Driving force of a car experiment

[Fred Hill]

Homework Statement

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...)

Homework Equations

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

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.

 

[PeroK]

View attachment 194718

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.

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

 

[Fred Hill]

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

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

 

[PeroK]

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

Okay, so what causes this force?

 

[Fred Hill]

Okay, so what causes this force?

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

 

[PeroK]

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

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?

 

[Fred Hill]

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?

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?

 

[PeroK]

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?

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?

 

[Fred Hill]

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?

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.

 

[PeroK]

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.

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?

 

[willem2]

View attachment 194717

View attachment 194718

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

 

[Fred Hill]

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?

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

 

[Fred Hill]

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

Thank you!