so welcome to another week of level one Nancy a mechanic’s interview this week we’re looking at section two of the course which is all about forces section one was covering acceleration and velocity and bloody time graphing distance time grabs those action to look at forces we’re gonna look at Newton’s law of motion and they’re not really explicitly mentioned in NCAA but the concepts are set so we talked about is three laws and number two we look at labeling forces and the net force concept and number three we’ll introduce some new formula here will introduce force equals mass times acceleration and we can also be reiterated as weight equals mass times gravity and then their fourth it factor we look at they would be pressure equals force over area the context for these probably bests and just look at some examples and then we’ll illustrate that at the point that we go so if we just lay down you can see law of motion first of all before we get started with any exam question as a Newton’s laws of motion so Isaac Newton came up with 83 laws of motion in the first law states that an object will keep doing what it’s doing just right is that men record okay so I’ve just written that it’s a sometim so Newton’s first law of motion and object will keep doing what it’s doing unless the forces acting on it become unbalanced so we’ll take a look at a question from 2013 and this question looks at the scenario of a runner and the graph got lucky time graphic just reproduced it right here that they decided and in this case they’re assessing the concept of equals first law of motion the idea for speed balanced or unbalanced so the question reads on the diagram below these three diagrams from section a B and C after velocity time graph draw and label of thrust and friction forces acting on the runner in your answer you should use our own Sushil the directions of thrust and friction forces beside each diagrams did it thrust is greater than friction crust is equal to friction or frost is less than four so this is all based on the first law where the forces are balanced or unbalanced so if we look at the first picture here section a on the graph on a velocity time graph this means acceleration so you have acceleration there section b would be constant speed and then section C would be deceleration or negative acceleration so if we go to a diagram then the forces that thrust force is what is acting this direction so this would be thrust and since in section a the runner is accelerating the friction force should be smaller than your thrust force so they said you died out show the directions we did that a mainstay to thrust is greater than friction so in this case thrust is greater than friction forces are unbalanced forces are unbalanced and therefore you get acceleration which is a change in motion so that follows Newton’s first law we take a look at the second picture section B and we’re moving at constant speed here constant speed means the forces are balanced for the positive thrust and friction in this case to show that balance in the forces we want to draw for gyros which are the same in size in this case thrust is equal to friction force is equal to friction the forces are balanced forces are balanced and therefore you have constant speed which means no change in motion and so the object keeps doing what it’s doing because there’s no on balancing in the forces and section C then we have deceleration in which case the thrust force is still there but the friction force is now larger and so crust is less than friction and we have forces are unbalanced again so we have a change in motion forces are unbalanced and therefore we get acceleration now that would be the answer to that first part that’s Part A Part B of the question is just above the graph here so we take a look at that and it continues on fartsy rolling belt starting Patsy and referring to your Forge diagrams in Part B explained the link between the net force acting on the runner in section of a B and C of the graph on the type of motion in your answer you should describe what is meant by net force command the length between net force and the motion for each section and compare the direction of the net force in the direction of the motion for each section okay so the easiest way to explain what net force is is factually just add some values to the forces so in the first section here a we

said that the first force was greater than the friction so we give those some baddies if we say the first force is equal to 50 Newtons and then the friction force is is less than that so let’s say it’s 30 Newtons then the net net force or F net is going to be 20 Newtons so 50 minus 30 and it’s going to be in the forward direction for which direction if we look at the second situation the forces are balanced so we could say that we have now got 50 Newton’s of first again but we also have 50 Newtons of friction so in this case the forces are not balanced the F net is equal to zero Newton and therefore there is no change motion so that was you know that voice and the last picture then we’re looking at the fact that friction is greater than thrust so let’s say friction is equal to 60 Newtons here and then the thrust forward forward is only 30 Newtons and therefore the F net in this case is going to be equal to 30 Newton and that’s going to be in the backwards direction so net force is just when you look at all the forces acting on the object and you add them all up and look at what the circle is force is it’s gonna be acting on the vehicle direction and you could also consider support in gravity here but since they’re balanced the whole time so it’s a they would 1418 for support and continued gravity when they effectively cancel each other right there in all cases so 40 and 40 so that’s what net force is you know if we go back up and look at what else the question is asking us he asked us to explain link between net force and the motion for each section which I effectively did and compare the direction of the net force in the direction of the motion for each section which we just looked at as well for a more detailed answer here’s what the an SI gear answer looks like so net force is the resultant force a net force auto-repair to the resultant boys when multiple forces interact there multiple forces we had here were friction gravity we also had support force there which is just a surface supporting in object and they also had thrust so when welcome