[Music] hi and welcome back to three sighs lessons cold dot uk' by the end of this video you should be able to describe what's meant by a resultant force you should then be able to calculate the resultant of forces acting in a straight line and if you were hired here student you should be able to draw a Freebody diagram to show the forces acting on an object now I should point out that in this video we're just looking at how to calculate resultant forces we're not looking at how those forces affect the motion of objects we were looking at that in a later video okay now remember that a force is a push or pull that acts on an object due to the interaction with another object forces of vector quantities because they've got both magnitude in other words size and direction so in this video were looking at resultant forces I'm sure you hear a man pushing a box along the floor the man's applying a force of 20 Newtons acting to the right and we can show that with this ro a friction force of 10 Newtons is acting to the left now the resultant force is a single force that has the same effect as all of the original forces acting together to work out the resultant force we subtract the smaller force from the larger force in this case there's a resultant force of 10 Newtons acting to the right okay this shows a car traveling along a road the driving force of the car is 10,000 Newton's acting to the left the car experience the friction force with the road of 4,000 Newtons acting to the right the force of air resistance also acts against the car this has a value of 5,000 Newton's acting to the right so how do we work out the resultant force in this case first we need to work out the total force acting to the right I don't together friction with the road and air resistance gives us a total force of 9,000 Newton's acting to the right subtracting 9,000 Newton's from the driving force gives us a resultant force of 1,000 Newton's acting to the left okay this shows a skydiver falling through the air at a constant velocity the skydiver experiences a force of 800 Newton's acting downwards this is the skydivers weight and other words the force due to gravity at the same time the skydiver experiences an upward force of 800 Newtons due to air resistance in this case the forces are balanced in other words the resultant force is zero now in the examples we've seen so far we've shown all the objects involved however it's much easier to draw a Freebody diagram I'm showing you the Freebody diagram for the skydiver here in a Freebody diagram the object is shown as a point the forces are drawn as are go starting at the point remember that the length of the arrow shows us the size of the force and the direction of the arrow shows us the direction of the force I'm showing you here an aeroplane flying at a constant velocity at a constant altitude and the word altitude means the height above the ground we're going to finish now by looking at the forces involved so here's a Freebody diagram as you can see the weight of the aeroplane is acting downwards towards the earth however as we said the aeroplane is at a constant altitude so that means that there must be a force the same magnitude as the weight but acting in the opposite direction that force is called lift now the airplane experienced the forward force provided by the engines and that force is called first and I'm showing that here at the same time the airplane also experiences the force of air resistance or drag acting in reverse direction now because the airplane is moving at a constant velocity the forward and backward forces must be balanced and we can see that by the length of the arrows remember you'll find plenty of questions on resultant forces in my revision workbook and you can get that by clicking on the link above [Music]