在磁感应强度为B的匀强磁场中，一个静止的放射性原子核发生了一次α衰变．放射出α粒子（ ${}_2^4\mathrm{He}$ ）在与磁场垂直的平面内做圆周运动，其轨道半径为R．以m、q分别表示α粒子的质量和电荷量．

    La Tomatina, Spain

    Tomato juice flows through the streets of Bunol on the last Wednesday of August as the worlds largest tomato fight happens. Festivities begin in the town square where there is a huge pole covered in soap and with a leg of ham at the top. Participants try to climb up the pole to get the ham as watchers throw buckets of water to them. After a gun is fired to signal the beginning of the fight, people catch the tomatoes and throw each other in the streets until the gun sounds again, marking the end of the event.

    Holi, India

    This spring festival is celebrated throughout the country in late February or early March. The two-day event celebrates the legend of Prahlad, who escaped unharmed from the fire of the evil Holika Dahan. People mark the event by lighting fires and throwing colored powder on one another Children load colored water into toy pistols called pichkari and take aim at passers-by. Holi is known as a happy celebration of the victory of good over evil.

    Burning Man, Nevada

    This infamous festival began in 1986 with the burning of a large figure to mark the summer solstice (夏至). It's since grown into a week-long annual event that draws more than 48,000 people to the Black Rock Dessert. It is held during the week before Labor Day. Participants set up camp in the desert to celebrate art.

    Songkran, Thailand

    This festival marks the Thai New Year, which is celebrated in the second week of April. This is the hottest time of the year in Thailand, so it makes sense that the biggest Songkran tradition is to throw water on people. Thais of all ages join in water fights, using buckets, hoses (软管) and water guns to celebrate the event.