forces interact if the forces are pointing in the same direction their thought is ad giving a larger net force so if we look at packet re-exam question for a second just to exemplify that you could have friction from the surface of the ground so this $3.00 per second this is your honor you could have you’re gonna have support force of the ground and it supports that object but you can also have a friction which is 20 surfaces in contact and then let’s say the wind takes off the head wind picks up there’s going to be an increase in air resistance so air resistance out here and if 30 forces are acting in the same direction which they are then the 20 Newton from a distance of the 30 Newton confliction on the surface will add an additional net force their the cross let’s say was 40 Newtons so first at 40 Newtons and the air resistance friction makes 50 Newtons then the F net in that case is going to be 10 Newtons in the backwards direction so that’s what they mean by net force it’s being force is being edited so the courses are it out the directions are forced to subtract giving us more a net force including and a zero net force net forces determine whether the donor is accelerating decelerating or maintaining a constant speed saturated with section a decelerating was section C and maintaining a constant lead with section D if the net force is pointing the same direction of motion the object accelerates if the net force is pointing in the opposite direction to the direction of fall should I update decelerates and if there is zero net force and the object maintains a constant speed or a stationary and so in Section eight it on here is accelerating this is because there’s a net force pointing forwards this occurs for the first pole created in the fiction section be the runner has a constant speed this is because there is no overall net force this occurs come towards the equal to friction and in Section C the runner is decelerating this is because there is no is there is a net force pointing in the opposite direction to the motion so you do need to be aware that when you’re on see these questions you need to write us for a detailed answer as possible what I did was quite short just using some diagrams you need to write the text support your answer but that Sam we go back to our objectives for today’s video we’ve looked at Newton’s first law of motion we’ve looked at labeling forces in the net force concept so we’re gonna now take a look at Newton’s second law which is about air force and mass applied sorry fourth mass acceleration let’s take a look at the section Karina okay

so I’m moving on to Newton’s second law you can second law of motion sees that the acceleration of an object is due to the force and mass applied which is essentially force equals mass times acceleration and so f equals force forces are all measured in Newtons myself to be in kilograms just remember that so if you gram per kilograms an acceleration of the hydrant section one is in meters per second squared very straightforward formula to use you just have to be able to recognize where it appears in exam question so if we take a look at 2011 we have a question about a runner again and there are two sections that we’ve looking at here section next section why a boy runs and on the track is shown above drink section X he runs with a constant speed of 2 meters per second for 15 seconds during section Y he runs with a constant acceleration of 0.2 m/s squared calculate the net force acting on the body with a mass of 60 kilograms during section Y and given appropriate unit for your answer so it’s simply using F equals M times a force equals mass times acceleration and where this is f net that we’re looking at and that’s going to be equal to 60 kilograms multiplied by 0.2 m/s squared which works at the announcer of 12 Newtons that’s how you use force equals mass times acceleration now sometimes they may ask you to rearrange the formula so if you had to do form a triangle for that you would have force on the top because force equals mass times acceleration and so force equals mass think acceleration up on the top and on the bottom multiplied by AIDS which you want to work at the acceleration in this case given the force you would say cover the one that you’re looking for so we would cover a in this case and we’re looking for acceleration is going to be equal to force divided by mass which in this case would be 12 Newton’s divided by 60 kilograms and that should give you your point two meters per second squared so you need to remember how to turn the formula force equals mass times acceleration into this triangle but that’s essentially what you can come across force equals mass times acceleration the other relationship between weight mass and gravity which is essentially the same formulas when we take a look at an example one of those questions okay so this is question three from 2013 and in this case we’re looking at the difference between weight and mass and then attacking the weight of a box a box in warehouse has a mass of two thousand five hundred kilograms so it’s in kgs already nugatory but there it’s way the difference between weight and mass it’s not often on the physics papers to get you to explain concepts but in this case you do so weight and mass the weight of an object the weight of an object is the force that’s in exerts on a surface the force that it exerts or applies on a surface due to gravity falling down on it so way to the and therefore its measured in Newtons not in kilograms so weight of an object the force the weight of an object is the is the force that it exerts in a surface due to gravity pulling down it whereas the mass is the amount of meant of matter in something you might have matter in an object and this is measured in kilograms now the great way to explain this is if you have a mass of 60 kgs on earth then your mass on the moon will also be 60 kgs because you don’t unless you remove part of your body chopping our mark or 1/4 or whatever you’re gonna have the same amount of matter making you up sea should have the same mass 60 kilograms however their your weight is different your way to earth would be 600 Newtons and your weight on the moon would be 1/6 1/6 of that there so you’d only be 100 Newtons and that’s because gravity on the moon is the 6th six last gravity is six times less powerful on the moon than it is on the earth and that’s due to the fact that the moon is much smaller much smaller so it’s your weight to changes as you move throughout the solar system not your mass and that’s the difference there if we look at the nca answer for that one weight is the downward force due to gravity that an object experiences while mass is a measurement matter about their

