How to Correctly Apply Newton's Laws to Determine Forces on a Dragged Log?

[Beretta]

To drag a 75kg log along the ground at constant velocity, you have to pull in it with a horizontal force of 250N. a) what is the resistive force exerted by the ground? b) what horizontal foce must you exert if you want to give the log an acceleration of 2m/s^2?

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Why my answer as -75kg as the resistive force exerted by the ground was concidered wrong? As well as 150N as the horizontal force that should be exerted to give the log an acceleration of 2m/s^2 was concidered wrong too.

 

[enigma]

this really should have been posted in the homework help forum, but...

a) kg is not a unit of force. Draw a free body diagram and see what magnitude the force has to be (zero net force = zero acceleration = constant velocity)

b) if it takes 250N of force just to keep moving at a constant speed, how is possible to accelerate at 2m/s^2 with 100N less force?

 

[Beretta]

We didn't cover the freebody digram yet. Would you please explain more?

 

[enigma]

Sure you did.

You
1) draw a picture of the object
2) draw in all of the forces
3) break the forces into X and Y components (they are all X in your problem)
4) add up the forces, taking into account + and - directions

If the object is moving at constant velocity the sum of the forces = 0

 

[Beretta]

Sigam Fx = Fnx + wx + Fx = Ma = 0
Fnx = -wx - Fx
Fnx = -75kg - Fx cos 0
Fnx = -325 N ?

 

[Beretta]

To drag a 75kg log along the ground at constant velocity, you have to pull in it with a horizontal force of 250N. a) what is the resistive force exerted by the ground? b) what horizontal foce must you exert if you want to give the log an acceleration of 2m/s^2?
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a) Is it 250N because of the constant acceleration?

b)
F = m*a
a = 2 m/s^2 and m = 75kg gives F = 150 N that must be 150 N over the 250 N the ground resists. so you must drag 400 N

Please help me out!

 

[HallsofIvy]

Originally posted by Beretta
To drag a 75kg log along the ground at constant velocity, you have to pull in it with a horizontal force of 250N. a) what is the resistive force exerted by the ground? b) what horizontal foce must you exert if you want to give the log an acceleration of 2m/s^2?
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a) Is it 250N because of the constant acceleration?

NO, it's 250N because of the constant SPEED! The acceleration is 0 so the "net" force is 0.

b)
F = m*a
a = 2 m/s^2 and m = 75kg gives F = 150 N that must be 150 N over the 250 N the ground resists. so you must drag 400 N

Please help me out!

Exactly right.

 

[Beretta]

First and second law

Thank you very much :)