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# Applying Newton's first law of motion

Newton's first law quiz
1. If the net force on an object is zero, its velocity won't change. (True)
2. An unbalanced force on an object will always impact the object's speed. (False)
3. Moving objects come to rest in everyday life because of unbalanced forces. (True)
4. An unbalanced force on an object will always change the object's direction. (False).
Created by Sal Khan.

## Want to join the conversation?

• For statement #3, does LIGHT also apply? •   Light is a bit tricky and although Newton did a lot to advance our understanding of light and optics on a basic level, you'll find out that its actual behavior isn't described by Newtonian physics. For example, Newton discovered that white light is composed of all the colors in the visible spectrum (he actually didn't know anything about invisible light, such as radio waves, microwaves, X-rays, etc) , but he had no good way of knowing the speed of light or what it is made up of. Today we know light is made up of things that have no mass called photons that act as both waves and particles depending on how they are observed. That alone can't be modeled by Newton's laws of motion and it's a pretty hard concept for anyone to really understand. Newton actually argued that light was made up of just particles. He wasn't aware of light's wave properties. Things like photons were first modeled in detail by quantum mechanics and the whole idea that photons are at once both waves and particles also came out of quantum mechanics.

Another thing Newton didn't know about was that light also always moves at the same speed regardless of your frame of reference, which come to think of it doesn't make sense. You would think that if you were moving fast enough, light would appear to move slower next to you, but it never does! Also, as it turns out from Einstein's equations, Newton's laws of motion can't be used to describe anything at the speed of light or anything approaching such a speed. Therefore, light can't be modeled by Newton's 1st law of motion or any of the 3 laws of motion. These sorts of problems related to the speed of light as well as what happens when something approaches the speed of light weren't really dealt with until Einstein formulated his Theory of Relativity.

So as you can see, the actual behavior of light is described by two branches of physics that didn't develop until the 20th century--- quantum mechanics, and Einstein's theory of relativity.

One last thing, microwaves radiowaves, etc, are light except at much longer wavelengths (lower frequencies) than visible colors, while UV light, X-rays, and gamma rays are light with higher frequencies than visual light. In fact, the type of light we can see and that we think of as light, is really a rather small sliver in a much broader spectrum. Note that wavelengths and frequencies are properties that result from the wave aspect of light's behavior.
• My teacher says that objects like the moon are in free fall, but why does it stay put and just rotate around the Earth? Is it like a vacuum or is it just the gravity is stronger than the free fall? • What is the difference between speed and velocity? I thought both were the same thing. • Speed and Velocity are quite different. Speed is a scalar quantity, i.e it has only magnitude but no direction. Therefore, Speed = distance/time.

But Velocity is a vector quantity, i.e. it has both magnitude and direction. Therefore, Velocity = displacement/time.

I highlighted distance and displacement because, distance is scalar and displacement is a vector.
• What exactly do we mean by an unblanced force??.....
It counteracts with Newton's third law of motion i.e. every action has an equal and opposite reaction

Sorry...the question may be a little stupid, but not enough to defeat my cuiosity . • The law about equal but opposite forces says that if you apply a force on something that it applies an equal but opposite force on you. The 2 forces are on different objects. When you are talking about unbalanced forces you are talking about the sum of forces on 1 object.

For example if you are pushing a block of wood across a table to the left so that it accelerates you have a force from gravity on the block down, a force from the table pushing up and you pushing left and the force of friction to the right. If the sum of these 4 forces leaves an amount to the left they are unbalanced and the block is accelerating to the left.

Newtons 3rd law would say that there is also a force up on the earth from the block, a force down on the table from the block, to the right on your hand from the block and to the left on the table from friction. All these forces are not on the same object so they can't be added.
• But doesn't friction decrease the magnitude of velocity i.e, speed? And friction is unbalanced. So how in the world doesn't an unbalanced force change the speed?? • The key word is always. An unbalanced force can change the object's speed but it can also change an its direction without changing its speed. So an unbalanced force does not always change the object's speed.
• What does 'velocity' mean? • Remember not to confuse "velocity" with speed!
In physics, velocity is a vector which means that the value of velocity can be negative, whereas "speed" is a scalar (it only gives the value, without direction) and therefore, can be only positive.
For example, if two cars are getting closer to each other (they are driving in opposite direction), we can say that the velocity of one of them is equal to 20m/s and the other one: -30m/s (minus sign because of the opposite direction), but their speed can be only positive so it is 20m/s and 30m/s respectively.
• Statement2: What unbalanced force would be able to change a moving objects direction without also changing its speed via gravity? • at , sal says if it was velocity instead of speed,statment 02 would be right..i donot get.does anyone please explain? • When an unbalanced force acts upon an object, it impacts its speed and not its velocity because if anything is moving in a circular motion, and suddenly it stops moving, though the speed will be the same, but instead of stopping in a circular direction, its direction turns in a way it makes it stop moving and we know that if the direction changes the velocity turns to zero.
• A common situation to describe inertia:

If a bus accelerates suddenly, the passengers will experience a backward push.

If inertia didn't exist, what would happen?

Pardon my english....  