Understanding Tension in Physics: Key Concepts and Examples

so tension when does tension exist well there are law of examples in nature

where tension exists it's rubber band tension these support chains involve the tension force

now as you saw in the video when don't we have tension well just because we have these objects it doesn't

mean they're always in tension if as you can see they're not pulled tight and they're just loose

as in these situations then the cable or the rubber band or the wire would not have any tension

in this situation mentioned previously there is no tension however at this point there is tension

and specifically the tension points a along the direction that the cable is in

in this situation a spider web also has a tension force and again tension force points along the direction

of the web in this case so let's look at our first example a superhero climbs

up a web if the maximum tension the web can support is 1500 newtons what is the

maximum acceleration the superhero can move along the web if his mass is 80

kilograms so let's draw a diagram and we have our superhero here

climbing a web and there's the force of gravity acting downwards

to get the force of gravity we use the formula fg equals mg where m is the mass and g

is the acceleration due to gravity 9.8 and so the force of gravity is 784 newtons however there's also a tension

force and it's this tension force that will allow him to climb up the web

notice the tension force is significantly greater than gravity and so whenever we do these physics type

style problems we always have to determine what direction is positive

when dealing with vectors and so the positive direction in this situation would be upwards

because ultimately the superhero will be traveling upwards and the positive direction is the direction

of movement or acceleration and so we start off with an f that

statement f that equals 1500 subtract gravity 1500 is positive because it points

upwards gravity is negative because it points downwards gravity opposes the motion in this

situation what is f not equal to according to newton's second law

f net is equal to m a substituting 80 for the mass and now solving for acceleration we get a value

of eight decimal nine five meters per second squared using significant digits and noting that

there's only one significant digit here our final answer would be 9 meters per second squared

what does this actually mean well if the superhero tries to accelerate

at a greater acceleration than this value then the web would snap

so beyond this acceleration the spiderweb would snap

now what if the superhero was not accelerating but was just hanging there just stationary

what would the tension force be acting on the web please pause video now

all right i hope you gave this some thought well if the acceleration is zero the

forces are now balanced and that's the case whenever

acceleration is zero the forces are always balanced and so in this situation the tension is 784 newtons it would be

identical to the force of gravity and notice in the diagram that these two arrows are the exact same

length it's important when you draw force diagrams that you represent the scale

and if this is for example two centimeters long then this line should also be two

centimeters long to show that the two vectors are equal finally why is there no normal force in

this situation i'd like you to give it some thought please pause video now

well it's because the character is not in contact with the ground to have a normal force the character or

a person has to be in contact with the ground in this situation the character is hanging on a web

i'd like you to redo this question however this time i'd like you to calculate tension force if the

acceleration of the character is only two meters per second squared all right i hope you tried this f that

equals tension subtract gravity m a equals tension subtract 784 the force of

gravity substituting this time we know our acceleration and we're trying to solve

for tension and rearranging we get 944 newtons or with significant digits only 900

newtons now does this make sense it does notice that for a low acceleration a

small acceleration the force is smaller and in general that's the case

as the acceleration of the object increases the force increases clearly

if the acceleration decreases then the force would decrease as well greater force means greater acceleration

all right here's example two a grocery bag is lifted out of the trunk of a car with an acceleration of three

meters per second squared the bag with its contents has a mass of 5 kilograms

will the bag break if the maximum tension force the handles of the bag can withstand is 80 newtons so once

again you're trying to figure out tension if it's bigger than 80 newtons then

unfortunately the bag will break and i know that's happened to me many times over the years

but if it's less than 80 newtons then the bag will not break so i'd like you to pause the video now

and give this a shot all right here's our diagram there's our bag

notice the tension force is drawn longer than the force of gravity force of gravity is determined with this

equation 5 times 9.8 mass times acceleration due to gravity we're going to define our positive

direction as up because we're lifting up on this bag negative direction then is opposite that

or down and here we go f net equals f t tension force is positive because it

points up subtract gravity gravity is negative because it points down

what is f that always equal to it's m a and continuing on 3 times 5 is equal to tension subtract gravity y3

well because the acceleration is 3 y5 because the mass is 5 and rearranging the equation we get 64

newtons as our tension force or in significant digits just 60 why 60 because we have one sig fig here

and so we can conclude that the bag would not break in this situation