# Homework Help: Driving force of a car experiment

1. Apr 14, 2017

### Fred Hill

1. The problem statement, all variables and given/known data
A car driving at a speed of 60km/h at a horizontal road.
The car accelerates to 65km/h, and then the engine gets deactivated. The speed then decreases to 55km/h in a time of 7,2 seconds.
The mass of the car is 1450 kg(the car, passenger etc...)

2. Relevant equations
What is the driving force of a car that's driving at the speed of 60km/h?

3. The attempt at a solution

Until here I'm not understanding so much, I thought maybe to subtract the sum of the forces by the mass, but I do not can't find a reasoning for that.

2. Apr 14, 2017

### PeroK

This force $F = -2012N$ that you have found. What do you think that is?

3. Apr 14, 2017

### Fred Hill

I think that is the net force, that tells me that the car is accelerating.

4. Apr 14, 2017

### PeroK

Okay, so what causes this force?

5. Apr 14, 2017

### Fred Hill

An unbalanced force in one direction. I think it is the friction force?

6. Apr 14, 2017

### PeroK

If you've ever ridden a bicycle, you should have some experience of air resistance. There will be other resisting forces, but for a car that is the main one.

The question is: do you think air resistance and other resisting forces apply when the car is moving at a constant 60km/h?

7. Apr 14, 2017

### Fred Hill

Yes, I think those resisting forces is what keeps a car at a constant speed. But then when the driving forces stop, the resisting forces increase, which causes the car to slow down?

8. Apr 14, 2017

### PeroK

That's an odd way to look at it! Why should the resisting forces increase when you stop driving?

What about if the resisting forces stay the same, whether you are driving or not?

9. Apr 14, 2017

### Fred Hill

Yes, you are right. The resisting forces won't increase I assume. If the car is still, it means the resisting forces and the driving forces are equal?

But then it's the driving force which is decreasing? While the resisting forces are the same. I may be very lost.

10. Apr 14, 2017

### PeroK

If the engine is not driving the car, then it will slow down (due to various resisting forces).

If the engine is driving the car, but the car is moving at constant speed, then the drving force of the engine must equal the resisting forces. Does that seem logical?

11. Apr 14, 2017

### willem2

You need some unit conversion. If you want to use F=ma with F in newtons and m in kg, a will have to have units of ms-2

12. Apr 14, 2017

### Fred Hill

Yes, according to Newtons first law I would think it is?

13. Apr 15, 2017

Thank you!