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Balanced & Unbalanced Forces

Balanced & Unbalanced Forces. I am so smart …. Newton’s Second Law of Motion says:. More Force = more Acceleration More Mass = more Force needed!. “To move a mass, you need a force!”. The greater the mass = greater inertia = more force needed!. Third Law of Motion.

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Balanced & Unbalanced Forces

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  1. Balanced & Unbalanced Forces

  2. I am so smart… Newton’s Second Law of Motion says: • More Force = more Acceleration • More Mass = more Force needed! “To move a mass, you need a force!” The greater the mass = greater inertia = more force needed!

  3. Third Law of Motion • “For every action there is an equal and opposite reaction.” • Rockets take off because of a force downwards from the bottom makes them accelerate in the opposite direction! Upwards reaction Downwards force

  4. Discuss: If your results/data didn’t seem to match these laws; what variables might have messed up your results/data in the tests? Neg. Force (Friction) Pos. Force (Thrust) Forces in the Balloon Lab: 2nd Law: The balloon will not move without air pushing outward from it. 3rd Law: When the air comes out, the balloon moves in the opposite direction of the air 2nd & 3rd Law: The more air you put in the balloon the faster it travels 2nd & 3rd Law: The more mass we add the slower the balloon travels (if the air force remains the same) SO: To make the balloon move the same speed (after adding mass) we have to use more force (more air)! Main Principal #5: The more mass an object has, the more force is needed to move the object! Gravity

  5. Forces and Accelerations • We have already learned that forces cause all accelerations (change of speed and/or direction) • But they don’t have to… • What if there are more than one force acting on an object at the same time (which is more realistic than just one)? • The forces might balance each other out! • How does this affect the motion of our object…? • Well…let’s explore that now…go to next slide:

  6. Balanced Forces: The forces in each direction are “equal”. • If more than one force is present, it does not have to cause an acceleration on an object. • If another force “balances” the first out, there will be no acceleration at all. Think: If both guys (who weight the same) pull on a rope in opposite directions, with an equal amount of force, how much will they move?

  7. Example: Ground pushes up Gravity pulls down Gravity pulls down on you… The ground pushes back up… THIS KEEPS YOU WHERE YOU ARE! If these football players push on each other equally as hard, will either one move?

  8. More Balanced Forces… 5 N 5 N Forces may cancel each other and produce no movement =No Acceleration! 5 N 5 N

  9. “Unbalanced” Forces If the multiple forces acting at one time are not balanced out (equal), then acceleration can/will occur on the object! Kicking the ball causes it to move quickly in a different direction If one side of the scale has more mass, then gravity will accelerate it down!

  10. Unbalanced ForcesCause Acceleration Multiple forces can combine to move an object that has too much inertia for one force alone. The forces “add together”! Adding Forces: Random Object 5 N What you just saw was like two people pushing on the same box. 5 N Notice that all the new forces are pointed in the same direction, and they add together! So, instead of only 5N of force pushing the object; now there are 10 N of force pushing! All by himself, one person might not be able to push a car. But with extra friends pushing, it becomes much easier! The forces add together.

  11. Unbalanced ForcesCause Acceleration If forces are not equal and are acting in opposite directions, a negative acceleration can/will occur. The forces will subtract from each other! Subtracting Forces: Object 5 N 5 N 5 N Notice that the forces are “unequal” and pointed in the opposite direction. So they are “unbalanced” and work against each other – or one partially cancels the other. The end result is that the forces on the left are slowed down by the single force on the right

  12. Why would you WANT a second force to slow down motion? Football players use force from their own bodies to slow down a player running with the ball! The opposite force from the first player will at least slow down the player with the ball. If you used the breaks to slow down the car; maybe you wouldn’t drive off the cliff you are aimed at…breaks create an opposite force to slow moving cars down. These are just three examples, but I bet you can think of a lot more if you tried… The atmosphere creates an opposite force on a spaceship slowing it down as it heads back down to Earth. If it didn’t it would be almost impossible to stop the spacecraft before it impacted the ground!

  13. Speaking of slowing down! Friction = When two surfaces “rub” against each other the force generated is called “Friction”. Breaks on a car use friction to slow the spinning of tires down. • Friction acts in “opposition” (against) to current forces and/or accelerations. • Main Principal #6: Friction slows velocity down! (Can cause a negative acceleration!) • If we want to overcome friction (for whatever reason) we need to add even more force! • EX: If you left the Emergency Break on your car while driving, the engine has to work harder to overcome the friction to make the car move! Discuss: What created friction in the Balloon Lab? (Hint: there are 2 main ones!) A: The air (gases) in the room and the straw sliding on the string! Both act to slow down the motion of the balloon!

  14. A: Since the acorn has more mass, it has more inertia! So, it’s current motion (falling) is harder to change! And the air friction doesn’t generate enough force to really slow it down as much as the leaf! Ta-Da! Thank you…Thank you… Gravity and Friction can combine in an unbalanced force: The leaf is colliding with the molecules of gas in the air! Which slows down the fall of the leaf! The acorn shown here is also falling due to gravity…strangely enough, the acorn falls faster than the leaf! But, the acorn is also colliding with the same molecules of air that created a lot of friction on the leaf. Discuss: Why does the acorn still fall faster than the leaf? Discuss: If the friction of the air and the pull of gravity were in perfect balance, what would the leaf be doing? Think hard…

  15. Balance between Gravity and Friction Q: If the friction of the air and the pull of gravity were in perfect balance, what would the skydiver be doing? A: The skydiver would be going a constant velocity (moving with no acceleration) down towards the ground until it hit the ground. The skydiver is colliding with the molecules of gas in the air! Which slows down the fall of the skydiver!

  16. How it works Constant Velocity Notice that as the skydiver leaves the plane, the force of gravity accelerates him faster towards the earth; however, the moment he jumps out of the plane, another force begins to oppose gravity – Air Resistance, (Friction). This friction with the air begins to slow down his rate of acceleration. Meaning, he’s still speeding up, just not as fast. Once the force of air friction equals the force of gravity, the skydiver steadily descends towards earth at a constant velocity. Accelerating Accelerating Air Resistance 30 m/s 0 m/s Speed: 28 m/s 25 m/s 20 m/s 10 m/s 0 m/s Constant Velocity Gravity

  17. Review Balanced Forces 10N 10N 10N 10N

  18. Review Un-Balanced Forces 10N 10N 10N 10N 10N

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