1 00:00:00,420 --> 00:00:04,550 So let us look at an example where we apply the principle of impulse and momentum. 2 00:00:04,580 --> 00:00:11,190 Now, what I've drawn here is a crate that moves along a floor while a force of F is 600. 3 00:00:11,190 --> 00:00:17,550 Newton is applied the crate as a mass of 80 kilograms of retinoic, efficient of friction. 4 00:00:17,550 --> 00:00:20,580 But for simplicity, let's ignore the coefficient of friction. 5 00:00:20,590 --> 00:00:25,230 I will tell you what you should do when you have sufficient or friction, but let's just ignore it for 6 00:00:25,230 --> 00:00:25,590 now. 7 00:00:26,190 --> 00:00:34,370 And the question is, if the crate is initially stationary, the term, the speed of the 20s, 10 seconds. 8 00:00:34,950 --> 00:00:40,440 So immediately, because time is mentioned here, these 10 seconds, that points to the fact that we 9 00:00:40,440 --> 00:00:44,610 need to use the principle of impulse and momentum to solve this problem. 10 00:00:46,000 --> 00:00:54,550 So let us write down the principle of impulse momentum, so M one, V one vector, plus I the impulse 11 00:00:54,550 --> 00:01:00,370 vector form is MTV to vector on the right and side. 12 00:01:00,380 --> 00:01:05,740 I have drawn my coordinate axis and I've chosen X to be in the horizontal direction and Y to be in the 13 00:01:05,740 --> 00:01:06,660 vertical direction. 14 00:01:07,090 --> 00:01:11,740 We're only going to have movement in the X direction because the forces only applied in the X direction. 15 00:01:12,010 --> 00:01:18,100 I can therefore simplify my vector equation by writing in one V one in the X direction. 16 00:01:18,370 --> 00:01:23,740 Plus the impulse in the X direction is aimed to to in the X direction. 17 00:01:25,320 --> 00:01:26,970 So let's plug in our values. 18 00:01:28,180 --> 00:01:34,360 M1, that's the mass is 80 kilograms, and I've said that the grade was initially stationary, so V1 19 00:01:34,360 --> 00:01:37,860 one is zero plus the impulse. 20 00:01:38,620 --> 00:01:40,150 How do we calculate the impulse? 21 00:01:40,150 --> 00:01:42,350 That's the force multiplied by the time. 22 00:01:42,820 --> 00:01:48,580 So that's six hundred multiplied by ten is equal to 80 multiplied by V2. 23 00:01:48,790 --> 00:01:55,390 And now it's simple to solve for V two, which is Civita that is six thousand divided by eight and that 24 00:01:55,390 --> 00:01:58,380 gives us seventy five meters per second in the X direction. 25 00:02:00,900 --> 00:02:10,590 So if we did not neglect the effects of friction, we would have had an impulse created by friction 26 00:02:10,590 --> 00:02:13,920 because friction is in the opposite direction of movement. 27 00:02:13,920 --> 00:02:15,990 So we would have had a negative impulse. 28 00:02:16,380 --> 00:02:21,000 So we would have gone and we would have calculated the frictional force. 29 00:02:21,000 --> 00:02:24,790 And as we know, that's the frictional coefficient multiplied by the normal force. 30 00:02:24,810 --> 00:02:30,330 So we would have taken the mass of the great multiplied by nine point eight to one that would give us 31 00:02:30,330 --> 00:02:31,470 the weight of the great. 32 00:02:31,950 --> 00:02:33,570 That would be the normal force. 33 00:02:33,960 --> 00:02:39,870 We would multiply that by the frictional coefficient and then we would get a frictional force that would 34 00:02:39,870 --> 00:02:42,540 work and the opposite direction of movement. 35 00:02:42,550 --> 00:02:44,790 So that would be the negative X direction. 36 00:02:45,420 --> 00:02:53,490 And so on our impulse and momentum equation, we would have had in one V, one X plus the sum of the 37 00:02:53,490 --> 00:02:54,230 impulses. 38 00:02:54,240 --> 00:03:00,930 So we would have had the positive impulse of the force that would be six hundred multiplied by ten seconds, 39 00:03:01,290 --> 00:03:07,020 and we would have the negative impulse of the friction and that would be the frictional force also multiplied 40 00:03:07,020 --> 00:03:13,890 by ten seconds, because that's the time that it works in on the crate would be into V to X and then 41 00:03:13,890 --> 00:03:17,920 we would have gotten a smaller velocity for V2. 42 00:03:18,720 --> 00:03:23,750 So if you understand that, then you understand the concept of impulse and momentum. 43 00:03:24,870 --> 00:03:30,210 What about a case like this where the force is not applied in the horizontal direction, but with an 44 00:03:30,210 --> 00:03:32,230 angle to the horizontal? 45 00:03:32,730 --> 00:03:38,310 Now there is a force that as a component in the X direction and the wind direction. 46 00:03:39,280 --> 00:03:46,960 So that means this force has an impulse in the X and the Y direction, note that that doesn't mean that 47 00:03:46,960 --> 00:03:49,840 the great will also move in the direction. 48 00:03:51,630 --> 00:03:58,590 That will depend on whether the weight of the crate is larger than the Y component of the force, if 49 00:03:58,590 --> 00:04:05,070 the weight of the crate is larger than the Y component of the force, then the crate won't be picked 50 00:04:05,070 --> 00:04:06,120 up from the ground. 51 00:04:06,130 --> 00:04:09,060 It will then just move in the X direction. 52 00:04:10,440 --> 00:04:17,490 And so then we would only have a X component of the velocity, but if the Y component of the force is 53 00:04:17,490 --> 00:04:21,900 larger than the weight of a crate, then the crate would move in the direction that we need to apply 54 00:04:22,470 --> 00:04:26,450 the equation of impulse and momentum in the X and the Y directions. 55 00:04:28,430 --> 00:04:34,520 And so when we look at the principle of work and energy, then work is only applied when the credit 56 00:04:34,520 --> 00:04:36,980 moves in the direction of the force. 57 00:04:37,010 --> 00:04:42,590 So for this example, there won't be any work in the Y direction unless the credit moves in the right 58 00:04:42,590 --> 00:04:43,070 direction. 59 00:04:43,820 --> 00:04:44,900 But impulse? 60 00:04:46,270 --> 00:04:51,490 Doesn't necessarily cause movement, and therefore you can have an impulse, even if the crowd does 61 00:04:51,490 --> 00:04:53,070 not move in that direction.