Main content

### Course: Physics library > Unit 3

Lesson 3: Balanced and unbalanced forces# Unbalanced forces and motion

Use your knowledge of balanced and unbalanced forces to evaluate four statements related to the behavior of objects when acted on by forces and to determine whether each statement is true or false. Created by Sal Khan.

## Want to join the conversation?

- The last question doesn't make complete sense to me. A satellite in orbit is being pulled by an unbalance force, gravity. But it is not moving in the direction of gravity, which would be straight down.(17 votes)
- The force of gravity
*is*accelerating the satellite downwards--if we were to remove the force of gravity then the satellite would move at a constant velocity along a straight path. In other words, the fact that the satellite's direction is constantly changing is evidence of the force of gravity.

Does this make sense?(35 votes)

- Sentence 4,5:35- objects always accelerate in direction of unbalanced force. If an object was accelerating left and an unbalanced force to the right acted on it, would it be correct to say that it decelerated to the left or that it accelerated to the right while simultaneously accelerating to a greater degree to the left?(16 votes)
- Yes, is correct say this because if you have a vector that has positive displacement to the right, an displacement to the left will be negative .The same occurs when you have a vector that has positive displacement to the left, the oposite side will be negative.(8 votes)

- I am a little confused about the last statement in the video (5:39and6:37). Could someone please explain why the object will accelerate if an unbalanced force is applied to it? Why doesn't it move in that direction at a constant speed, why does it increase in speed?(10 votes)
- Because forces are, by definition, the things that make objects move. I you get a net force on an object, it accelerates according to Newton's Second Law.

You can think of it like this: if you push a book lying on a table really hard (an easy example of unbalanced force), it will begin to move. This means that it*accelerates*from lying still to moving. Then it might*continue*to move at a constant speed (if there was no friction or air resistance, at least), but is has to accelerate in order to start moving at all.

It is the same with an already moving object: if it doesn't increase in speed or change direction as you push it, it simply means that nothing happens (which in turn means that the forces are still balanced due to friction resisting your push).(14 votes)

- At2:49, would the object move if the object was moving with a force of 1N, and I am pushing the object the opposite way with the force of 1N.(7 votes)
- if the object had a force of 1N on it and then you apply another force of 1N on it in direction opposite to it then the object will experience a balanced force. thus its motion will continue as it was without accelerating any further(6 votes)

- in the 4th statement, if the unbalanced force is constant, then the acceleration is uniform, am i right?(9 votes)
- It depends on how much force is applied to it and at whether the force is increasing gradually or staying exactly the same. For example if I’m pushing a rock at 4N constantly, it will continue to move provided that no other forces act on it. But if I start pushing at 4N then start increasing the force to 5N then 6N etc, the acceleration will also increase likewise.

I hope this helps.(1 vote)

- to the 4th statement; what if an object having a force of 4N to the right would be acted on by an unbalanced force of 6N upwards its direction would be neither upwards nor roght but in a diagonal direction between those two vectors right? how could u still say it would move in the direction of the unbalanced force?(6 votes)
- You're absolutely right! Although you don't need to use the word "unbalanced", since it's clearly unbalanced if the forces are acting perpendicular to one another. The only way it would balance is if they were 180 degrees to one another and the magnitude of the forces were equal.

If they're at a right angle, it's easy to solve for the resulting force. To find the magnitude of the resulting force, c, you can use the Pythagorean theorem (4^2 + 6^2 = c^2) and to find the exact angle (theta) you would have to use simple trigonometry, specifically tangent: tan(theta) = 6/4

Once you solve this, you can say that the resulting force is*__N at __*degrees above the horizontal.(5 votes)

- In the last example, what if we have a force of 5N moving towards the right and a force of 6N hits it from the front (not the left or right direction but from the front- kind of at 90 degrees). The 5N force will begin to move in the direction of the 6N force but its magnitude will decrease, won't it? in that case won't this scenario be false?(4 votes)
- The statement won't be false, the object will move diagonally upwards, and actually the momentum will increase as we can prove using vector addition:

Since the angle is 90˚, we can apply the Pythagorean theorem to find the length of the diagonal of a rectangle with side lengths 6 and 5:

6^2 + 5^2 = x^2

36 + 25 = x^2

61 = x^2

x = √61 ≈ 7.81N(5 votes)

- If lets say for the 4th statement we had an object with a constant velocity of 5m/s to the right, and then applied a force of 4N to the left, wouldn’t you say that eventually the object would start to go backwards after a moment of still going to the right? To add a question: Does that disprove the 4th statement?(3 votes)
- The force would be to the left and the acceleration would be to the left, so the acceleration would be in the same direction as the force.(3 votes)

- For the last statement, wouldn't centrifugal motion be a example of a unbalanced force that accelerates a object in a vector that does not match the unbalanced force?(3 votes)
- If that is correct than the last statement would be false.(2 votes)

