As shown in the figure, stroboscopic photos of tennis ball moving from left to right on the horizontal ground are taken. The known flash flashes once every 0.02s and is recorded according to the photo

As shown in the figure, stroboscopic photos of tennis ball moving from left to right on the horizontal ground are taken. The known flash flashes once every 0.02s and is recorded according to the photos. According to the picture, at the same time, the distance of tennis is getting shorter and shorter, and according to the formula v=st, the movement speed is getting smaller and smaller; According to the diagram, the distance that tennis moves at the same time is different, so tennis moves in a variable speed straight line.

So the answer is: reduce; Variable speed straight line.