cars and back to the question they want you to calculate the weight of a box now the weight formula is equal to mass times gravity and this is essentially a reiteration of the force equals mass times acceleration formula because the value of g is tilt and mass is still in kilograms and weight is still in Newtons so they’re effectively the same formulas the value of g though we’re looking at here is actually the force due to gravity with the 10 units per kilogram in this formula whereas this formula here it’s acceleration due to gravity which is 10 meters per second square so the unit’s just changed but if that could be it’s the same number I’m so the weight of the box here weight at the box is going to be equal to the mass which was 2,500 kilograms and that was given up top here so 2,500 kilograms by gravity which is 10 Newton’s per kilogram and that will get rid of your and kgs there which will give you an answer of 25000 Newtons and that’s the weight at the box in Newtons so that’s the w equals M times G format so the last thing we want to look at this morning is the pressure equals force over area formula so P static pressure and that’s going to be Newton’s per meter squared because we have force which is measured in Newtons being divided by area which in this case is being measured in liters squared so pressure equals force over area you get a formula and it essentially to calculate the pressure exerted by not bigger than usually have to explain the concept of surface area and the spreading of pressure okay so for the final section where we’re looking at pressure equals force over area we’re looking at this question for from 2011 and the question for from 2011 what’s a better pair of football boots one pair with studs and one pair without stud the question is as follows a student of mass 40 kilograms uses the football boots on the bug wandered without studs has a surface area of 165 centimeters squared which is if you divide that by 100 that’s 0.01 65 meters squared in contact with the ground one booth with six coats on the surface area only six centimeters squared or point zero zero zero six meters squared in contact with the ground so this surface area here surface area here is much smaller than the surface area above if you don’t have the studs calculate the pressure P exerted if the student stands on one port now the significance of saying one caution is that you only have to do the calculation for one thought if it was both beaten he’d have the times it times the surface area by two on a hard surface for the boot without stones and for the blue crystals so without studs the pressure equals force over area now the force of the student is going to be worked out using force equals mass times G which in this case were using the weight for my so W equals n times G so that’s equal to 40 kilograms multiplied by the force of gravity which we just had with 10 Newton’s per kilogram and that gives you value of four hundred Newtons for the weight of this student for the weight of this student is going to be constant in this case so the pressure is going to be equal to four hundred Newton’s divided by the area in this case in meters squared which is zero point zero one six five meters squared and that gives you pressure or the food without studs up twenty four thousand two hundred forty two Newton’s per meter squared look the answer that would go there and then with studs pressure is equal to force over area so in this case it’s four hundred Newtons and that’s being exerted over a surface area which is smooth much smaller of only 0.006 meters squared and I give you pressure 666666 I’m beat it squared so the pressure is much greater with the studs than without the studs and then the question goes on and says or what is the advantage of having studs and so the suppose the advantage gained by the student when running on South Crosse football field while wearing the boots which stores compared with wearing the boots at the same size without sort in your answer you should compare the pressure exerted on the ground by the booth with the stores in the blue collector stood so with the studs we just had the pressure was six six

hundred and sixty six thousand six hundred and sixty thousand you my meter squared and then the booth at the stove just had the pressure would equal to twenty four thousand two hundred and forty two Newton’s per meter squared and an explained the relationship between surface area and the pressure exerted and explain how the difference and pressures would help the students going on Excel to certify class so effectively because the pressure is much much higher on the booth with the studs their boot is going to push those doors into the soft brass and then you get better traction and then for the boot without the studs the pressure is much less and as a result the boot is going to really grip that circulates in baffle you want too much traction to on right and play football if we look at the answer from instant year and that one they say for 2011 excuse me my dear Levin we’re looking at here’s the comparison I’ll just zoom in a bit okay so the pressure exerted by the boots those is greater than the boots don’t we just looked back icky bodies right there and they’re smaller the surface area which is their force which the ports leaders over the greater the pressure exerted on the ground and how it happened glass as a student way to the plate only smaller surface area with the boots which studs it needs to a greater pressure on the blinds this means that the studs was thinking to the grind more which will enable the student to get a good better grip or traction on the glass when running lettuce constant this question comes up and every year it’s just that we changed the context sometimes it’s skis and snow sometimes snowshoes and snow in this case it was a football cleats on grass and then all they do is change the contacts media but that is a review of all the teaching of gone this week and a little bit into next week it’s all about section two forces and then after you come back from the holidays we look at the final section for level one mechanics which will be on energy changes