in real life we constantly deal with uh multiple forces at the same time so we have to understand that there are different kind of uh forces acting on this on the object may be uh result in different movement okay so the first one is the linear Force linear force is two or more forces on acting on the same line so it can be on the same direction or uh to the opposite direction when is uh at the same direction basically um the force is act adding to each other uh the force will cancel uh with each other when they're in a opposite direction um this is very ideal right but in reality U the force doesn't act on the same line that often say if they are acting on the same plan like this example there are two forces acting in this direction and then another counter force acting in this direction you know basically this three point May reach to a a balance um status so in this in this uh sense we call these three uh forces as the parallel uh forces there are other terms that we use to describe uh multiple forces for example the concurrent forces the concurrent forces is two or more forces act on an object with uh from different directions but on a common point so it is different from the parallel because parallel basically they're acting on the different point but the count the concurrent forces they're actually uh acting on the same um a common points for example there's a force uh pushing this object in this to this direction and there's another uh Force that's pushing the object to this direction basically the two forces are acting on the same um spot right here so what would happen is you know one direct one force here one force there the the resultant Force um uh we will be uh in this direction um so why uh we need to understand this though so remember uh I briefly touch based on uh different forces that in clinical uh I may use to uh um manipulate the joint if if the joint space is too close I may apply the tensal force to separate them right so this will I have to be really careful to design um an activity that that's acting on the same line so that I can uh Pro uh provide this kind of traction Force to uh The Joint same thing I may need to uh design an activity that I can provide this compression force to keep the tissue together but again on the same line so to some um um degree and unfortunately this happen very frequently that uh especially for people uh sitting on a wheelchair or using a wheelchair um because the weight um is kind of pull pulling um the person down but the texture of the a wheelchair or the ankle of the wheelchair may cause this person to slide so you can see it's kind of like a two different uh forces on the surface and that create this kind of sheer force that will cause this person to slide down um the the chair um sometimes this is intentional but a lot of times trust me it's not so um we will need to choose a proper texture for the wheelchair to create this kind of action reaction force or the friction so that this person don't slide down or we can change the anle of the seating so that this person don't slide down just because we uh understand the forces uh impact to the the human movement um when we are trying to train our patient for another example here uh we can um think about this so if if I have a weak uh if I have a patient who has weak shoulder uh motion for example the weight of the upper extremity let's say 6 kilogram I can use um I can um put some weight let's say 5 kilogram to the pce system here to counter uh the six kilogram here so that means is this person can only need to generate uh 1 kilogram of force to maintain the balance or to stop this arm from going down or they can generate more than one kilogram so that this person can rise up the arm just because uh from the assistance um um of the weight that I add to it so I can modify my training intensity uh to to my client to uh gradually increase um their tolerance or muscle strength uh of using their their body if this person is kind of progressing say um the the weight of the bucket that is full of water let's say again 5 kilogram with 6 kilogram here um I can add uh weight and the weight can be seen as you know additional weight right so basically the two forces the weight of the bucket and the weight for the weight cuff can add in together so this person need to uh generate a force that can counter both for forces that's acting that's pulling um their arm down so this require this person to generate a lot more forces or gradually lower the bucket down without SP the water out so this is another way that um when in uh clinical practice um we can use uh the concept of force to design an activity for our client what about multiple forces that's acting on the same um point so um this that's simplify this um to a simple math uh question here so there's one force acting in this direction there's another force acting in this direction so when we simplify it at to a math equation to get the resultant force in this sense is C we can basically use equation a s + b² = c² so that we can get the U answer for the resultant force in this and why do we need to know um this equation now if we spend time to look at our own hand when we Bend our finger like the like the the um like the picture here um the center of these lines doesn't uh fall you know at the middle of the hand it actually deviate a little bit to uh the radial side to be specific they all pointed to the scaffo U right here so why this matters that means when we designing a splint to uh uh align or to facilitate hand movement the motion um the the direction of the motion has to be consistent with the natural alignment that uh um the move the the the finger movement should be so that being said we would need to create this kind of resting force from different uh directions so that when we have one elasri Bend here another uh Bend right here then we'll create uh the resultant Force that's consistent to the movement that we want to do so this is very important um concept that we need to be uh really careful and then really keep in mind when we apply this um our knowledge of uh forces to our clinical practice