Newton's 3rd Law

Transcript

1. Newton's 3rd Law • states that forces act in equal but opposite pairs • for every action there is an equal but opposite reaction • when one object exerts a force on another object, the second object exerts the same size force on the first object • example: when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted
2. Newton’s 3rd Law • whenever 2 objects interact with each other, they exert forces upon each other • sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body • two forces resulting from this interaction - a force on the chair and a force on your body. • two forces are called action and reaction forces
3. Bumper Car Collision... In the collisions, the first car exerts a force on the second one. the second exerts tje same force in the opposite direction on the first car. QUESTION: DURING THE COLLISION, WILL BOTH CARS HAVE THE SAME ACCELERATION? The acceleration depends on the mass of the cars. The car with the smaller mass accelerates more.
4. Why don't action and reaction forces cancel each other out? • forces that 2 objects put on each other are called the action-reaction force pair • either force can be considered the action or reaction force • action-reaction forces do not cancel because they act on different objects • forces can only cancel if they act on the same object Action and reaction forces in action... The action from the bowling ball acts on the pins. The pins will fly in the direction of the force. The reaction force from the pin acts on the ball. It causes the ball to slow down.
5. How does a bird rely on Newton's 3rd Law to Fly? • When a bird flies, its wings push in a downward and a backward direction. This pushes air downward and backwards. By Newton's 3rd Law, the air pushes back on the bird in the opposite directions, upward and forward. This force keeps the bird in flight and propels it forward. What other forces act upon birds as they fly? • lift • air resistance • air currents • gravity 3rd Law Check Your Understanding
6. How do action-reaction force pairs work on large and small objects? • Why doesn't the Earth appear to move when you push down on it with your foot? • When you walk forward you foot pushes down on the Earth and the Earth pushes back up on you propelling you forward. Earth has so much mass compared to you that it does not appear to move when you push on it. How do rockets take off? A rocket launch is an example of Newton's third law. Fuel in shuttle's engine is ignited and a hot gas is produced. The gas molecules collide with inside walls on the engines. Walls push gas out of the bottom of the engine, the action force. The gas molecules put reaction forces on the walls of the engine, which pushes the engine and rocket forward. This is called thrust.
7. • Blowing up balloon and letting it go • Balloon rockets • Chairs with wheels tug of war Weightlessness While in space, astronauts are said to be weightless, as if Earth's gravity was not pulling on them. But Earth's gravity is what keeps a shuttle in orbit. Why do they seem weightless in space then? Newton's 3rd Law can explain why they float as if no forces were acting on them.
8. How is weight measured? • as you stand on a bathroom scale, your weight pushes down on the scale • this causes the scale pointer to show your weight • Newton's 3rd Law tells you that the scale pushes back up on you with a force equal to your weight • this force balances the downward pull of gravity on you Your weight pushes down on the scale as the scale pushes back up at you.
9. How does free fall cause weightlessness? • free fall is free descent of a body in which the gravitational force is the only force acting on it Feather vs. Hammer on the Moon • Why did they fall at the same rate? • Who is Galileo and what were his contributions to the field of physics? • Who was Galileo and his contributions? • Write down 3 facts about Galileo's contributions to the field of physics.
10. Weightlessness cont.... • imagine standing on a scale in an elevator that is falling • an object is in free fall when the only force acting on it is gravity • the elevator, you, and scale are all in free fall • in free fall the scale does not push back on you, because gravitational force is the only force acting on the scale • according to Newton's 3rd Law you are also not pushing down on the scale so the scale remains at 0lbs • you seem to be weightless , however Earth's gravity is still pulling down on you • because nothing is pushing up on you, you have no sensation of weight
11. Why are spacecrafts in orbit weightless? • objects will orbit the Earth as it follows the curvature of the Earth's surface • gravity keeps pulling the object down, but forward motion keeps it from falling straight down • objects that orbit the Earth are in free fall • objects inside the shuttle are also in free fall, this makes the shuttle and everything inside it seem weightless • as an astronaut releases a ball in space it will not move unless a force is applied to it • the ball does not move because the ball, astronaut, and shuttle are all falling at the same speed Newton's 3rd Law Review