Front end shakes at 50mph but stops at55mph why

Because one or both of the front wheels are out of balance.
1. You need to have the front wheel/tire package rebalanced by a competent balancer. Go to a good tire retailer. They have the equipment to do the re-balancing for you. Don't go to the cheapest shop you can find and hope they will do a great balance job for you.
2. If you still have the vibration after re-balancing (assuming it is done well) you will either have a faulty tire or tires (out of round or a heavy spot in the tire) or there is a problem with one or both of the wheels. If either of the front wheels have been slightly bent out of shape from abuse you will get a vibration in the front end of the car and in that event the wheel needs to be replaced. If you have a cheap brand of tires fitted to the car and the re-balancing does not solve the vibration problem I would suspect the tires first. Cheap tires will typically give you problems like this if you buy them but better brands can have tire faults also.

3. Do not let someone tell you that a front wheel alignment will solve a balance problem with the wheels/tires. It will not because the vibration is caused by the rotating wheel and tire. By all means have a wheel alignment done again - because you need to do this on the car about every 10-12000 miles in any event - but doing this will not solve a balance issue.

4. If you want to run a test before doing any work have the rear wheels on both sides moved to the front of the car and the fronts to the rear. Then check if you still have exactly the same vibration. If not then you know where the problem is. If you still get exactly the same vibration or a similar front vibration with the rear wheels fitted on the front, then the balance on the rear wheels/tires will need to be checked and re-done as well.