That means person inside car is either too heavy or something like a toy is caught in engine
While driving, apply the brakes gently for a short distance, as you maintain your speed.
you have to be more specific for a proper answer... at what speed does it shake?.... 40mph> ? what do you mean mist?
You have a wheel balance problem. Did you hit a curb? Did the wheel get encased in mud? Wheel balance fall off? Try changing positions of the wheels to isolate the problem to a specific wheel or tire? Or is the shake connected with engine speed? Vehicle speed? While using the brakes it gets worse or no effect? See how specific you can get the problem and tell your mechanic what you know.
Check your wheel bearings, tie rod ends, balljoints and CV axles. A bent rim will also cause a shake at high speed.
shifted or broken belt in the tires
needs wheel balance .maybe needs air in tires
The most likely cause is a warped rotor. It can also be other things so have you brakes and suspension inspected.
Mass. Speed. Condition of road surface and road wheels. Awareness. Well serviced brakes.
Speed
Because you will skid and lose control of the vehicle. Unless it is equipt with anit-lock brakes.
Could be,Tire(s) out of balance, A worn tie-rod end (s),
If the back end of your vehicle starts to skid do not panic and apply the brakes because if you do that the problem's gonna worsen ! All you have to do is continue driving but at a lower speed because this is gonna happen mostly on highways or a long road cause that's when you're faster and this happens. So continue reducing your speed gradually until you can safely use your brakes.