# Newton's Second Law Question

#### apples

1. A 2.0 kg otter starts from rest at the top of a muddy incline 85 cm long and slides down to the bottom in 5.0 s. What net force acts on the otter along the incline.

2. Relevant equations- F=ma, maybe some more too.

3. The attempt at a solution
I attempted this question for one and a half hour, but I couldn't get it. This is coming in my exam TOMORROW, but I just don't get it.
The answer is 14.0 N, but I just don't know how to get it. And it's too late to ask help from a school teacher. I've done this question before, but someone took my physics copy to copy all the assignments and never gave it back...
Now I don't know what to do.

#### marlon

##### nereida gallardo
you can calculate the acceleration along the incline using

$$x-x_i = v_i t + \frac{at^2}{2}$$

$$x_i$$ is the initial position along the incline : 0 m
$$v_i$$ is the initial velocity along the incline : 0 m/s

marlon

#### apples

The initial x position isn't 0 m, it's an incline :S

#### PhanthomJay

Homework Helper
Gold Member
The initial x position isn't 0 m, it's an incline :S
You are looking for the acceleration and the net force in the direction of the incline; therefore, you can choose the slope of the incline to be the x axis, and set x_initial = 0, as previously noted. Also, recheck the value of the correct answer.

#### marlon

##### nereida gallardo
The initial x position isn't 0 m, it's an incline :S
Yes it is. Check how i defined the reference frame (Like PhantomJay explains).

Again just take the origin at the starting point. The x axis is along the incline. You can chose the X and Y axis anyway you want.

marlon

#### apples

I still don't get it
Suppose i take the incline as the x axis...
xf-xi=.85m
but vi=0 m/s
and the acceleration in the x direction is 0 m/s... or not??
This is the part i'm getting confused in
There's no accelration in the x direction.
Suppose i take the condition as it is.
According to my frame of reference the height of the incline is on the y axis.
the incline is a horizontal line with a negative slope...
then the acceleration in the x direction is 0 so is vi...
I don't understand. how can 0=0.85....

#### cristo

Staff Emeritus
I still don't get it
Suppose i take the incline as the x axis...
xf-xi=.85m
but vi=0 m/s
and the acceleration in the x direction is 0 m/s... or not??
This is the part i'm getting confused in
There's no accelration in the x direction.
What makes you think there is no acceleration in the x direction? You are looking for the net force acting on the otter.. this force will provide the acceleration. If you consider, like marlon says, taking x_i = 0 and v_i = 0, then there must be an acceleration in order for the otter to move!

Using Marlon's equation

you can calculate the acceleration along the incline using

$$x-x_i = v_i t + \frac{at^2}{2}$$

$$x_i$$ is the initial position along the incline : 0 m
$$v_i$$ is the initial velocity along the incline : 0 m/s

marlon
work out the acceleration. Then you should be able to find the net force

#### Doc Al

Mentor
I still don't get it
Suppose i take the incline as the x axis...
xf-xi=.85m
but vi=0 m/s
and the acceleration in the x direction is 0 m/s... or not??
Why in the world would you think there's no acceleration down the incline? If there were no acceleration, the otter would just sit there. But he slides down!

Taking x to be along the incline, use the given kinematics equation to solve for the acceleration.

#### apples

I think i'm confusing it a bit with projectile motions....
i was thinking that that the otter slid down the incline because of gravity. and since there is an incline, he slides down it, but... errr.. I don't know, I think I"m confusing myself. Let me see if I can solve this now.
EDIT: can someone explain why the acceleration in a projectile motion without air resistance is zero, and why the acceleration here isn't zero?

Last edited:

#### Doc Al

Mentor
can someone explain why the acceleration in a projectile motion without air resistance is zero, and why the acceleration here isn't zero?
The acceleration of a projectile without air resistance is 9.8 m/s^2 down. Not zero!

The only force on a free projectile is gravity, and that acts in such a way that all objects experience the same acceleration--called the acceleration due to gravity.

#### apples

Yeah so the accelration along the y axis is 9.81 m/s^2, and the acceleration along the x axis is 0!...
So why wasn't the only force acting on the otter here gravity? Please explain, I hope this answer would remove all my confusions.
Thanks a lot

#### Doc Al

Mentor
Yeah so the accelration along the y axis is 9.81 m/s^2, and the acceleration along the x axis is 0!...
For a free projectile, where the y-axis is vertical and the x-axis is horizontal, this is correct.

(1) Gravity is not the only force acting on the otter. The incline exerts a force on the otter! That makes a big difference: something sliding down a ramp is not a projectile.
(2) The x-axis is not horizontal, it is parallel to the incline. This is done for convenience, since we know that the only motion will be parallel to the incline.

#### apples

Thanks a lot! I think I'm clear about all this now. :)

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