Video: Force and Motion

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Force and Motion

Video Transcript
How do forces affect the motion of an object? And how can a force diagram be related to a kinematic representation of an object ... like a dot diagram or a motion graph? Well I'm Mr. H. and I have some answers for you.

Force Diagrams
Forces are pushes or pulls that act upon an object as the result of its interaction with the surroundings. We represent forces by force diagrams. Each arrow represents a force. The length of the arrow indicates the strength of the force; a long arrow is used for a strong force. The direction of the arrow represents the direction of the force. There is no limit on the number of forces.

Balanced Forces
When viewing a force diagram, it is important to distinguish between balanced forces and unbalanced forces. When oppositely-directed forces are of equal strength, we say the forces are balanced. Balanced forces cause objects to keep on doing what they are doing. If at rest, they stay at rest. If in motion, they continue in motion with the same speed and direction. Balanced forces never cause objects to accelerate. So when you see this ... or that ..., you can expect a constant velocity. When forces are balanced, the object could be at rest and staying at rest as shown by these kinematic representations ... . Or the object could be moving and continuing to move with an unchanging velocity as represented by these representations... .

Newton's First Law
Without ever stating it, I've been describing Newton's First Law of Motion. Objects at rest remain at rest; objects in motion continue in motion with the same speed and direction ... as long as the forces are balanced. But do be careful. Not everybody believes it's true. Consider this. Here's a force diagram with an up and a down force. Can this object be moving to the right? Or how about this force diagram with four equal-strength forces. Can this object be moving to the right? The answer is ... YES! Either one of these objects could be moving to the right. Or to the left. You can't tell which direction an object is moving from the forces which act upon it. You can only tell how it is moving -and these objects move with constant speed. When a moving object has balanced forces, you know it is moving in some direction with a constant speed. There doesn't need to be a force to the right to continue moving to the right. Once in motion, it would continue in motion with unchanging speed as long as the forces are balanced.

Unbalanced Forces
Unbalanced forces result in a change in motion ... an acceleration. These diagrams show unbalanced forces. Unbalanced forces occur when oppositely-directed forces are of unequal strength. We say there is an unbalanced force or a net force. 
When there is an unbalanced force, there will be an acceleration in the same directionas the unbalanced force or net force. So Object 1 will accelerate to the right ... Object 2 to the left ... Object 3 upward ... and Object 4 downward. But this is when you need to be careful, because accelerating to the left doesn't mean the object moves to the left. Let me explain.
Here's two dot diagrams - one for an object moving to the right and one for an object moving to the left. Both objects are accelerating to the right. When an object moves left and slows down, we describe its acceleration as being to the right. Each dot diagram shown here is consistent with more force to the right than to the left ... as shown in the force diagram. Object B is like a leftward moving car that is skidding to a stop. What slows it down? More force to the right than to the left. So these force diagrams are consistent with a rightward acceleration, and can be represented by any of these kinematic descriptions (Pause) since each description shows a rightward acceleration. (Pause)
Here's two more force diagrams. Notice that there is a leftward unbalanced force. With more force to the left than the right, the object accelerates to the left. It could be moving to the left and speeding up as shown by this dot diagram ... or by these graphs. But these same force diagrams could also explain an object that is moving to the right and slowing down. (Pause) In each case there is a leftward acceleration. A leftward unbalanced force always causes a leftward acceleration.

Newton's Second Law
Without stating it, I've been describing a portion of Newton's Second Law of Motion. An unbalanced or net force causes objects to accelerate in the direction of the net force. Once more you need to be careful. Unbalanced forces don't cause objects to move in a given direction. They cause objects to accelerate in a given direction. Unbalanced forces cause objects to speed up or slow down. More force against the motion than with the motion causes objects to slow down. More force with the motion than against it causes objects to speed up.

Vertical Motion
We can repeat these ideas for vertical motion. More downward than upward force causes the object to accelerate downward. It could move upward and slow down or move downward and speed up. Like this ball. It could move downward and speed up ... or it could move upward and slow down. And more upward force than downward force causes the object to accelerate upward. Just like horizonatal situations, unbalanced force ALWAYS cause objects to accelerate in the direction of the unbalanced force.
The relation between force and motion can be one of the most difficult concepts to grasp and you need to give it some practice. There are a variety of interactive exercises on our website that allow you to check to see if you got this.  You can find links to them in the Description section of this video. Give one of them a try. Hey I'm Mr. H. Thanks for watching!



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