Balanced & Unbalanced Forces

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

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! Downwards force Upwards reaction

Main Principal #5: The more mass an object has, the more force is needed to move the object!

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

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?

Example: 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? Gravity pulls down Ground pushes up

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

“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!

Unbalanced Forces Cause Acceleration Adding Forces: 5 N Random Object 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! Multiple forces can combine to move an object that has too much inertia for one force alone. The forces “add together”! 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. What you just saw was like two people pushing on the same box.

Unbalanced Forces Cause Acceleration Subtracting Forces: 5 N Object 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 If forces are not equal and are acting in opposite directions, a negative acceleration can/will occur. The forces will subtract from each other!

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 brakes to slow down the car; maybe you wouldn’t drive off the cliff you are aimed at…brakes create an opposite force to slow moving cars down. 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! These are just three examples, but I bet you can think of a lot more if you tried…

► 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 Brake on your car while driving, the engine has to work harder to overcome the friction to make the car move! Speaking of slowing down! Friction = When two surfaces “rub” against each other the force generated is called “Friction”. Brakes on a car use friction to slow the spinning of tires down.

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! 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… 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… The acorn shown here is also falling due to gravity…strangely enough, the acorn falls faster than the leaf!

Balance between Gravity and Friction The skydiver is colliding with the molecules of gas in the air! Which slows down the fall of the skydiver! 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.

How it works force of gravity Air Resistance 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. force of air friction equals the force of gravity Once the force of air friction equals the force of gravity, the skydiver steadily descends towards earth at a constant velocity. Gravity 0 m/s 30 m/s Air Resistance Constant Velocity Speed: 0 m/s 10 m/s 20 m/s 25 m/s 28 m/s Accelerating Constant Velocity Accelerating

Review Balanced Forces BalancedPush i.e. Pushing a Car No Acceleration BalancedPull i.e. Tug-o-war No Acceleration 10N

Review Un-Balanced Forces Add Together Un-Balanced Same Direction FasterAcceleration Subtract from each other Un-Balanced Opposite Direction SlowerAcceleration 10N