# Newtonian motion

1. Apr 12, 2016

### Ab17

1. The problem statement, all variables and given/known

You are playing with your daugther in the snow. She sits on a sled and asks you to slide her across a flat , horizontal field. You have a choice of (a) pushing her from behind by applying a force downward on her shoulders at 30 degrees below the horizontal (b) attaching a rope to the front of the sled and pulling with a force 30 degrees above the horizontal. Which would be easier for you and why

2. Relevant equations
F =ma

3. The attempt at a solution
I think both situations yield the same net force thus the same acceleration

2. Apr 12, 2016

### Staff: Mentor

Consider the case where the sled has a constant velocity. Do you still need a force to keep it moving? What about that effect (it is also relevant while still accelerating)?

3. Apr 13, 2016

### Ab17

Both options are not in constant velocity. Both options have one force in the horizontal direction

4. Apr 13, 2016

### Staff: Mentor

It does not matter.

There is an effect you missed so far. The force won't all be used for acceleration. There is something else to consider.
Hint: it is snow, not ice.

5. Apr 14, 2016

### Ab17

Friction

6. Apr 14, 2016

### Staff: Mentor

Correct. How do you calculate the friction force? Does pushing/pulling at an angle change it?

7. Apr 14, 2016

### Ab17

I think for both cases it will be -Fcos@

8. Apr 14, 2016

### Staff: Mentor

What is F, and where does the sign come from, and in which direction?

9. May 7, 2016

### Ab17

F is the applied force

10. May 7, 2016

### Ab17

The sign is a mistake

11. May 7, 2016

### nrqed

I think that what mfb wants to make you think about is the definition of the friction force. What is the fundamental equation that gives the kinetic friction force on a sliding object?

Last edited: May 7, 2016
12. May 8, 2016

### Ab17

Fk = ukN

13. May 8, 2016

### nrqed

Right. So what can we say about the normal force in the first case compared to the second case?

14. May 8, 2016

### Ab17

It has a component in one case?

15. May 8, 2016

### Staff: Mentor

What does "it has a component" mean? All forces can always be split into separate components. But where do those components point to?

16. May 10, 2016

### Ab17

I mean the force is at an angle so it will result in a horizontal and vertical comp.. but the thing is both forces are at angles. But the components of both forces have different direction

17. May 10, 2016

### Ab17

Are you trying to point that?

18. May 10, 2016

### Staff: Mentor

Right.
So what is your conclusion?

19. May 10, 2016

### Ab17

The y comp of first case will be up while down in second case....so maybe more friction in second case? Is that right

20. May 10, 2016

### Staff: Mentor

The second case is you pulling up. You expect more force on the ground there?

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