image in your sig, for some reason it makes myIE2 crash whenever I see it. I beg of you, please change it

human "cannonball" is shot from a cannon and follows a parabolic path into a net. At the top of her trajectory,



her energy is purely potential
her energy is purely sound
her energy is purely heat
her energy is purely kinetic
... None of the other answers are correct

--------------------------------
I think none.

Potential.

When she reaches the peak, she is at a complete rest for a moment, having nothing left but potential (stored) energy. When she begins her decent, it is converted into kenetic, along with sound due to the changes in air pressure (though probably not audable), heat due to the friction. And kenetic energy, as you probably already know, is released energy, or energy being released.

i dont understand anything here and i have to take this class next quarter........................ :(

none of the above. parabolic means there's also horizontal motion, gogitta

thats what I thought second., but she keeps going, so i wasn't sure about momentum, but yea, thanks.

a) that answer was wrong

oh well
new questions

1. A physics student pushes a vacuum pump across the lab with a force of 56 N at an angle of 26.1 degrees. If he moves the pump a distance of 8 meters What is the work done in the process?

2. A sled and rider together weigh 813 N. They slide down a frictionless hill through a vertical distance of 11 m. Use conservation of energy to find the kinetic energy of the sled-rider system at the bottom of the hill. Assume the sled and rider start from rest.

Re: Re: Homework Help - Physics
 

that is, of course, assuming that the friction stated in this problem is surface to surface friction with the cement. It could just as easily be an external force field or drag(since velocity is constant, drag force is constant)

s0be

Theoretically, it's a. purely Potential

But in reality, it's a combination of Potential Energy and Sound Energy. SoundSince the cannonball is shot with a high velocity, all it's atoms are moving at high velocity & when they come in contact with air, due to air friction, some atoms have reduced velocities compared to some others, which in turn causes these atoms to internally 'collide' with each other & thus producing small amount of Sound and Heat. Hence, the answer is None of the above four.

actually, since she was shot out in a parabola, at the top of her path, she has 1/2*M*Voy^2 potential energy, and 1/2*M*Vox^2 kinetic energy, which remains constant until reaching the net. Along with the negligble sound energy a person wolud make in the form of a perfect sphere, in a vaccum.

s0be

Acceleration is ALWAYS -9.8m/s^2
Note the NEGATIVE sign......unless down is positive it is negative. Also, when you throw up somthing its falling up:)!

[edit]ok....i mean when you throw up a ball or something...not throw up...[edit]

Actually, it is the sum of the two energies that remains constant, and not the two of them individually. Both PE & KE keep changing continuously, but one compensates for the other, thereby keeping the Total Energy, TE = PE + KE always constant.

Sorry man, my Physics is disgrace.
I was always behind the door in Physics class.

all the kinetic energy from a 1620 kg car traveling 64 miles per hour was converted to heat (and indeed, the kinetic energy of the car did start as heat from burning gasoline), how much would the temperature of the car rise? Assume the car is made mostly of steel, with a specific heat of 448 J/(kg deg C).

I hate temperature. :hang:

i figured it out. i switched specific heat and heat energy in teh equation. duh! :igor: :rolleyes:

ok. i have a big problem here. I just calculated the acceleration of gravity to be 800. :igor: not good. heres from my lab where i took the data and found these values:


d avg avg final
time velocity velocity
0 0 0 0
20 .213 93.90 187.80
30 .251 119.52 239.04
40 .296 135.14 270.28
50 .331 151.06 302.12
60 .357 168.07 336.14
70 .388 180.41 360.82
80 .406 197.04 394.08
90 .439 205.01 410.02
100 .464 215.52 431.04

i might have calculated final velocity wrong although i'm pretty sure its right. I should be able to get the acceleration by finding the slope of the graph of time and final velocity. Maybe i did something wrong converting cm/s to m/s i dunno. all i know is that's some prety fast gravity and this is due tomorrow. :mad:

what are your units?

sorry:

d = cm
avg time = seconds
avg velocity = cm/s
final velocity = cm/s