Suppose you throw a small rock over a lake so that it sails over the surface of the water. The rock has a mass of 0.090 kg and is initially moving at 8.0 m/s. How much net force does it take to bring the rock to a stop in 1.00 s?

A. 89 N

B. 0.120 N

C. 0.72 N

D. 0.011 N

**What do y'all think? :) thanks!!!

Question

Suppose you throw a small rock over a lake so that it sails over the surface of the water. The rock has a mass of 0.090 kg and is initially moving at 8.0 m/s. How much net force does it take to bring the rock to a stop in 1.00 s?

A.   89 N

B.   0.120 N

C.   0.72 N

D.   0.011 N

**What do y'all think? :) thanks!!!

loser66 · Answer

dan?? help, please.

abhisar · Answer

ok, you need to calculate the deacceleration first using the equaions of motion.

anonymous · Answer

okie! which ones are those again?

abhisar · Answer

use, v=u+at
plug in u = 8
v=0
t=1
and calculate a. 
Then multiply it with the weight of the stone to get force.

anonymous · Answer

okay:)

so 0=8+a(1)
0=8+a
a=-8 ?

abhisar · Answer

yep, now multiply it with weight.

anonymous · Answer

so -8 * 0.090 kg?
= -0.72 ?

anonymous · Answer

would that make the answer C.  0.72 N ?

abhisar · Answer

yes, the negative sign shows that you need to apply the force in opposite direction of the motion of stone.

anonymous · Answer

ahh okay awesome!! thank you!!! :D do you get this one possibly as well? :/ 

The same net force that acts on Rock A produces three times the acceleration when it acts on Rock B. What can you say about the mass of Rock B?

A.     Rock B has three times the mass of Rock A.

B.     Rock B has one-third the mass of Rock A.

C.     Rock B has the same mass as Rock A.

D.     Rock B has a mass of zero.

anonymous · Answer

not quite sure what that means :/ I think it would be either A or B? :/ since rock a is three times the acceleration? but not very sure!! :(

abhisar · Answer

F=mass X acceleration,
So for same force, mass is inversly proportional to acceleration. Greater the acceleration, smaller the mass.

abhisar · Answer

Got it ?

anonymous · Answer

okay, so would this be the correct solution?

B.     Rock B has one-third the mass of Rock A.

anonymous · Answer

not sure if i understood correctly? haha:P

anonymous · Answer

oh wait oops it should be A right?

abhisar · Answer

B

abhisar · Answer

Rock B has one third the mass of Rock A

anonymous · Answer

ohh haha yeah makes more sense because of the greeter acceleration smaller mass right?

anonymous · Answer

ahh okay thank you @Abhisar !!! :D

abhisar · Answer

Yep, :)

perl · Answer

good job everyone, i just noticed this question