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# DEMO - PowerPoint PPT Presentation

DEMO. You have a heavy medicine ball. You throw it off the back of the cart. What do you think will happen? Why do you think this will happen?. AFTER DEMO. What has the greater force acting on it - you or the ball? Who has the greater acceleration you or the ball? Why?

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## PowerPoint Slideshow about ' DEMO' - zahina

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Presentation Transcript

• You have a heavy medicine ball. You throw it off the back of the cart.

• What do you think will happen?

• Why do you think this will happen?

• What has the greater force acting on it - you or the ball?

• Who has the greater acceleration you or the ball? Why?

• How could you increase the acceleration of the cart? (HINT - there is more than one way!)

• In what ways is this similar to how a rocket works? (HINT - think of the hot gas as molecular balls of gas)

• Does a rocket need ground to push off of in order to launch?

• Did the balls you threw off the back the rolling chair need something to push off of?

Newton’s 3rd law of Motion

• forces exist in pairs

• every action has an equal and opposite reaction

The crew team uses Newton's third law of motion to move its boat. When an oar is put into the water, the water exerts an equal force on both sides of the oars, the surface of the flat side of the oars pushes against the water. The water pushes back on the oars with an equal and opposite force. The boat moves in the opposite direction of the oars with a force that is equal to that of the oars as they push against the water. The boat moves because the forces against it are unbalanced.

The force on a bullet and the force on the gun is same when the gun is fired, yet the bullet and the gun have very different accelerations? Why is this?

Remember Force = mass x acceleration the gun is fired, yet the bullet and the gun have very different accelerations? Why is this?

Force gun= Force bullet

Mgunx Agun = Mbullet x Abullet

The bullet has a much greater acceleration due to its small mass/inertia

Q – Why aren’t bullets as massive as guns?

Q- What would happen if you fired a gun on a skateboard?

diving boards the gun is fired, yet the bullet and the gun have very different accelerations? Why is this?

ice skating the gun is fired, yet the bullet and the gun have very different accelerations? Why is this?

• What will happen to these 2 ice skaters when they push off each other?

How does a rocket work?

Do rockets require air to accelerate?

• Rockets shoot molecular balls of gas

• The rocket pushes on the gas molecules and the gas molecules push back with the same force.

• Because there is a small mass to each gas molecule you need to push them out with a great acceleration which requires a great force (explosion)

• Because the gas has a large force on it, the gas pushes the rocket back with the same large force

• By increasing the amount of gas shot out per second, you increase the mass of gas

• To accelerate a larger mass of gas requires a greater force and thus a larger reaction force

Assessment Question pushes rocket

• Joe the fisherman attempts to leap from his small fishing boat to the dock his boat is next to, but ends up in the water between his boat and the dock, why?

• You bring your boat close to the dock. Your boat is very light and is on a frictionless body of water. You want to get off the boat but the dock is just far enough away so you have to jump off, however it looks close enough where you could easily do it. You jump… what happens? Will you move forward? Will you make it?