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## Physics library

### 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. •  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?
• Sentence 4, - 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? • I am a little confused about the last statement in the video ( and ). 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? • 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).
• At , 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. • in the 4th statement, if the unbalanced force is constant, then the acceleration is uniform, am i right? • 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? • 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.
• 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? • 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   