Newton's First Law of Motion Concepts A little quiz on some of the ideas in Newton's first law
Newton's First Law of Motion Concepts
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- Now that we know a little bit about
- Newton's First Law.
- Let's give ourselves a little quiz.
- What I want you to do, is figure out which of
- these statements, are actually true.
- Our first statement is:
- "If the net force on a body is zero, its velocity will not change."
- 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 four:
- "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.
- Our first statement right over here "If the net force on a body is zero, its velocity will not change."
- This is absolutely true. This is actually another way of rephrasing Newton's First Law.
- If I have some type of object that's just travelling 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
- that there's no gravitational interaction, there will always be some minuscule one.
- But we'll assume no gravitational interaction, absolutely no particles that it's bumping into.
- Absolute vacuum-less space, this thing will travel on forever.
- Its velocity will not change, neither its 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". 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 body will always impact the object's velocity." That would be true
- but we wrote speed here. Speed is the magnitude of velocity. It doesn't take into account the direction.
- And to see why the second statement is false, you can think about a couple of things.
- We'll do more videos on intuition of centripetal acceleration and centripetal forces, inward forces,
- if this does not make complete intuitive sense for you at this moment.
- But imagine we're looking at an ice skating rink from above.
- And there is an ice skater, this is the ice skater's head.
- And they're travelling in that direction.
- Now imagine, right at that moment they grab a rope that is nailed to a stake in the ice skating rink.
- We're viewing all of this from above, and this right over here is the rope.
- Now what is going to happen?
- Well the skater is going to travel, their direction is actually going to change.
- And they can hold onto 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 in whatever direction they were travelling in when they let go.
- They'll keep going on in that direction and if we assume there is a very small amount of friction
- from the ice skating rink, they'll 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 skater's direction.
- So an unbalanced force doesn't necessarily have to impact an objects speed, it often does, but in this situation
- it would have only impacted the skater's direction.
- Another situation like this, once again this involves centripetal acceleration, inward forces, inward acceleration.
- Is a satellite in orbit or any type of thing in orbit.
- So if that is some type of planet, and this is one of the planet's moons right over here.
- The reason 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 is its speed right here, if the planet wasn't there it would just keep
- going on in that direction forever and ever.
- But the planet over here, there is an inward force of gravity.
- And we'll talk more the force of gravity in the future.
- But this inward force of gravity is going to accelerate this object inwards while it travels.
- So after some period of time, this object's velocity vector, if you add the previous velocity with how much it has changed.
- It's new velocity vector--
- Now this is after it has travelled a little bit.
- It's new velocity vector might look something like this...
- and it's travelling at the exact right speed so that the inward force or the force of gravity
- is always at a right angle to its actual trajectory.
- It's also at 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 off 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 incorrect 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, 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.
- The grinding of the surface of the book with the grinding of the table.
- If I'm inside of a pool, even if there is absolutely no current in the pool, and if I were to try and push
- some type of object inside the water, it eventually comes to a stop because of all of the resistance
- of the water itself. It's providing an unbalanced force in a direction opposite its motion.
- That is what slowing it down.
- So in our everyday life, the reason why we don't see these things go on
- forever is that we have these frictions, these air resistance 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 block over here,
- and it's travelling some velocity in that direction 5 metres per second.
- If I apply an unbalanced force in that same direction...
- so that's my force right over there, 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'm not always necessarily changing the object's direction.
- So this is not true. An unbalanced force on an object will
- not always change the object's direction.
- It can like in these circumstances, but not always.
- So always is what makes this very very very wrong.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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