1 00:00:14,000 --> 00:00:17,000 Have you ever noticed that it's harder to start pedaling your bicycle 2 00:00:17,000 --> 00:00:20,000 than it is to ride at a constant speed? 3 00:00:20,000 --> 00:00:23,000 Or wondered what causes your bicycle to move? 4 00:00:23,000 --> 00:00:26,000 Or thought about why it goes forward instead of backwards or sideways? 5 00:00:26,000 --> 00:00:29,000 Perhaps not, and you wouldn't be alone. 6 00:00:29,000 --> 00:00:31,000 It wasn't until the 17th century 7 00:00:31,000 --> 00:00:34,000 that Isaac Newton described the fundamental laws of motion 8 00:00:34,000 --> 00:00:37,000 and we understood the answer to these three questions. 9 00:00:37,000 --> 00:00:40,000 What Newton recognized was that things tend to keep on doing 10 00:00:40,000 --> 00:00:43,000 what they are already doing. So when your bicycle is stopped, 11 00:00:43,000 --> 00:00:46,000 it stays stopped, and when it is going, 12 00:00:46,000 --> 00:00:48,000 it stays going. 13 00:00:48,000 --> 00:00:50,000 Objects in motion tend to stay in motion 14 00:00:50,000 --> 00:00:52,000 and objects at rest tend to stay at rest. 15 00:00:52,000 --> 00:00:55,000 That's Newton's First Law. 16 00:00:55,000 --> 00:00:59,000 Physicists call it the Law of Inertia, which is a fancy way of saying 17 00:00:59,000 --> 00:01:03,000 that moving objects don't spontaneously speed up, slow down, or change direction. 18 00:01:03,000 --> 00:01:09,000 It is this inertia that you must overcome to get your bicycle moving. 19 00:01:09,000 --> 00:01:12,000 Now you know that you have to overcome inertia to get your bicycle moving, 20 00:01:12,000 --> 00:01:14,000 but what is it that allows you to overcome it? 21 00:01:14,000 --> 00:01:17,000 Well, the answer is explained by Newton's Second Law. 22 00:01:17,000 --> 00:01:20,000 In mathematical terms, Newton's Second Law says 23 00:01:20,000 --> 00:01:24,000 that force is the product of mass and acceleration. 24 00:01:24,000 --> 00:01:27,000 To cause an object to accelerate, or speed up, 25 00:01:27,000 --> 00:01:29,000 a force must be applied. 26 00:01:29,000 --> 00:01:31,000 The more force you apply, 27 00:01:31,000 --> 00:01:34,000 the quicker you accelerate. And the more mass your bicycle has, 28 00:01:34,000 --> 00:01:36,000 and the more mass you have too, 29 00:01:36,000 --> 00:01:39,000 the more force you have to use to accelerate at the same rate. 30 00:01:39,000 --> 00:01:43,000 This is why it would be really difficult to pedal a 10,000 pound bicycle. 31 00:01:43,000 --> 00:01:48,000 And it is this force, which is applied by your legs pushing down on the pedals, 32 00:01:48,000 --> 00:01:51,000 that allows you to overcome Newton's Law of Inertia. 33 00:01:51,000 --> 00:01:54,000 The harder you push down on the pedals, the bigger the force 34 00:01:54,000 --> 00:01:56,000 and the quicker you accelerate. 35 00:01:56,000 --> 00:01:58,000 Now on to the final question: 36 00:01:58,000 --> 00:02:00,000 When you do get your bike moving, 37 00:02:00,000 --> 00:02:02,000 why does it go forward? 38 00:02:02,000 --> 00:02:05,000 According to Newton's Third Law, for every action, 39 00:02:05,000 --> 00:02:07,000 there is an equal and opposite reaction. 40 00:02:07,000 --> 00:02:11,000 To understand this, think about what happens when you drop a bouncy ball. 41 00:02:11,000 --> 00:02:13,000 As the bouncy ball hits the floor, 42 00:02:13,000 --> 00:02:15,000 it causes a downward force on the floor. 43 00:02:15,000 --> 00:02:17,000 This is the action. 44 00:02:17,000 --> 00:02:21,000 The floor reacts by pushing on the ball with the same force, 45 00:02:21,000 --> 00:02:24,000 but in the opposite direction - upward - 46 00:02:24,000 --> 00:02:27,000 causing it to bounce back up to you. 47 00:02:27,000 --> 00:02:29,000 Together, the floor and the ball form what's called 48 00:02:29,000 --> 00:02:32,000 the action/ reaction pair. When it comes to your bicycle, 49 00:02:32,000 --> 00:02:35,000 it is a little more complicated. As your bicycle wheels spin 50 00:02:35,000 --> 00:02:39,000 clockwise, the parts of each tire touching the ground 51 00:02:39,000 --> 00:02:41,000 push backwards against the earth - 52 00:02:41,000 --> 00:02:45,000 the actions. The ground pushes forward with the same force 53 00:02:45,000 --> 00:02:48,000 against each of your tires - the reactions. 54 00:02:48,000 --> 00:02:53,000 Since you have two bicycle tires, each one forms an action/ reaction pair 55 00:02:53,000 --> 00:02:56,000 with the ground. And since the Earth is really really really big, 56 00:02:56,000 --> 00:02:59,000 compared to your bicycle, it barely moves 57 00:02:59,000 --> 00:03:02,000 from the forced caused by your bicycle tires pushing backwards - 58 00:03:02,000 --> 99:59:59,999 but you are propelled forward.