What if two children push a swing at the same time? Would the swing go higher? Yes, together their force would be more. It is common for forces to be combined. Most objects on Earth have at least two forces acting on them. Do you know what one of them is? Of course, that force is gravity. How many forces do you have on you right now? Gravity pulls you down toward the center of Earth. Your legs exert a downward force. They hold you up against the pull of gravity. Consider the example in Figure 1.3. A book is resting on a table. Gravity pulls the book downward. It has a force of 20 newtons. At the same time, the table pushes the book upward. Its force is also 20 newtons. The table opposes the pull of gravity.

At what level of force does the table oppose gravity?

(CNN) -- Britain's Chris Froome won the 17th stage of the Tour de France on Wednesday to strengthen his grip on the yellow jersey. Froome completed Wednesday's 32-kilometer time trial in 51 minutes 33 seconds, nine seconds ahead of Spain's Alberto Contador with his compatriot Joaquim Rodriguez Oliver finishing a further second behind in third. "I couldn't believe it when I got over the line and saw I had the fastest time," Froome said after his third stage win of this year's tour. "I went into today thinking I was going to try and limit my losses, thinking about the days to come now. So to go through the finish line with the fastest time, I really didn't see that coming." On mountainous terrain along a frequently twisting route most of the riders opted for their traditional road bikes over the more aerodynamic machines used in time trials. But Froome opted to use both on the stage, switching to a time trial model for the final 12-kilometer descent. The swap proved decisive as the Briton clawed back a 20-second advantage Contador had built up earlier in the stage. The Spaniard now moves up to second overall, four minutes 34 seconds behind Froome with just four stages remaining. Contador's Saxo-Tinkoff teammate Roman Kreuziger is now third and trails Froome by four minutes and 51 seconds. Thursday sees the riders tackle the gruelling 172.5-kilometer 18th stage which concludes with a double ascent of Alpe d'Huez over the last 60 kilometers.

How long did Fromme bike on the 17th stage before the final descent?

As for engravings, Durer's work was restricted to portraits and illustrations for his treatise. The portraits include Cardinal-Elector Albert of Mainz; Frederick the Wise, elector of Saxony; the humanist scholar Willibald Pirckheimer; Philipp Melanchthon, and Erasmus of Rotterdam. For those of the Cardinal, Melanchthon, and Durer's final major work, a drawn portrait of the Nuremberg patrician Ulrich Starck, Durer depicted the sitters in profile, perhaps reflecting a more mathematical approach. Despite complaining of his lack of a formal classical education, Durer was greatly interested in intellectual matters and learned much from his boyhood friend Willibald Pirckheimer, whom he no doubt consulted on the content of many of his images. He also derived great satisfaction from his friendships and correspondence with Erasmus and other scholars. Durer succeeded in producing two books during his lifetime. "The Four Books on Measurement" were published at Nuremberg in 1525 and was the first book for adults on mathematics in German, as well as being cited later by Galileo and Kepler. The other, a work on city fortifications, was published in 1527. "The Four Books on Human Proportion" were published posthumously, shortly after his death in 1528.
How many books were produced by Durer in total?