# Forces of motion.

1. Oct 15, 2006

### AznBoi

I need help with two more problems, thanks! I have work for one of the problems, please see if its close to being correct. Ty.

1. The driver of a speeding empty truck slams on the brakes and skids to a stop through distance d. a) If the truck carried a load that doubled its mass, what would be the truck's "skidding distance"? b) If the initial speed of the truck were halved, what would be the truck's "skidding distance"?

My work: a) If the truck carried a load that doubled its mass, the truck’s “skidding distance” would be half of the original distance in which the truck carried nothing.
b) If the initial speed of the truck were halved, the truck’s “skidding distance” would also be close to half of the original distance when the truck had 2 times more initial speed.

2. A truck loaded with sand acclerates along a highway. If the driving force on the truck remains constant, what happens to the truck's accerleration if its trailer leaks sand at a constant rate through a hole in its bottom?

I don't get how the sand would influence the acceleration of the truck if it is going down into a hole at the bottom. I mean the sand's inertia is accelerating forward but how would it affect going down the bottom?

2. Oct 15, 2006

### Staff: Mentor

You gave answers, but you didn't show how you got those answers. Hints: What force stops the truck? How does that force depend on the truck's mass? What acceleration does the force produce?

Hint: The force remains constant, but does the mass?

Last edited: Oct 15, 2006
3. Oct 15, 2006

### AznBoi

1) If the mass on the truck doubles, then its inertia would be less than before. Therefore it would be harder to stay in its original motion

If the initial speed were halved, then the distance would be half because the initial force is half?

2) ok, I get it I think. Because it's mass decreases as the force remains constant, its inertia increases, so does the acceleration of the truck?
Is it because the accerleration of an object is inversely proportional to its mass??

4. Oct 15, 2006

### AznBoi

Am I on the right track?

5. Oct 15, 2006

### Staff: Mentor

Inertia is proportional to mass. But try to answer my earlier questions as precisely as possible.

6. Oct 15, 2006

### AznBoi

1. What force stops the truck:
The ground friction stops the truck.

How does that force depend of the truck's mass:
The more mass it has the smaller the inertia, so friction would stop it easier than if the mass was smaller, having more inertia (original line of motion)

What acceleration does the force produce:
F=ma?? so a= F/m ?

2. If sand falls below the truck at a constant rate, the driving force would increase proportionally. ?

Wait are the hints for two seperate questions? lol

7. Oct 15, 2006

### Staff: Mentor

Exactly: Kinetic friction stops the truck.

How does that friction force depend on the mass of the truck? (Stop using the word inertia. )

How the force and mass changes determines how the acceleration changes. The acceleration (and speed) determines the stopping distance.

You are told that the driving force remains constant. But a changing mass would affect the acceleration.

8. Oct 15, 2006

### AznBoi

Oh I see it is because you need more acceleration when carrying a truck with more mass. So if you decrease the mass, the driving force would still remain constant, but the accleration would increase because there isn't that much weight right? Is that what you mean?

9. Oct 15, 2006

### AznBoi

The more mass a truck has, the more friction force it takes. lol, this is just an intro hw to Newton's laws, I haven't really learned about kinetic friction yet.

10. Oct 15, 2006

### Staff: Mentor

If the force remains the same (which is given), the acceleration (given by a = F/m) will increase as the mass decreases.

11. Oct 15, 2006

### Staff: Mentor

If the mass doubles, the friction force doubles. But what happens to the acceleration? (a = F/m)

12. Oct 15, 2006

### AznBoi

I see, the acceleration decreases. Thanks for your help.