# Car starting to move, friction and forces

1. Feb 5, 2015

### Karol

1. The problem statement, all variables and given/known data
A car of weight 17[kN] starts to move. the coefficient of friction is 0.6. the dimensions in the drawing.
The height of the CoM, point C, is 0.5[m]. the front wheels drive the car.
What are the pressing forces on the front and rear wheels and what is the friction force on the front wheels.

2. Relevant equations
Friction: f=μN
Moments: F1L1=F2L2
3. The attempt at a solution
Rf is the reaction on the front and Rr is on the back. the reactions caused by the weight:
$$R_f=\frac{2}{3}\cdot 17=11.33$$
The friction force on the front: f=0.6⋅11.33=6.8. it should be 4.36.
The friction "pulls the carpet" under the CoM and creates a moment which is balanced by the rear wheels:
$$0.5\cdot 6.8=R_r\cdot 2\rightarrow R_r=1.7$$
This force adds to the reaction to the weight: Rr=(17-11.33)+1.7=7.37
Of course it's not true. it should be Rr=4.47. the pressing force on the front wheels should be Rf=7.27
But the motor reacts to the torque applied to the front wheel and this moment increases the pressure on the rear wheels, but for that i need the radius of the wheel.[/SUB]

[Moderator note: repaired subscripting tags - gneill]

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2. Feb 5, 2015

### Svein

The drawing suggests that 2/3 of the "weight" of the car is on the front wheels. This will give you the maximum friction force from the road on the car.

3. Feb 5, 2015

### Karol

Yes, that's what i have done in my second and third lines in my calculations

4. Feb 5, 2015

### Svein

Sorry, but most of your reasoning is all but unreadable, since it is inside a "SUB" clause.

5. Feb 5, 2015

### Staff: Mentor

@Svein : Subscripting tag balance has been restored.

6. Feb 5, 2015

### jbriggs444

You can solve angular momentum problems by choosing an arbitrary axis. You are not required to choose an axis about which something is actually rotating. You are not required to choose an axis through which an axle passes. This makes the wheel radius irrelevant.

In the usual case, almost any axis will work. You want to choose one that will simplify the problem by eliminating one or more terms from the resulting equations. Good choices are:

An axis positioned at the center of mass.
An axis positioned at the point of application of a key force.

7. Feb 5, 2015

### Staff: Mentor

Is that the exact and entire problem statement?

Are you not told how the car starts to move? Surely an acceleration would make a difference in the moments.

8. Feb 5, 2015

### Staff: Mentor

Yes. I agree. I think what the problem statement is implying is that the acceleration is so high that the front wheels are on the verge of peeling out.

Karol:

If the rear wheels are freely rotating and have negligible mass, then the torque on the rear wheels is (a) negligible or (b) not negligible.
Based on this answer, is the frictional force of the ground on the rear wheels (a) negligible or (b) not negligible.

What direction is the frictional force exerted by the ground on the front wheels? (a) rearward or (b) forward?

Chet

9. Feb 5, 2015

### Karol

I don't think it's a problem of angular momentum since they didn't teach this complex subject and the chapter is about moments and forces.
And also i think i need the moment that is exerted on the front wheels and for that i need the radius, but it's not given.
These are all the details given in the question, no acceleration, so i guess it's on the verge of slipping.
Let's assume the frictional force on the rear wheels=0 and with negligible mass. these assumptions are adequate to the material learned.
I guess the frictional force is forward since the car starts to move forward.

10. Feb 5, 2015

### Staff: Mentor

This is all correct, except for your indication that there's no acceleration. If the frictional force on the front wheels is forward, then the car is accelerating forward.

Chet

11. Feb 5, 2015

### Karol

yes, correct, forward, simple

12. Feb 5, 2015

### jbriggs444

If they are teaching you about moments then you definitely do need to consider weight transfer from front to rear.

But the radius of the wheels is still irrelevant. Moments can be computed about an arbitrary chosen axis. However, in order to avoid talking about angular momentum, the chosen axis in this case would need to be at the car's center of mass. (With any other axis choice, the car would have an angular momentum that changes over time and one would be hard pressed not to talk about it).

13. Feb 5, 2015

### Staff: Mentor

I respectfully disagree. One can take moments about any axis, even if the center of mass is accelerating, if one includes the moment of the pseudo force -ma. However, in this problem, it is best to take moments about the center of mass.

Chet

Last edited: Feb 5, 2015
14. Feb 5, 2015

### Karol

There isn't angular momentum in this problem since the material is for a simpler degree than engineering, it's for practical engineers who study 2 years.
I solved 25 problems in this chapter and none with angular momentum. it's not even mentioned.
The chapter is the second in this book which is Statics, the first chapter is about vector algebra, center of mass and etc.

15. Feb 5, 2015

### Staff: Mentor

Yes. I agree. So where do you stand now on the solution to this problem? What is your force balance in the vertical direction, and what is your moment balance about the center of mass?

Chet

16. Feb 6, 2015

### Karol

I repeat the solution i posted in #1:
Rf is the reaction on the front and Rr is on the back. the reactions caused by the weight:
$$R_f=\frac{2}{3}\cdot 17=11.33$$
The friction force on the front:
$f=0.6\cdot 11.33=6.8$ it should be 4.36.
The friction "pulls the carpet" under the CoM and creates a moment which is balanced by the rear wheels:
$$0.5\cdot 6.8=R_r\cdot 2\rightarrow R_r=1.7$$
This force adds to the reaction to the weight: $R_r=(17-11.33)+1.7=7.37$
It should be Rr=4.47. the pressing force on the front wheels should be Rf=7.27
I don't feel comfortable at all with the solution but i had to do something.

17. Feb 6, 2015

### Staff: Mentor

Let NR be the normal force on the rear wheels and let NF be the normal force on the front wheels. Let μNF be the frictional force on the front wheels. With this notation, what is the force balance in the vertical direction and what is the moment balance about the center of mass?

Chet

18. Feb 6, 2015

### jbriggs444

I agree with your disagreement, but was trying not to include that pseudo-force.

19. Feb 6, 2015

### Staff: Mentor

In posts #1 and #16, you omitted the moment about the center of mass caused by the frictional force.

20. Feb 6, 2015

### Karol

$$\left\{ \begin{array}{l} NF+NR=17 \\ NF(0.5\cdot\mu+1)=2\cdot NR \end{array} \right.$$
$$\rightarrow NF=10.3, \ NR=6.7, \ \mu NF=6.18$$
It should be: NF=7.27, NR=4.47, μNF=4.36

Last edited: Feb 6, 2015