Newton’s Second Law Continued

You learned that when you are calculating force and have to take gravity into account you must add 9.8 m/s2 to the acceleration What must you do when you are calculating Acceleration? You must multiply 9.8 m/s2 to the mass and then subtract it from the original force!! Why? This will convert your mass into a force and allow you to adjust your force for the effect of gravity Remember:1 kg = 9.8 kg x m/s2 or 9.8N If you have an object that has a mass of 50 kg how many N is that: 9.8 m/s2 x 50 kg = 490 kg x m/s2 or 490 N

Example You apply a force of 50 N to lift a package with a mass of 4 kg. Calculate the resulting rate of acceleration of the package.

Solution F = M x A or A = F/M What do we know? F= 50 N M = 4 kg We can’t just plug gravity into the formula!!!! Remember the gravitational constant is an acceleration rate!!!! However 1N = 9.8 kg x m/s2 So by multiplying your mass times the gravitational constant you now can deal with gravity 4kg x 9.8 m/s2 = 39.2 kg x m/s2 or 39.2 N A = (50N – 39.2N)/4kg A = 2.7 m/s2

Newton’s Second Law Continued Question? If you throw object from a certain height and also drop an object from the same height which one will hit the ground first? Neither – they both hit at the same time!!

Projectile and Circular Motion What is a projectile? Anything that is thrown or shot through the air Horizontal Motion Once you throw a ball no other force accelerates it forward (ignoring a tail wind and air resistance) so its horizontal velocity is constant. Vertical Motion When the object thrown starts being pulled downward, horizontal motion has turned into vertical motion Now the ball has constant horizontal motion (due to inertia) however it has increasing vertical velocity

So what is the answer to the question? Penny demonstration. They will both hit at the same time. Gravity still exerts the same acceleration rate

Question? How does a roller coaster work?

What about curves? If you are moving at a constant speed in a straight line there is no acceleration!!! However, if you enter a curve, even if you have not changed speed you have accelerated!!!

Curves The change in direction of the velocity is toward the center The acceleration towards the center is known as centripetal acceleration

Roller Coasters In order for acceleration to occur when in a curve there must be some unbalanced force acting on the object. Centripetal Force – the force acting toward a center of a curved or circular path Examples: A car driving on the road. The centripetal force is the friction between the tires and the road Roller coaster – the friction between the track and the wheels of the car