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# Applying Newton's first law of motion

## Video transcript

now that we know a little bit about Newton's first law let's give ourselves a little quiz and what I want you to do is figure out which of these statements are actually true and our first statement is if the net force on a body is zero its velocity will not change interesting statement number two an unbalanced force on a body will always impact the object's speed also an interesting statement statement number three the reason why initially moving objects tend to come to rest in our everyday life is because they are being acted on by unbalanced forces and statement for an unbalanced force on an object will always change the object's direction so I'll let you think about that so let's think about these statement by statement so our first statement right over here if the net force on a body zero its velocity will not change this is absolutely true this is actually even another way of thinking about what another way of rephrasing Newton's first law if I have if I have some type of object that's just traveling through space with some velocity so it has some speed going in some direction and maybe it's deep space and we can just for purity assume it there's no gravitational interactions there always be some miniscule one but we'll assume no gravitational interactions absolutely no particles that it's bumping into absolute vacuum of space this thing will travel on forever its velocity will not change neither at speed nor its direction will change so this one is absolutely true statement number two an unbalanced force on a body will always impact the object's speed and the key word right over here is speed if I had written impact the object's velocity then this would be a true statement an unbalanced force on a on a body will always impact the object's velocity that would be true but we wrote speed here speed is the magnitude of velocity does not take into account the direction and to see why the second statement is false you can think about a couple of things and we'll do more videos on the intuition of centripetal acceleration and centripetal forces inward forces if this does not make complete intuitive sense to you just at this moment but imagine we're looking at an ice skating rink from above and you have an ice skater this is the ice skaters head and they're traveling in that direction now imagine right at that moment right at that moment they grab a rope that is nailed to a stake in the ice skating rink right over there reviewing all of this from above and this right over here is the rope now what is going to happen well the skater the skater is going to travel their direction is actually going to change and they can hold on to the rope and as long as they hold onto the rope they'll keep going in circles and when they let go of the rope they'll start going they'll start going in whatever direction they were traveling in when they let go they'll keep going in on in that direction and if we assume it very very very small frictions from the ice skating rink they'll actually have the same speed so the force the inward force the tension from the rope pulling on the skater in this situation would have only changed the skaters direction so an unbalanced force doesn't necessarily have to impact the object speed it often does but in that situation it would have only impacted the skaters direction another situation like this and once again this involves centripetal acceleration inward inward forces inward acceleration is a satellite in orbit or any type of thing in orbit so if that is if that is some type of planet and this is one of the planets moons right over here the reason why it stays in orbit is because the pull of gravity keeps making the object change its direction but not its speed its speed is the exact right speed so as it travels so this was its speed right here if the planet wasn't there would just keep going on in that direction forever and forever but the planet right over here there's an inward force of gravity and we'll talk more about the force of gravity in the future but this inward force of gravity is going to accelerate this object inwards while it travels and so after some period of time this object's velocity vector if you add the previous velocity with how much it's changed with how much it's changed it's new velocity vector now this is after it's traveled a little bit it's new velocity vector might look something like might look something like this and it's traveling at the exact right speed so that the inward force or so that the force of gravity is always at a right angle to its actual to its to its actual trajectory it's the exact right speed so it doesn't go off into deep space and so it doesn't plummet into the earth and we'll cover that into much more detail but the simple answer is unbalanced force on a body will always impact its velocity it could be its speed its direction or both but it doesn't have to be both it could be just the speed or just the direction so this is an in correct statement now the third statement the reason why initially moving objects tend to come to rest in our everyday life is because they are being acted on by unbalanced forces this is absolutely true and this is the example we gave the reason why if I take an object if I take my book and I try to slide it across the desk the reason why it eventually comes to stop is because we have the unbalanced force of friction but the grinding of the surface of the book with the grinding of the table if I if I'm inside of a pool and even if there's absolutely no current in the pool and if I were to if I were to try to push some type of object inside the water it eventually comes to a stop because of all of the resistance all of the resistance of the water itself it's providing an unbalanced force in a direction opposite its motion that is what's slowing it down so in our everyday life the reason why we don't see these things go on and on forever is that we have these frictions these air resistance the the the or or or the friction with actual surfaces and then the last statement an unbalanced force on an object will always change the object's direction well this one actually is maybe the most intuitive we always have the situation if I have a let's say I have a block right over here and it's traveling with some velocity in that direction five meters per second if I apply an unbalanced force in that same direction in that same direction so that's my force right over there if I if I apply it in that same direction I'm just going to accelerate it in that same direction so I won't necessarily change it even if I were to act against it I might decelerate it but I won't necessarily change its direction I could change its direction by doing something like this but I don't necessarily I'm not always necessarily changing the object's direction so this is this is not true an unbalanced force on an object will not always change the object's direction it can like these circumstances but not always so always is what makes this very very very wrong