Download Warm up Forces Sir Issac Newton Questions to think about What is

Warm up Have you ever tried to pull something that just wouldn’t budge? Describe a situation in which you pulled or tried to pull something. What made the job easier? Sir Issac Newton Newton said that the force that pulls an apple to the ground is the same force that helps keep the moon in orbit around the Earth. He was able to show this force keeps the planets in their orbits around the sun. What is force? • A push or a pull • Gives energy to an object causing it to start moving, stop moving, or change the direction. • EX: 1. To open a door, you exert a force on it to cause it to move. 2. The moon pulls on the oceans creating the tides. 3. A negative and positive particle are attracted to each other Forces Questions to think about 1. What holds the moon in place around Earth? 2. Do you play tennis or baseball? Have you ever raked a pile of leaves or shoveled snow? What about riding a bicycle? A nuclear explosion? What do all of these have in common? Combining Forces • Forces in the same direction—add their forces – This is called an unbalanced force – Unbalanced forces change the motion of an object • Forces in the opposite direction—subtract their forces – If the forces are equal in size it is called balanced forces – There is no change in motion
1 Example • Net force—the combination of all of the forces • If the net force is zero: the object does not accelerate—it’s balanced Tug of War • If both sides are pulling at the same force, it is balanced and no one moves. • If one side is pulling harder (at a different force than the other), it is unbalanced and that side wins. • http://www.newtonsapple.tv/video.php?id= 902 Questions to think about Warm Up 1. How is your heart able to produce a force? 2. Why is this force vital to life? • Rub the palms of your hands together. • What is happening to your hands? • What would cause your hands to get warm as you rub them together? Friction Types of Friction • A force that acts in a direction opposite to the motion of the object. • Sliding: solid objects slide over each other • Rolling: solid objects roll over each other – Will cause the object to slow down and finally stop • EX: Car brakes • You must exert a force greater than friction to move an object! – Less friction than sliding • Fluid: fluids (liquids or gases) exert force upon an object – Less than sliding • EX: air resistance
2 Newton’s Laws • The change in momentum for an object is greater then the force when the force is larger or when the force acts over a longer time. • Lubricants are used to reduce friction • Three laws that describe all states of motion—rest, constant motion, and accelerated motion. – Oil in your engine • You would not be able to walk without friction. – Walking on ice Newton’s First Law • An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force. • Inertia—the property of matter that tends to resist any change in motion. Demo for 1 st law • Coin on index card – The more massive an object is, the more difficult to change its motion. – EX: Car stopping suddenly on a road Newton’s Second Law • Shows how acceleration and an unbalanced force are related. Example How much force is needed to accelerate a 1400 kg car 2 m/s 2 ?
Force (F) = mass (m) x acceleration (a) F=ma Force is measured in Newton (N) 1N = 1 kgm/s 2 3 Solution F = ? m = 1400 kg a = 2 m/s 2 Example 2 A baseball accelerates downward at 9.8 m/s 2 . If the gravitational force acting on the baseball is 1.4 N, what is the baseball’s mass? F = m x a F = 1400 kg x 2 m/s 2 = 2800N Gravity Solution 2 F = ma m = F/a m = (1.4 N)/ 9.8 m/s 2 * Remember that 1 N = 1 kgm/s 2 m = 0.14 kg •
Legend has it…
•In 1500s Galileo dropped 2
cannonballs from the top of the
Leaning Tower of Pisa in Italy.
•One had 10 times the mass as the
other one.
•Which one lands first?
Neither. They landed at the same time! Falling Objects • Why was this important? • To Newton, it meant that both objects were falling at the same rate regardless of their masses. (They accelerated at the same rate.) • There must be a force present because of Newton’s 2nd Law! It was gravity. • The force of attraction that depends on the mass of 2 objects and the distance between them­­gravity. 9.8 m/s 2 Air Resistance • Any falling object meets
air resistance.
• As the object falls, the air
resistance gradually becomes equal
to the pull of gravity--balanced
forces.
• When the forces are balanced there
is no acceleration. It continues to
fall at a constant velocity.
4 Newton’s Law of Universal Gravitation • Newton calculated the acceleration
of the apple and compared it with
the acceleration of the moon.
• He concluded that the force acting
on the moon was the same force
that was acting on the apple-gravity.
• This was the 1st universal law of
forces!
• Law of universal gravitation--all
objects in the universe attract
each other by the force of gravity.
• Depends on 2 factors: mass of the
objects and the distance between
them.
• EX: There is a force of gravity
between you and your book. Why is
the book not pulled toward you? Centripetal Force • A centered force that causes an object to move in a circular path. • How does a satellite orbiting Earth not end up crashing to Earth or flying off into space? – Gravity keeps it from flying off and velocity keeps it from crashing into Earth. Weight and Mass • Weight is a measure of the force of
gravity on an object.
• Unit: Newton (N)
• A medium sized car weighs between
7000 and 9500 N.
• Your mass does not change, unless
you diet and exercise or eat a lot.
• It is your weight that changes from
one place to another.
• EX: You would weigh less on a
mountain top than at sea level.
5 • Weight = Mass x gravity
• w=mxg
• EX: If your mass is 50 kg, your
weight would be 490 N.
w = (50-kg) (9.8 m/s2)
w = 490 N
• What would be the weight of a
100-kg mass? Newton’s Third Law • For every action, there is an equal and opposite reaction. – Forces work in pairs • EX: As you walk, your feet push against the floor. At the same time, the floor pushes with an equal but opposite force against your feet. • EX: A space shuttle shoots gases out before lift off. These gases are what propel the shuttle into space 3. A block is attached to a rope and hung from the ceiling. If gravity pulling down on the block is the action force, what is the reaction force? The rope pulling on the block 4. You toss a ball into the air. Before you release the ball (while your hand is moving) what forces are acting on the ball on its way up? On its way down? Up: The force of your hand, air resistance, and gravity Down: Air resistance, and gravity Demo for 3 rd law • Water in a circle Questions to think about 1. If a person holds a book, what forces are acting on the book? Is there a net force present? Gravity and the force of your hand; the net force is zero. 2. What forces are acting on a book which is setting on a table? Is there a net force present? Gravity and the force of the table; the net force is zero. 5. When a car is moving down a road at a constant speed, what forces are acting on the car? Is there a net force or are the forces in equilibrium? The force of the car’s engine, friction of the road, and air resistance; forces are in equilibrium—constant speed 6. If a car is accelerating, what forces are acting on the car? Are the forces balanced? The car engine, friction, and air resistance; forces are not balanced—accelerating
6 Three Laws of Motion
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