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Transcript
Force and Motion
ISCI 2002
Force
1. Force: ‘push’ or ‘pull’ on an object
2. Objects in motion stay in motion unless enacted
upon by a ‘unbalanced’ force.
• Newton’s First Law
Force
3. Forces: friction (drag),
thrust, etc.
4. Are you telling me that if
an object in motion
continues at a constant
velocity unless …….
• Forces change the velocity
of an object in motion
Law of Inertia
1. The first law relates to inertia.
2. The tendency for an object to remain in whatever
state of motion that it is in is referred to as ‘inertia’
3. Consider the following:
•
The greater the mass the _________ the inertia.
4. Mass – how much inertia an object has
Weight and the Force of Gravity
1. Weight: (mass)(force of gravity)
2. Mass is constant. Weight depends
on the force of gravity upon an
object.
• Is the weight of a 60-kg rocket at the
surface of the Earth equal to the
weight when at 2,600 kilometers
above the Earth?
• No. Why?
The Newton
3. The ‘Newton’
• (kg)(m/s2) -------- kg x m/s2
Relationship between Force, Mass and Acceleration
1. Newton’s Second
Law
• The acceleration of an
object is directly
proportional to the
net force acting on it
and inversely
proportional to its
mass.
• a = Fnet / mass
Second Law
2. What happens to acceleration when:
(a). The force upon it doubles? (mass is
constant)
(b). The force is decreased by ½? (mass is
constant)
(c). Mass is doubled (force is constant)
Newton’s Third Law
1. For every action there is an equal and opposite
reaction.
2. Forces always come in pairs.
3. Features:
•
•
•
They are equal in magnitude
Opposite in direction
They act on different objects
Third Law
Newton’s Third Law
• If you punch the wall with a 10 N force, the wall
exerts a force of __________ upon your fist.
• Opposite in direction?
• Act on different objects?
Speaking of Acceleration due to Gravity
• Galileo – objects in ‘free-fall’.
• Objects falling freely fall with the same acceleration
(9.8 m/s2) – near Earth
• So speed or velocity changes 9.8 m/s every second.
• The instantaneous speed or velocity of a body
dropped from rest:
• V = (9.8 m/s2) time
• V = at (if the initial velocity is 0)
Free Fall
Freely Falling Objects
• At 2 seconds an objects velocity fall is ____________.
• If a ball is thrown upward at an initial velocity of
29.4 m/s, what is it’s velocity at 1.0 seconds? 2.0
seconds? 3.0 seconds? 4.0 seconds? 5.0 seconds? 6.0
seconds?
Free Fall
• Galileo rolled balls down a ramp.
• He measured the distance they moved during time intervals.
• This slowed down the ball in order to take accurate
measurements.
• He found that:
• d = ½ at2
• If an object falls (from rest) for a total of 2 seconds, how far
has it fallen?