IB Physics 11 Mr. Jean September 16 th, 2014

The plan: Video clip of the day Relative Motion Free Fall Motion –Solutions to in class problems 2D motion Introduction to Projectiles 2D motion questions.

The Effects of Air Resistance Air resistance depends on 2 things –Surface area –Velocity Air resistance increases as surface area increases Air resistance increases as the velocity increases

Terminal Velocity As an object falls through the air, it accelerates, due to the force of attraction of the Earth. This force does not change. As the velocity increases, the air resistance, the force opposing the motion, increases, therefore the acceleration decreases.

If the object falls for long enough, then the air resistance (a force acting upwards) will equal the force of attraction of the Earth (the weight) (a force acting downwards) Now there are no net forces acting on the object (since the two forces balance) so it no longer accelerates, but travels at a constant velocity called its terminal velocity.

Terminal velocity depends on –The size –Shape –And weight of the object A sky diver has a terminal velocity of more than 50ms -1 (100 miles per hour)

Relative Motion If you are stationary and watching things come towards you or away from you, then your stating velocities is easy. If, however you are in motion, either moving towards or away from an object in motion, then your frame of reference is different

In this case the relative velocity is the velocity of the object relative to your motion. Examples include –cars overtaking –Trains going passed platforms

Free Falling Objects: When an object is dropped. –1) There is NO account for air resistance. –2) All Free-Fall objects fall at the same rate –What happens when there is no gravity?

A ball is dropped from the top of CN tower which is 440m above ground. The acceleration of gravity is -9.81m/s 2. –A) How long does it take for the ball to hit the ground? –B) At what velocity does it hit the ground?

A) How long does it take for the ball to hit the ground?

B) At what velocity does it hit the ground?

2D Motion

x – axis (left & right motion) is determined by the cosine of the angle multiplied by the velocity. v ix = v i *cos(θ) y – axis (up & down motion) is determined by the sine of the angle multiplied by the velocity. V iy = v i *sin(θ)

Projectile Motion:

Check it out: ectile-motionhttp://phet.colorado.edu/en/simulation/proj ectile-motion games/kittencannon.jsphttp:// games/kittencannon.jsp

Variables: s x = horizontal distance (m) s y = vertical distance (m) v y = velocity (m/s) up & down v x = velocity (m/s) left & right g = acceleration due to gravity (-9.81m/s 2 ) –Gravity only influences y-axis!!!! θ = angle in degrees

Projectile symmetry:

Important Assumptions: No air resistance. Object does not hit anything while in flight. Ground is level unless stated differently. You are on Earth unless stated differently. Gravity field is constant. The object is not moving at light speed.

Projectile Motion:

Free Fall Question  Projectile An object on Earth starts at rest and free falls for 15seconds. How fast is it falling after 15 seconds? –How fast in Km/h is it falling? How far did it fall after 15 seconds?

How fast is it falling?

How far did it fall? ∆height?

Free Fall Question  Projectile An object on Earth, is rolling with an initial horizontal velocity of 15m/s. It rolls off the edge of a building and free falls for 15seconds. What was the objects initial velocity? –Velocity in X? –Velocity in Y? How fast is it falling after 15 seconds? –Velocity in X? –Velocity in Y?

What was the objects initial velocity? –Velocity in X? –Velocity in Y?

How fast is it falling after 15 seconds? –Velocity in X? –Velocity in Y?

Free Fall Question  Projectile An object on Earth, is rolling with an initial horizontal velocity of 15m/s. It rolls off the edge of a building and free falls for 15seconds. How far did it fall after 15 seconds? –S x ? –S y ?

How far did it fall after 15 seconds? –S x ? –S y ?

Pages to read: Giancoli –Pages: 45 to 63 –Pay special attention to the approach laid out by this author. –Go over the examples