Exploration Series. PIRATE SHIP Interactive Physics Simulation Page 01

Transcription

1 PIRATE SHIP Interactive Physics Simulation Page 01

2 Why does a cannon recoil when it fires? The world can be viewed as a complex set of interactions between isolated objects. We will describe the cannon and cannonball as interacting through a THRUST force. The thrust force acts on both the cannon AND the cannonball. Likewise, the tension in the rope pulls on BOTH the pirate AND the sail. All forces come in pairs of this kind. We call them Newton s Third Law pairs. Understanding them is one of the great challenges of introductory physics. To access this physics simulation visit: Page 02

3 Interaction Diagram - This kind of diagram is known as an interaction diagram. It shows all the forces acting on each of the objects. What is great about this type of diagram is it shows the force pair acting between two objects. For example, the thrust force acts on both the cannonball and the cannon - the force is leftward on the ball and rightward on the cannon. We show this as a SINGLE line drawn from the ball to the cannon on this diagram. You can click on any force to see the objects it is acting on, and you can click on any object to see all the forces acting on it. To access this physics simulation visit: Page 03

4 Challenge ME! Which is stronger, the thrust force acting on the cannon or on the cannonball? What is the third law pair to the downward gravitational force acting on the pirate? In which direction does the tension force on the pirate point when a gust of wind rises up? Need Help? Check out the Pirate Ship Walkthrough video at: To access this physics simulation visit: Page 04

5 If every force comes in equal and opposite pairs, how can the horse and cart move forward? It is true that when the horse pulls on the cart, the cart pulls on the horse with an equal force. So how do they move? Well, the EARTH pushes on the horse with a static friction force acting on its hooves. The horse walks forward by attempting to slide her hooves backward across the ground; the frictional force resisting that slide allows the system to move forward. Remember: the two forces in a mutual interaction (3rd Law) pair act on TWO DIFFERENT OBJECTS, so they can never 'cancel' each other. Is the normal force acting on an object at rest the third law pair for the gravitational force? No! That's a common misconception. In fact, these forces only happen to be equal in magnitude and opposite in direction - but they do NOT make a 3rd law pair. In fact, if you simply DROPPED the object onto the table, the normal force would exceed the weight of the object while bringing the object to rest. Then the two forces are NOT equal in magnitude, demonstrating that they cannot be a third law pair. Remember: two forces have to be of the same TYPE to be a third law pair - and they have to act on two DIFFERENT objects. Do I feel a gravitational attraction to the objects in the room near me? Yes! But it is an extraordinarily small force - one you'd never notice. Gravity is a very weak force - the only reason we notice it at all is because we are very, very near an astronomically large object (the Earth!) I can lift my friend and carry them around - why can't I lift myself and fly? That's a silly question but gets at important physics: when you lift your friend, they are pushing you down with the same amount of force. If you attempt to lift yourself, you'll feel BOTH the upward and downward forces, and these will cancel. It's too bad: it would be fun to fly around To access this physics simulation visit: Page 05

6 Physics Concepts Click on the link below to learn more. Types of Forces - Newton's Third Law - Free Body Diagrams - To access this physics simulation visit: Page 06