Transcript Newton`s Second Law Contineud

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