For example, a car hits a brick wall. But the car was traveling at constant speed. Zero acceleration.
In: Physics
At the moment of impact the speed goes from something to nothing. That is (negative) acceleration.
While not hitting the wall (in a vacuum), constantly speeding car doesn’t assert force to anything.
A force acted upon it
It takes less force to accelerate a Corvette to 60mph than it does a semi
As the car drives, it keeps moving via the engine accelerating it as fast as the friction it is experiencing decelerates it (Well, at least in a hypothetical scenario where that is possible)
If the car hits a brick wall, part of its’ speed, possibly all of it, is transferred onto the brick wall, which is why it deforms. The total force at the moment of impact doesnt change. Part of it is transferred, thus causing the wall to deform and such, but it remains the same all over. The car is slowed in the process
You are confusing momentum (p=mv) with force (f=ma). The relationship between the two can be seen with the impulse. Change in momentum, delta_p = mass.x.delta_v (change in velocity). Divide both sides by change in time, delta_p/delta_t = mass.x.delta_v/delta_t. The right side of the equation can be rewritten as mass.x.acceleration (delta_v/delta_t = a). Bringing it all together, this means delta_p/delta_t = ma = f, or rewritten delta_p = f.x.delta_t ([Wikipedia – impulse physics definition](https://en.m.wikipedia.org/wiki/Impulse_(physics)))
In plain english, this equation just says that the change in the momentum of an object can be established by a force applied externally for a period of time. In this case the brick wall applies a stopping force upon contact. However, it’s not instantaneous, as that would mean the period of time would be 0, and the force would be infinite.
To reduce the force felt from the impact by the passengers, vehicle manufacturers add crumple zones (aka bumpers). They increase the time it takes for momentum of the car to decrease to zero, and thereby reduces the force felt.
Edit~ sorry about the formatting for the first paragraph.
the force happens when the car hits the brick wall, which causes a pretty hight deacceleration, but you use it as if it was positive
>For example, a car hits a brick wall. But the car was traveling at constant speed. Zero acceleration.
Could you elaborate on how you arrive at zero acceleration?
A car moving at a constant 30m/s, then hitting an obstacle and coming to a full stop over the course of a second experiences an acceleration of -30m/s^2 during that crash.
The answer is they don’t. Until they collide with something else and accelerate (negatively in this case)
The car is no longer traveling at a constant speed. The car is accelerating from, say, 60mph to a dead stop. An object going at a constant speed experiences no net force.
When the car makes contact with the wall, a force (normal force) acts on the car, causing a deceleration. That’s what changes the speed of the car to 0 and does damage to the car. Newtons third law (equal and opposite reactions) explains what the wall feels (the same magnitude of force exerted on it – damaging the wall).
You can think of the same situation in terms of energy. A moving mass has energy (kinetic). When a force is applied to it, energy is transfered. In this case, the speed (and therefore the kinetic energy) goes to 0, so that energy is converted to other forms -which are observed in the aftermath of the crash (sound, friction heat, deformation of metal etc).
Forces cause energy transfer and change the state of motion of things
Well, it has momentum even if its moving at constant velocity, and when it collides with another object then the momentum transfers, and new contact forces are subjected to it
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