แรงโน้มถ่วงสากล
และแรงเสียดทาน
กฎความโน้มถ่วงสากลของนิวตัน (Newton's law of universal gravitation)
นิวตันได้ระบุในงานของเขา “Philosophiae Naturalis Principia Mathematica” เกี่ยวกับการดึงดูดของจุดมวลในเอกภพว่า
“แต่ละจุดมวลในเอกภพจะดึงดูดจุดมวลอื่นๆ ด้วยแรงที่มีขนาดเป็นสัดส่วนโดยตรงกับผลคูณของมวลทั้งสอง และเป็นสัดส่วนผกผันกับค่ากำลังสองของระยะห่างระหว่างกัน” ซึ่งก็คือ กฏความโน้มถ่วงสากลของนิวตัน เขียนความสัมพันธ์ได้เป็น
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โดย
คือ ค่าคงตัวความโน้มถ่วงสากล มีค่าเป็น
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คือ ค่ามวลของวัตถุแรกกับวัตถุที่สองตามลำดับ
คือ ระยะห่างระหว่างมวลสองก้อน
แรงเสียดทาน (Friction)
คือ แรงที่ต้านการเคลื่อนที่ของวัตถุ เกิดขึ้นระหว่างผิวสัมผัสของสองวัตถุและมีทิศตรงข้ามกันกับทิศของการเคลื่อนที่ แบ่งออกเป็น 2 ประเภท คือ
- แรงเสียดทานสถิต (Static friction) เกิดขึ้นเมื่อมีแรงกระทำกับวัตถุหนึ่งแต่ยังไม่เคลื่อนที่ และจะมีค่ามากที่สุดตอนที่วัตถุเริ่มเคลื่อนที่บนผิวหนึ่ง โดยขนาดของแรงเสียดทานมีค่าเป็น
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2. แรงเสียดทานจลน์ (Kinetic force) เกิดขึ้นเมื่อผิวของวัตถุหนึ่งเคลื่อนที่บนอีกผิวหนึ่งที่มีอัตราเร็วคงตัว โดยแรงเสียดทานจลน์จะมีค่าน้อยกว่าแรงเสียดทานสถิต และมีค่าเป็น
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ลักษณะของแรงเสียดทาน
- แรงเสียดทานนั้นไม่ขึ้นกับจำนวนพื้นที่ผิวสัมผัส
- แรงเสียดทานนั้นไม่ขึ้นกับความเร็วที่วัตถุเคลื่อนที่
และยังมีทิศตรงข้ามการเคลื่อนที่ของวัตถุ - แรงเสียดทานขึ้นอยู่กับแรงที่วัตถุกดพื้นในแนวที่ตั้งฉากหรือแรงปฏิกิริยาของพื้นในแนวที่ตั้งฉาก
- แรงเสียดทานขึ้นกับพื้นผิวสัมผัส เช่น เรียบหรือขรุขระ
ปัจจัยที่มีผลต่อแรงเสียดทาน
แรงเสียดทานจะมีค่ามากหรือน้อยขึ้นอยู่กับปัจจัยเหล่านี้คือ
- แรงกดที่ตั้งฉากกับผิวสัมผัส ถ้าแรงกดตัวฉากกับผิวสัมผัสมากจะเกิดแรงเสียดทานมาก ถ้าแรงกดตั้งฉากกับผิวสัมผัสน้อยจะเกิดแรงเสียดทานน้อย
- ลักษณะของพื้นผิวสัมผัส พื้นผิวสัมผัสที่หยาบและขรุขระจะเกิดแรงเสียดทานมากกว่าพื้นผิวสัมผัสที่เรียบลื่น
- ชนิดของผิวสัมผัส เช่น คอนกรีตกับเหล็ก เหล็กกับไม้ จะเห็นว่าผิวสัมผัสมีความหยาบขรุขระ เรียบลื่น เป็นมันแตกต่างกัน ทำให้เกิดแรงเสียดทานไม่เท่ากัน
การลดแรงเสียดทาน
การลดแรงเสียดทานสามารถทำได้หลายวิธีดังนี้
- การใช้น้ำมันหล่อลื่นหรือจาระบี
- การใช้ระบบลูกปืน
- การใช้อุปกรณ์ต่างๆ เช่น ตลับลูกปืน
- การออกแบบรูปร่างของยานพาหนะให้เพรียวลม
ทำให้ลดแรงเสียดทาน
การเพิ่มแรงเสียดทาน
การเพิ่มแรงเสียดทานมีประโยชน์ในด้านความปลอดภัยของมนุษย์ เช่น
- ยางรถยนต์มีดอกยางเป็นลวดลายเพื่อเพิ่มแรงเสียดทานระหว่างล้อกับถนน
- การหยุดรถต้องเพิ่มแรงเสียดทานที่เบรก เพื่อหยุดหรือทำให้รถแล่นช้าลง
- รองเท้าบริเวณพื้นต้องมีลวดลายเพื่อเพิ่มแรงเสียดทานทำให้เวลาเดินไม่ลื่นหกล้มได้ง่าย
- การปูพื้นห้องน้ำควรใช้กระเบื้องที่มีผิวขรุขระเพื่อช่วยเพิ่มแรงเสียดทาน