Newtons Laws Take 2 take out your 3s February 13, 2009
Newtons 2 nd Law proves that different masses accelerate to the earth at the same rate, but with different forces. We know that objects with different masses accelerate to the ground at the same rate. However, because of the 2 nd Law we know that they dont hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s
Terminal Velocity – force of friction = force of gravity air resistance = weight – acceleration = 0 m/s 2 – F net = 0 N Who has a higher terminal velocity, a person who has a mass of 200 kg or a person with a mass of 100 kg? 200 kg
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Free Body Diagrams - A picture that shows all of the forces acting on an object What are the forces acting on this object? http://lectureonline.cl.msu.edu/~mmp/applist/si/plane.htm FfFf FnFn FgFg FpFp
Tension Practice Problem A stop light with a mass of 25 kg is supported by 2 wires. What is the tension in the wire that is 15˚ from the normal? What is the tension in the wire that is 50˚ from the normal? 15˚ 50˚
Tension Practice Problem A stop light with a mass of 25 kg is supported by 2 wires. What is the tension in the wire that is 15˚ from the normal? What is the tension in the wire that is 50˚ from the normal? 15˚ 50˚ F g = m g F g = (25 kg) (9.8 m/s 2 ) = 245 N
Arnold Strongman and Suzie Small are having a tug of war contest. They pull on opposite ends of a rope. Who exerts a greater force on the rope? Rope tension must be the same throughout the whole rope. Therefore, Arnold can pull no harder on the rope than Suzie. The most important factor in a tug of war is not how hard you pull on the rope, but how hard you push on the ground.
Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. 1. How does the force exerted by the spring on the 1m-cart compare to the force exerted by the spring on the 2m-cart? The forces are equal 2. How fast does the 2m-cart roll compared to the smaller 1m- cart? It would roll half-as fast since it experienced half the acceleration when released from the spring.
Tension Force Transmitted through a string, rope, cable, or wire Pulled tight by forces acting from opposite ends Force is directed along the length of the wire and pulls equally on the objects on the two opposite ends of the wire.
Example) Will mass 1 slide down the inclined plane? Mass 1 = 300 g and Mass 2 = 150 g The same principles apply but now we have a force pulling the object up the ramp versus down! –To simplify things, we will assume the inclined plane and pulley is frictionless and the rope is massless.
What force (s) are acting upon mass 2? What force (s) are acting upon mass 1?
What force (s) are acting upon mass 2? What force (s) are acting upon mass 1? Will mass 1 slide down the inclined plane? No
3 rd Law According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.
3 rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.
Newtons 3rd Law in Nature Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).
3 rd Law Flying gracefully through the air, birds depend on Newtons third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.
Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). Action-reaction force pairs make it possible for birds to fly.
Other examples of Newtons Third Law The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).
3 rd Law Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.
3 rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.