Early Thoughts on Force
 While modern scientists understand the relationships between force and motion, it actually took about 2000 years to develop this understanding.
Aristotle
 Aristotle was a Greek ________________ and ________________ who lived from 384 – 322 B.C.
 He made many scientific discoveries through the use of __________________ and logical
__________________.
 He had the first idea of the need of a _____________ to cause objects to move.
 Unfortunately, he incorrectly proposed that you need a force to keep an object moving.
 Aristotle’s incorrect ideas about force were considered to be correct for almost 2000 years.
 It was difficult for anyone to find fault in Aristotle’s proposal, since there were so many observations that seemed to follow his ideas.
Galileo
 Galileo was an Italian _________________ who lived from 1564 – 1642.
 He challenged the views of Aristotle and devised several experiments to help disprove incorrect notions of force and motion.
 One of Galileo’s famous experiments was dropping a large rock and a small rock from the top of the leaning tower of Pisa.
 When he dropped these rocks, Galileo found that the two rocks reached the ground at the
____________ _____________.
 This disproved the notion that large objects fall faster than small objects.
 Another experiment that Galileo devised was rolling a ball up and down a series of ramps and watching the effects.
 Galileo reasoned that since a ball’s speed increased when going down a ramp and decreased when going up a ramp, if he rolled a ball on a horizontal surface it should continue at the
_____________ _______________.
 Galileo believed that the ball did not continue rolling at the same speed because of tiny imperfections on surfaces. He called the effect of slowing an object’s motion ______________.
 This idea was a major breakthrough in the understanding of force and motion. An idea which was later built upon by ______________ _________________.

Isaac Newton
 Isaac Newton (1642-1727) was an English _____________________ and _________________ who revolutionized the way we think about the world.
 In the late 1600’s he postulated three laws of motion that are the basis for describing the motion of all large objects in the universe.
 In addition to formulating the laws of motion, Isaac Newton had many other claims to fame:
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Newton’s 1 st Law
 An object at rest stays at rest and an object in motion will continue to move at the same speed in a straight line unless a force acts on the object.
 This law is also called the law of _______________.
 ______________ – The tendency for an object to maintain its state of motion.
 Thus, according to the property of inertia, an object at rest wants to stay at rest.
 An object in motion wants to stay in motion.
Examples:
If you are riding the school bus with your books sitting next to you, why do your books slide off the seat when the bus comes to a quick stop?
Why do you lean into the door of a car when rounding a sharp turn?
Newton’s 2 nd Law
 Force = mass × acceleration
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 Force is measured in units of ________________.

 Newton’s 2 nd Law is simply an equation that describes the relationship between ____________,
______________ and ____________________.
Examples:
How much force is required to accelerate a 25-kg mass at 4-m/s 2 ?
What is the acceleration of a 20-kg cart when it is pushed with a force of 200-N?
A car accelerates at 3-m/s 2 from a force of 4500-N. What is the mass of this car?
Weight
 Calculating weight is an application of Newton’s 2 nd Law.
 Weight = mass × gravitational acceleration
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 Remember that ________________.
 When calculating weight in this manner, weight is measured in ________________.
Examples:
What is the weight of a 65-kg person?
What is the mass of a person who weighs 490-N?
Newton’s 3 rd Law
 For every force there is an equal and opposite force.
 This law is also called the law of _________________________ forces.
 An example is when you stand on the floor, your body weight pushes down on the floor, and the floor pushes up on your body with the same force, but in the opposite direction.

Examples:
What are the action-reaction forces when you are sitting in a chair?
What are the action-reaction forces when you are swimming through the water in a pool?
What are the action-reaction forces of a rocket moving through space?
 Not all action and reaction forces cancel.
 In the case of the swimmer and the rocket, even though the forces are equal and opposite the objects are still able to move.
 The reason for this is the difference in ____________.
Example:
If an apple falls to the Earth, we say that the Earth is pulling the apple. However, if this is the case, there needs to be an opposite force. What would be the opposite force?
 The reason why we see the apple move, but we do not see the Earth move is because of the huge difference in ____________.
 The apple has a very ____________ mass compared to the Earth.
 This means the apple will accelerate at a ___________ rate, while the Earth accelerates at a very
___________ rate.