Please show your work / steps how to solve the problem and which equations/formula you are using. Willing to pay 20fg per questions answered. So question #1 has 6 questions, i will be paying 120fg etc..)


#1) A free-falling object has a net force (its weight) of 100 N. Its free-falling acceleration is 9.8 m/s^2 downward. What is the object's mass?


#2) The moon's mass is 7.4 x 10^22 kg. The moon's acceleration is 0.002 m/s^2 in a direction toward the EArth. What is the magnitude of the net force felt by the moon?


#3) A person holds a pendulum at 0.2m above its lowest height. He shoves it off from there at a velocity of 3/ms. The mass of the pendulum is 2kg.

a. What is the initial potential energy of the pendulum?

b. What is the initial kinetic energy of the pendulum?

c.Add parts (a) and (b) to get the total energy of the pendulum.

d.(Multiple choice) Where is the pendulum when v = 0?
(i) At its lowest height (ii) Midway up (iii) At its maximum height

e. What is the magnitude of the velocity at its lowest height?

f. What is the maximum height?


#4) A railway car with mass m1= 1000kg is rolling along at v1 =2 m/s. It rolls into bigger car at rest. The bigger car has mass m2 = 3000kg.

a. What is the initial momentum of car 1?

b.What is the initial momentum of the system?


The cars collide. Suppose car 1 is sent rolling backward at 0.1 m/s (that is, v1 = -0.1 m/s.)

c. What is the new momentum of car 1?

d. What is the new momentum of car 2?

e.What is the velocity of car 2?

This post was edited by Stolem on Nov 8 2013 05:34pm

1. F = ma
m = F/a
m = 100N/9.8m/s^2
m = 10.2kg

(check to see if they want specific rounding - it could just be m = 10kg)

pm'd

3a Right before the pendulum is released (at rest) it has maximum PE and zero KE. its maximum PE = mgh = (2kg)*9.8*0.2

Right before the pendulum gets to its lowest point it has maximum KE and zero PE. KE = 1/2 mv^2 = 0.5 *2*(3^2)

c)

d) the velocity is zero right as it reaches its max height and has to switch directions

e) mech energy is conserved so we can set KEi + PEi = KEf + PEf

Right before the pendulum is released it has all PE and 0 KE. before its about to stop its all KE and 0 PE

then PEi = KEf
mgh = 1/2 mv^2
m's cancel

gh = 0.5 v^2
v = sqrt(2gh)
= sqrt(2*9.8*0.2)
= 1.97 m/s

not sure about this says he shoves it with initial v = 3 m/s. might wanna ask someone else here



4a lowercase p = mv = (1000kg)*2 = 2000 kg * m/s

b i believe answer is same as A since they haven't collided yet. also, car 2 is at rest so only car 1 is contributing to the momentum of the entire system

c) p =mv
= 1000*-0.1
= -100

if we defined positive as right, negative means that car1 hit car2 and is now going left (negative)





These are most likely incorrect. Can someone explain why I'm getting two different values for the velocity of car2?

d) momentum is conserved
p(initial) = p(final)
m1v1 + m2v2 = m1v1 +m2v2
1000*2 + 3000*0 = 1000 (-0.1) + 3000v2
2000 = -100 + 3000v2
2100 = 3000v2
v2 = 0.7 m/s


e) in elastic colisions, KE is also conserved.

KEi = KEf

1/2 m1v^2 + 1/2 m2v^2 = 1/2 m1v^2 + 1/2 m2v^2
1/2 (1000)(2^2) + 1/2 (3000)(0^2) = 1/2 (1000)(-0.1^2) + 1/2 (3000)v^2
2000 + 0 = 5 + 1500v^2
v = sqrt(1.33)
v = 1.15 m/s

There is no reason to assume this is an elastic collision (imo).

This post was edited by Azrad on Nov 9 2013 12:26pm

(+.1)*

This post was edited by saber_x3 on Nov 9 2013 01:47pm

That was the only thing I could think of.

In inelastic the two objects collide and then "stick" together as one mass after the collision
In elastic the two objects collide and then "bounce" off each other and depending on which one was bigger, behave certain ways after the collision


How should I be thinking of it otherwise?

won't change the answer since that negative number is squared anyway

Those are the idealized collisions. Most collisions create heat which---if not accounted for---ruins the conservation of kinetic energy. Of course this problem does not tell us what kind of collision it is, or if there is friction.

This post was edited by Azrad on Nov 9 2013 05:28pm

Done.

This post was edited by Stolem on Nov 9 2013 05:44pm