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### Course: Middle school physics > Unit 1

Lesson 2: Forces and acceleration# Force, mass and acceleration

When a net force acts on an object the object's motion will change in the direction of this net force. The larger the force is that acts on an object, the larger the change in motion. However, the greater the mass of the object, the greater the force needed to have the same change in motion.
Created by Sal Khan.

## Want to join the conversation?

- Im confused. There is no gravity in space so how will the asteroids weigh anything. So the asteroid with one rocket each will go at the samed speed..... right?(17 votes)
**Weight**and**mass**are not the same thing.

All matter (you, the earth, astroids, and the sun, etc) has mass, but unless they're under the influence of gravity they won't have a weight.

For example, your weight on the moon is much less than your weight on earth, but your mass is always the same everywhere.

Here are the proper definitions:

- Mass: is the measure of the amount of matter in a body.

- Weight: is the measure of the amount of force acting on a mass due to the acceleration of gravity.(58 votes)

- at3:04, I don't understand the term 'force equals mass times acceleration. could some one please explain? thanks.(5 votes)
- They say that because The amount of F (Lets say force is F) depends on how much mass the object has and how much acceleration is acting on it. Let's say that a ball has a mass of 100 grams. And there is another ball that weighs 50 grams. You push each ball with enough force to push a ball that weighs 200 grams. The first ball would move half as fast as the second because 50 is half of 100.

Did that help?(11 votes)

- Wouldn't adding more rockets add more weight also like the rock with 2 boosters should be going a bit slower than the big rock right?(2 votes)
- Not exactly, I think he's talking about the rockets in a way that the weight of the rocket doesn't matter. But your also right, the weight of the rockets will affect how fast the rocks are going, because the rocket engine also needs to carry the weight of the actual rocket, which would make the rocket go slower (and if there's a certain speed those rocks need to be going, the rockets need to generate more force). Basically what we're talking about is the power/weight ratio.

Good question.(3 votes)

- I'm here early lol

if the mass increases, then doesn't the force increase as well because F= ma and the bigger m gets the bigger F gets(1 vote)- Yes, you need a bigger force to get a bigger mass to undergo the same acceleration.(5 votes)

- Can somebody please explain to me more clearly what a net force is?(2 votes)
- Net force is the total amount of all force being acted on an object(4 votes)

- what can force change?(1 vote)
- Force can impact quite a number of things. For example, if an object is in motion, a force applied may change it's speed or direction. Force can also stop or start making an object move.

If force is not applied on a moving object, that thing will move forever. That's why astronauts must be really careful about their movements in space. One wrong move, and they may never see Earth again.

Hope this helps! :D(5 votes)

- f=ma is like e=mc(1 vote)
- Yes they're both formulas. The difference is that 'f=ma' is the formula of force and 'e=mc2' is the formula of energy.(4 votes)

- In space mass is neglected(0 votes)
- What is the net force he keeps talking about?(1 vote)
- Based on my understanding, net force is the combination of all the forces acting on an object, with the direction(1 vote)

- he says that the more mass there is the more force is needed to move it this is false mass and weight are two different things for example if there are two rockets that are the same and one is pushing a large rubber duck and the other is pushing a smaller rock then the one pushing the bigger object well move faster(3 votes)
- Yep exactly!(0 votes)

## Video transcript

- So I have three different
asteroids over here and they have different masses. And we'll talk a lot more
about what mass means. But one way to think about it is, how much stuff there is there. There's other ways to think about it. And so let's say that this first asteroid is twice the mass of either
of these two smaller ones. And these two smaller
ones have the same mass. Now, we've attached the back of a rocket to each of these asteroids. In fact, this one over
here has two rockets, and we're going to assume that all of the rockets are equivalent and we ignite them all. And so they all exert the same
force each on the asteroid. So for example, we have a net force acting
leftward on this large asteroid. We have the same net force
acting on this smaller asteroid, also going to the left. And on this other smaller asteroid, we have two times that net
force acting to the left. So what I want you to
do is pause this video and think about which of these asteroids is going to be accelerated the most, and which of these asteroids is going to be accelerated the least. All right. So you might have an intuition
that the larger the force, the more acceleration you might see. So let me write it like this. So you might get a sense that
if you increase your force, that that's also going to
increase your acceleration. And it does turn out that
that is indeed the case. Now, the other notion that you might have is that the more of the
stuff that there is, the more mass that you have, the harder it is to accelerate it. So if you're mass is larger, than your acceleration is lower. And it turns out that these
things are all proportional. So for example, if we just compare these
two masses right over here, they have the same net force acting on it. And I keep saying net force, that means you just net
out all of the forces acting in a certain dimension. For example, if I had
another identical rocket acting in the opposite direction, they would net out, and this
asteroid right over here wouldn't be accelerated at all. But going back to our example here, we have the same net force acting on each of these asteroids. But the first asteroid has twice the mass of the second asteroid. So how do you think the
accelerations will relate? Well, as you might imagine, the acceleration on the larger asteroid is going to be half the
acceleration on this asteroid. Or another way to think about it, this asteroid is going to
have twice the acceleration as this first asteroid. And that's because it has half the mass. And one way you can relate
force, mass, and acceleration, and this is one of the
most important equations in all of physics, is that force is going to be equal to
mass times acceleration, or I could say the magnitude of the force is equal to the mass times the magnitude of the acceleration. So notice, in this
example right over here, our forces are the same, but
the masses are different. If I have half the mass
as I have over here, I'm going to have twice the acceleration. And that might make intuitive sense if you've ever tried
to apply the same force to something that has a small mass versus something that has a large mass. Now, if we compare these two asteroids, they have the same mass here, but the force here, the net force, acting in that left direction, is double. So if you double the force,
don't change the mass. Well, then you're going to
have twice the acceleration. So this is going to have twice
the acceleration of this one, and this one's going to have twice the
acceleration of that one. But the important thing to realize is how force, mass, and
acceleration are connected.