Acellerating car vs constant speed car

• Lars Krogh-Stea
In summary, the determining quantity for the force of impact is the speed at the time of crashing. The kinetic energy of the car must be dissipated upon impact, and for normal cars, the force from the engine is much smaller than the collision force at that speed. However, there could be scenarios where this matters more, such as a very soft car with a strong engine. In this case, both impacts are the same, as disengaging the clutch does not have an impact on the speed at the time of crashing. The acceleration goes to zero immediately upon disengaging the clutch and the velocity follows a trajectory determined by acceleration and resistance. The highest speed is reached at the edge of the ramp, even though the motorcycle follows a trajectory.
Lars Krogh-Stea
Hi, I've been dsicussing this with som friends, and would be thankful if someone took the time to explain it to me :)

The scenario:
Car A travels at constant velocity (80km/h) along the road and hits a brick wall. Car B acellerates upward towards 100km/h, and at excactly the moment it passes 80km/h it hits a brick wall. Which car hits the brick wall with the most force/takes most damage?

If you'll include formulas and calculation, I'' be even more thankful :)

Best Regards, Lars Krogh-Stea

The speed ##v## at the time of crashing is the determining quantity. The kinetic energy is ##{1\over 2}mv^2## and that has to be dissipated: converted into deformation of steel and what have you.

Lars Krogh-Stea and billy_joule
Lars Krogh-Stea said:
Car A travels at constant velocity (80km/h) along the road and hits a brick wall. Car B acellerates upward towards 100km/h, and at excactly the moment it passes 80km/h it hits a brick wall. Which car hits the brick wall with the most force/takes most damage?

For normal cars the force from the engine is much smaller than the collision force at that speed, so there won’t be much difference. But you could construct scenarios where it matters more (very soft car at very low speed with a very strong engine that keeps applying a force during the collision).

Lars Krogh-Stea
A.T. said:
For normal cars the force from the engine is much smaller than the collision force at that speed, so there won’t be much difference. But you could construct scenarios where it matters more (very soft car at very low speed with a very strong engine that keeps applying a force during the collision).

In my example the driver disengages the clutch at the moment of impact, I should have mentioned that.. If there wasn't an impact my believe is that the car would reach maybe 85 or 90 km/h before it starts to slow down as the car doesn't imediately start to slow down, but that the velocity rather follows a trajectory determined by accelleration and resistance.. Doesn't that potential for extra velocity translate into to something at impact? I would believe that the input amount of energy is grater to set a car in motion that has a "velocity trajectory" that reaches 85km/h than 80km/h.. But then again, I'm open to the fact that I still don't understand this.. ;)

Lars Krogh-Stea said:
In my example the driver disengages the clutch at the moment of impact,
Then both impacts are the same.

Lars Krogh-Stea said:
If there wasn't an impact my believe is that the car would reach maybe 85 or 90 km/h...
No, it wouldn't

Lars Krogh-Stea said:
...before it starts to slow down as the car doesn't imediately start to slow down,
Yes it does.

Lars Krogh-Stea
I get that the accelleration starts to slow down at the time you disengage the clutch, but not that it goes instantly to zero.. I will test this in my car after work, minus the brick wall ;) Thanks for the answers :)

Acceleration goes to zero immediately (in fact air & other friction even makes it negative). You feel it: your body and the seat have adjusted to the acceleration -- if it stops you feel pushed forward (especially if you are not the driver).
Test carefully, preferably where there is no other traffic...

Lars Krogh-Stea
Lars Krogh-Stea said:
I get that the accelleration starts to slow down at the time you disengage the clutch,
It goes below zero due to drag and rolling resistance. Ingoring those you get constant speed in both cases, so still the same impact.

Lars Krogh-Stea said:
but not that it goes instantly to zero..
Disengaging the clutch is not instantaneous. But at the moment the clutch is completely disengaged (assuming an ideal clutch), the acceleration is zero or less.

Last edited:
Lars Krogh-Stea
I get it now... If a motorcycle is accellerating off an upward ramp, the highest speed is on the edge of the ramp, even though the bike follows a trajectory. Seems quite logic now that you explained it to me :) Thank you!

Lars Krogh-Stea said:
I get it now... If a motorcycle is accellerating off an upward ramp, the highest speed is on the edge of the ramp, even though the bike follows a trajectory.
Yes, velocity is different from position. When you throw a ball upwards, the vertical position keeps increasing for a while, but the vertical velocity starts decreasing as soon it leaves your hand.

Lars Krogh-Stea

1. What is the difference between an accelerating car and a car with constant speed?

An accelerating car is one that is increasing its speed over time, while a car with constant speed maintains the same velocity without changing.

2. How does acceleration affect a car's performance?

Acceleration plays a crucial role in a car's performance as it determines how quickly the car can reach a desired speed and its ability to overtake other vehicles on the road.

3. What are the factors that affect a car's acceleration?

The factors that affect a car's acceleration include the power of the engine, weight of the car, friction and air resistance, and the quality of the road surface.

4. Can a car maintain a constant speed without any acceleration?

No, a car cannot maintain a constant speed without any acceleration. In order to overcome air resistance and maintain a steady velocity, a car needs to apply a certain amount of force which is provided by the engine's acceleration.

5. How does acceleration impact fuel efficiency of a car?

Generally, a car with constant speed will have better fuel efficiency compared to an accelerating car. This is because acceleration requires more energy and fuel consumption from the engine. However, modern cars are designed to optimize fuel efficiency even while accelerating, so the difference may not be significant.

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