Find the object's acceleration

[Kerdo]

Hello!
Would love a quick explanation of these two exercises :)

1. Homework Statement
1) Find objects acceleration, when mass is 200kg and it is being pulled on a horizontal plane with a force of 2000N. How much work is done on 400m road?
2) An object fell from 200m. How much of its energy was converted to work, which deformed land when air resistance was 50N? What was the velocity of the object when it hit the ground? Which would have been the velocity if there was no air resistance?

Homework Equations

F = ma;
Ek = mv^2/2;

The Attempt at a Solution

1) Found the acceleration with the formula of Newton's second law, but couldn't go forward.
2) Don't get the air resistance part.

 

[QuantumQuest]

Hello Kerdo

1) Found the acceleration with the formula of Newton's second law, but couldn't go forward.

What do you mean you couldn't go forward? If you're referring to work what is the relevant equation?

2) Don't get the air resistance part.

What have you tried for this so far?

 

[PeroK]

Hello!
Would love a quick explanation of these two exercises :)

1. Homework Statement
1) Find objects acceleration, when mass is 200kg and it is being pulled on a horizontal plane with a force of 2000N. How much work is done on 400m road?
2) An object fell from 200m. How much of its energy was converted to work, which deformed land when air resistance was 50N? What was the velocity of the object when it hit the ground? Which would have been the velocity if there was no air resistance?

Homework Equations

F = ma;
Ek = mv^2/2;

The Attempt at a Solution

1) Found the acceleration with the formula of Newton's second law, but couldn't go forward.
2) Don't get the air resistance part.

For 1) What do you know about work? What is work?

 

[Kerdo]

Hello Kerdo



What do you mean you couldn't go forward? If you're referring to work what is the relevant equation?
What have you tried for this so far?

Thanks for your reply!

1) I was able to calculate the acceleration of the object which was (10 m/s^2). But didn't know how to go forward, I know that the object had kinetic energy, but there I need the velocity and for velocity I need time.

2) I haven't done any exercises with air resistance, it seems like: object is falling with velocity x, so I have to calculate the force and subtract the air resistance?

For 1) What do you know about work? What is work?

Thanks for your reply!

I know that work is the energy that gets lost when an object does something (am I correct?).

 

[CWatters]

There is an equation relating work to force and something else that is listed in the question.

This equation will help with both questions.

Work = force * ?

 

[QuantumQuest]

I know that work is the energy that gets lost when an object does something (am I correct?).

No. Have you learned about work? Work expresses transfer of energy or conversion of energy from one form to another. There is an equation for work, as pointed out by CWatters, relating work, force and one more thing. Can you write it down?

 

[Kerdo]

There is an equation relating work to force and something else that is listed in the question.

This equation will help with both questions.

Work = force * ?

Thanks for your reply!

work = force * distance

that means, 100% of potential energy is converted to work? And from there I can calculate the force and subtract the air resistance?

 

[Kerdo]

For the second exercise I got:

Ep = mgh, Ek = mv^2/2
Ep = Ek, mgh = mv^2/2 => gh = v^2/2 => v = sqrt(2gh)
so v ~= 62,6m/s without the air resistance.

How do I subtract the air resistance from there?

Sorry, have been studying for preliminary exam for like 4 hours and it's all so confusing :D

 

[QuantumQuest]

work = force * distance

The equation is Work = Force • Displacement • Cosine(theta) or $W = F\cdot s\cdot \cos(\theta)$ where $\theta$ is the angle between force and displacement. If the displacement takes place horizontally$^*$ then this angle is $0^{\circ}$ and the equation becomes $W = F\cdot s$. Can you finish the first question?

$^*$
and the force is collinear and in the same direction

 

[Kerdo]

I think I figured this out!

1)
a = F/m => a = 2000/200 = 10m/s^2
A = Fs => A = mas => A = 200 * 10 * 400 = 800000J = 800kJ
And it seems to be correct.

For the second one I'm not sure how to use the air resistance.

 

[QuantumQuest]

I think I figured this out!

1)
a = F/m => a = 2000/200 = 10m/s^2
A = Fs => A = mas => A = 200 * 10 * 400 = 800000J = 800kJ
And it seems to be correct.

Yes it is correct. There is no need to recalculate $F$, it is given.

For the second one I'm not sure how to use the air resistance.

Note that the problem does not give the mass of the object, so you cannot calculate the force exerted on the object downwards. You're given only the air resistance. So what does this mean in terms of the method you will use?

EDIT: It is also a good idea to learn to use Latex. You can find help using the link which is right below your post editor on the left bottom ("?LaTeX / BBcode Guides").

 

[CWatters]

For the second one I'm not sure how to use the air resistance.

The air resistance is a force and you know the displacement.