For example, a car hits a brick wall. But the car was traveling at constant speed. Zero acceleration.
In: Physics
> For example, a car hits a brick wall. But the car was traveling at constant speed.
It was. It wasn’t when it hit the wall. It accelerated when it hit the wall and slowed, probably to a stop, probably very quickly.
(by the way, remember that in physics, “acceleration” doesn’t just mean “getting faster”. Slowing down and changing direction are also forms of acceleration–any change in velocity is acceleration)
This is why crumple zones work. If you increase the time it takes for the car to slow down, then the acceleration is reduced, so the force on the passengers is reduced.
F = ma can be read out as “the acceleration that something feels is proportional to the force that is exerted upon it”. The proportionality is something we call “mass”, and goes hand-in-hand with the concept of “inertia”. Also note that acceleration is the change to velocity, which includes both how fast something is going, and where it is moving.
When a car is undergoing constant motion, it does not change direction nor the speed at which it is going. This means that there are no external forces that accelerate the car at that moment, and thus zero acceleration at that moment. However, that does not mean that acceleration was always zero throughout the history of the car’s movement. At some point, the car had to accelerate to go from not moving to moving. However, once this force is removed, the car will continue moving at that final speed that it settled at, from the *initial* force that enabled it to accelerate from rest.
In the example of the brick wall, the car is moving with constant velocity until it hits the wall and will go from travelling at constant speed in a particular direction, towards stopping (or bouncing off, etc.) Because the velocity (speed and direction of movement) changed from the collision, this means there was necessarily acceleration. However, F = ma does not tell you *where* that acceleration came from, just that some force F must exist to do so, during the time interval of the collision (i.e. how long the car and wall are touching/interacting).
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