Sự phát triển của khoa học kĩ thuật thế giới nửa đầu thế kỉ 20 đã có những tác dụng và hạn chế gì ? Mỗi ý lấy 1 ví dụ để chứng minh

take five to 10 minutes to warm up and cool down properly.

plan to start slowly and boost your activity level gradually unless you are already exercising frequently and vigorously.

be aware that training too hard or too often can cause overuse injuries like stress fractures, stiff or sore joints and muscles, and inflamed tendons and ligaments. sports prompting repetitive wear and tear on certain parts of your body — such as swimming (shoulders), jogging (knees, ankles, and feet), tennis (elbows) — are often overuse culprits, too. a mix of different kinds of activities and sufficient rest is safer.

listen to your body. hold off on exercise when you're sick or feeling very fatigued. cut back if you cannot finish an exercise session, feel faint after exercise or fatigued during the day, or suffer persistent aches and pains in joints after exercising.

if you stop exercising for a while, drop back to a lower level of exercise initially. if you're doing strength training, for example, lift lighter weights or do fewer reps or sets.

for most people, simply drinking plenty of water is sufficient. but if you're working out especially hard or doing a marathon or triathlon, choose drinks that replace fluids plus essential electrolytes.

choose clothes and shoes designed for your type of exercise. replace shoes every six months as cushioning wears out.

for strength training, good form is essential. initially use no weight, or very light weights, when learning the exercises. never sacrifice good form by hurrying to finish reps or sets, or struggling to lift heavier weights.

exercising vigorously in hot, humid conditions can lead to serious overheating and dehydration. slow your pace when the temperature rises above 70°f. on days when the thermometer is expected to reach 80°f, exercise during cooler morning or evening hours or at an air-conditioned gym. watch for signs of overheating, such as headache, dizziness, nausea, faintness, cramps, or palpitations.

dress properly for cold-weather workouts to avoid hypothermia. depending on the temperature, wear layers you can peel off as you warm up. don't forget gloves.

what do you mean

paul willis (born 1950) is a british social scientist known for his work in sociology and cultural studies. paul willis' work is widely read in the fields of sociology, anthropology, and education, his work emphasizing consumer culture, socialization, music, and popular culture. he was born in wolverhampton[1] and received his education at the university of cambridge and at the university of birmingham. he worked at centre for contemporary cultural studies and subsequently at the university of wolverhampton. he was a professor of social/cultural ethnography at keele university. in the autumn of 2010, he left keele university and is now a professor at princeton university.

paul willis’s work has focused mainly, but not exclusively, on the ethnographic study of lived cultural forms in a wide variety of contexts. from highly structured to weakly structured ones, willis examines how practices of `informal cultural production` to produce and construct cultural worlds `from below`.[2]

trained in literary criticism at cambridge,[2] paul willis received his phd in 1972 from the centre for contemporary cultural studies at birmingham university where he remained as senior research fellow until 1981. he gave a paper at the 9th symposium of the natiohnal deviancy conference in january 1972 entitled 'a motor-bike subculture

during the 1980`s willis served as youth policy adviser to wolverhampton borough council in the english midlands. there he produced the youth review (published by the council and ashgate) which formed the basis for youth policy and for the formation of the democratically elected youth council, both still functioning. during the 1990s he served first as head of the division of media, communications and cultural studies, and then as a member of the professoriate at the university of wolverhampton. in 2000 willis co-founded the sage journal, ethnography. in 2003 willis was hired as a head professor of social and cultural ethnography at keele university.