Optimized Force of Gravity Homework - Answers

[Jonathan Teas]

Homework Statement

ok this is the scenario: just supppose you had a block of ordinary ice on a frozen lake. Of course, it is free from all friction. Now, suppose some continuous, constant force acts on the block in a horizontal direction. after some time, the force is stopped so the speed of the block will have increased a certain amount.

A:if orignal force is left unchanged, but the mass is doubled, the the speed increase will be
a:unchanged,b:doubled,c:tripled,d:cut in half,e:four-fold
B:if the mass and action time is unchanged but the force is doubled, then the change in speed will be:
a:unchanged,b:doubled,c:trippled,d:cut in half,e:four-fold
C:the force the and the mass are the same, but somehow the force of gravity is doubled. the the speed increase will be:
a:unchanged,b:doubled,c:trippled,d:cut in half,e:four-fold

Homework Equations

f=ma

The Attempt at a Solution

I think, that for A it should be e, only b/c if the mass is doubled then the speed will decrease heavily.
For B, i think that it should be quadrupled. but I am really not sure.
For C, I think that if the force of gravity is doubled the the speed should be halfed and not 4-fold b/c g is never squared but I am really not sure.

 

[betel]

There is another relevant equation relating speed v and acceleration a.

All your answers are wrong.
For A: Why do you say four-fold if on the other side you say the speed will decrease?
For C: How does gravity act on your block?

 

[Jonathan Teas]

by four-fold doesn't that mean its divided by four which would indicate speed decreasing.
for c, gravity is doing double the pull on the object so if gravity is doubled would speed be halfed or...what other relavent equation is there...speed=d/t, d=1/2at^2,a=d/t

 

[betel]

Four fold means times four. Either way it would be wrong.
What is pulling at the block? Imagine (or draw) the situation. You have block of Ice lying on a lake. How does gravity act on the block.

There is one equation relating the speed after some time with acceleration a.
Find this equation and then combine it F=ma to so you only have speed, acceleration, time and force in the equation. Then you can read off the behaviour needed.

Btw. your last equation a=d/t is wrong.

 

[HallsofIvy]

The "force of gravity" has nothing at all to do with this problem. It acts only vertically which you can ignore in this problem.

 

[Jonathan Teas]

What do you mean by something pulling at the block all foces are gone besides the blocks own weight.a = Δv/Δt i think that's the equation that you are referring too.

 

[betel]

v=a \cdot t is indeed the equation I was referring to.

Gravity does not play a role for this problem. The block is on the surface of a frozen lake. This means effectively the block is sliding on a horizontal plane without friction. As gravity is always directed downwards it cannot move the block horizontally. Therefore there has to be some external force accelerating the block. What kind of force this is, is not specified.
Furtermore, for gravity it would not make sense to say the force is doubled or stopped, as you cannot simply do this with gravity.