- At3:28what does the net force mean?(2 votes)
- The net force is the sum of all the forces acting on an object.(3 votes)

## Video transcript

I have four statements here and I want you to think about which of of these statements are true and the first statement is an object in motion will slow down unless it is acted on by an unbalanced force in the direction of motion - interesting second statment, an object in motion will maintain it's speed and direction forever, unless acted on by an unbalanced force- also very interesting. Third statement, an object at rest will stay at rest, unless acted on by an unbalanced force. And then the fourth statement, an object acted on an by a unbalanced force will always acclerate in the direction of the unbalanced force So, i'll leave you to think about those statements, and figure out which of those statements are actually true. It might be none of them, it might be all of them, or some combination of them. Now that's try to think about each of these statements. So first of all an object in motion will slow down, unless it is acted on by an unbalanced force in the direction of motion. Well this, at least, on a first cut kind of seems consistant with our everyday experience. If i'm pushing some furniture across the room, so let's say that this is, that this is the carpet of my house, and i'm pushing a television set -let's say it's one of those old school heavy television set- (so i'm pushing a television set), the only way that I can keep it in motion is that I keep pushing on it. So let me draw my force of pushing (my force of pushing). So let's call that right there the force of Sal pushing the television set. So the only way that I can keep (the only way that I can keep) my television set moving seems like if I keep applying this pushing force on it. And I were to stop doing that, it seems like it will slow down. So on a first cut everday experience, this seems like it would actually maybe be true. It only seems that way, because i'm actually not accounting all of the forces in this situation. There is not just the force of me pushing. There is also the force of friction. (There is also the force of friction.) So i'll call that, there is also the force of friction. And if I want to keep this TV moving at a constant veloctiy, my force of pushing (that make that the vectors look about the same magnitude, becuase they need to be the same magnitude) If I want to keep my television set at a constant velocity in the direction that i'm pushing, I have to exactly offset the force of (the force of) friction. If my force of pushing is less than the force of friction, then the TV set will to decelerate. If my force of pushing is larger than the force of friction, then my TV set will accelerate. But in the scenario right now, I have an object in motion, but there's actually a balanced force. My pushing is completely balanced by the force of friction. Another scenario, if we were to go to deep space (i'm not even thinking about air resistance right here), but if we were to go into space, where, especailly deep space, no planets around, complete vacuum, if an object is in motion (if an object is in motion), and there is absolutely no forces balanced or otherwise, there are absolutely no forces acting on it, then that object is going to stay in motion. The only way it might change its direction or decelerate or accelerate is if there is a net force on it. And just by going back to this example, the only reason why it will slow down, if I were to stop pushing, is becuase there is a net force on it, the force of friction. And if you want to think about it, there would probably be very minor amount of air resistance. So this actually isn't true. An object in motion does not have to slow down ,unless it is acted on by an unbalanced force. An object in motion can stay in motion, as long it is being acted on either by a balanced force, or (i guess) a set of forces that balance each other, or no forces at all. So this is not (that is not) true. An object in motion will maintain it's speed and direction forever unless acted on by an unbalanced force. This is true! This is exactly what's happening right over here in the case of a (I guess) a block moving through space in a vacuum. If there are no forces acting on it (no net forces acting on it) it is not going to change its velocity. Similarly, you could have this situation, where you actually have forces acting on it, but they balance each other out. In this situation, as long as I can keep pushing forever, and the floor doesn't change, and so the friction doesn't change, this thing will keep moving in that direction, theoretically forever. The only way to change its direction or accelerate in one direction or another direction, would be to have an unbalanced force. Maybe if my force of pushing were larger than the force of friction, or the force of friction were larger than my force of pushing. So this right over here is absoluely true. Third statement, an object at rest will stay at rest unless acted on by an unbalanced force. And this is actually pretty intuative, I think, for people. If you just leave something alone (if you just leave something alone) it's just going to stay alone. It's not going to start spontaneously moving. And so, this is true. It's not just the situation where there is no forces; it could also be the situation that's it's going to stay at rest, if there are balanced forces acting on it. And so the only way you can actually get this thing to come out of that rest position is if you have a net force, an unbalanced force. If, let's say, this force right here is (is) any amount larger than this force right over here, if they don't completely balance out. So this, once again, is true. Last statement, an object acted on by an unbalanced force will always accelerate in the direction of the unbalanced force. So if I have an object, and there's an unbalanced force, and if we net out all the forces, let's say, the direction is in the force's net out and we get a force acting in this direction. This is actually true! The object will always accelerate if it has an unbalanced force acting on it in that direction, it will accelerate if there's a net force in that direction. And there's another way of thinking about it. If you see an object that is not accelerating, like my TV that's moving at a constant velocity or this thing that's just traveling through space, if you see something that is not accelerating, that means that there must not be an unbalanced force acting on it. So could have balanced forces acting on it, or you could have no forces on it at all, but you can't have an unbalanced force. So if you have an unbalanced force, the thing will be accelerating.