Newton’s Laws. --Don’t cross the Newton. You’ll get figged . Newton’s laws. An object in motion tends to stay in motion unless acted upon by an outside, net force An object accelerates proportional to the net force exerted upon it, inversely proportional to its mass.
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Newton’s Laws
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Newton’s Laws --Don’t cross the Newton. You’ll get figged.
Newton’s laws • An object in motion tends to stay in motion unless acted upon by an outside, net force • An object accelerates proportional to the net force exerted upon it, inversely proportional to its mass. • For every action, there is an equal and opposite reaction.
Newton’s First Law • …also known as “inertia” • An object in motion tends to stay in motion, an object at rest tends to stay at rest…
Newton’s First Law • …also known as “inertia” • An object in motion tends to stay in motion, an object at rest tends to stay at rest… …unless acted upon by an outside force!
Decrease air resistance • Streamlining • Slowing down (Professional cyclists shave their legs!) Increase air resistance • Parachutes • Sails • Airfoils
Frames of reference • Suppose a kayaker can paddle at 1.8 m/s, and she is on a river flowing at 1.5 m/s. • If she is paddling downstream, how fast is she moving with respect to: • The water? • The riverbank? • If she is paddling upstream, how fast is she moving with respect to: • The water? • The riverbank?
Definition --A push or a pull. --must be exerted on an object
Forces • A force is a push or a pull. • It has a DIRECTION! 5N East 7N West 15N North
Forces • A force can cause acceleration. • …this object accelerates to the east 5N East
Forces • A force can cause acceleration. • …or not. The object’s weight pushes it against the surface
Forces • The acceleration of an object is a result of the forces acting upon it. A object’s weight pulls it down The cart starts to move west as the horse begins to pull
Forces • The units on a force are Newtons • (Remember? Newton’s Laws of motion?)
Forces • A one Newton force will accelerate a 1 kg object at one m/s2 1N=1kgm/s2 1 kg A 1 N force will accelerate a 1kg object at 1 m/s2
Forces • A one Newton force will accelerate a 1 kg object at one m/s2 1N=1kgm/s2 .5 kg A 1 N force will accelerate a .5kg object at 2 m/s2
Forces • A one Newton force will accelerate a 1 kg object at one m/s2 1N=1kgm/s2 2 kg A 1 N force will accelerate a 2kg object at .5 m/s2
Newton’s Second Law • The acceleration of an object is proportional to the net force acting on it, and inversely proportional to the object’s mass. a=F/m or F=ma
Centripetal force An object moving in a curved path is acted upon by a centripetal force
Weight Weight is the force exerted upon an object by the acceleration of gravity.
Weight is a force! • What is the weight of a 5.8 kg object? 2) What is the weight of a 865 g object? 3) What is the mass of a 580 N object?
The acceleration of gravity = 9.8 m/s2 • What is the weight of a 5.8 kg object? 57N 2) What is the weight of a 865 g object? 8.5 N 3) What is the mass of a 580 N object? 59 kg
What is the acceleration? 1) A 850 kg car gets a force of 2700 N (from the friction between the tires and the road) 2) A tugboat provides a force of 70,000 N of tension on its tow cables to pull a ship with a mass of 15,000,000 kg away from the dock. 3) The Saturn-V rocket has a mass of 2.8x106 kg, while its engines provide 3.3x107 N of thrust
What is the acceleration? 1) A 850 kg car gets a force of 2700 N (from the friction between the tires and the road) 3.2 m/s2 2) A tugboat provides a force of 70,000 N of tension on its tow cables to pull a ship with a mass of 15,000,000 kg away from the dock. .0047 m/s2 3) The Saturn-V rocket has a mass of 2.8x106 kg, while its engines provide 3.3x107 N of thrust 12 m/s2
What is the force? 1) A 72 kg sprinter leaves the blocks at 15 m/s2 2) A fighter jet with a mass of 4500 kg accelerates at 35 m/s2 while taking off. 3) A quarterback throws a 1.2 kg football at 21 m/s, accelerating it from rest in .32 s
What is the force? 1) A 72 kg sprinter leaves the blocks at 15 m/s2 1100 N 2) A fighter jet with a mass of 4500 kg accelerates at 35 m/s2 while taking off. 160,000 N 3) A quarterback throws a 1.2 kg football at 21 m/s, accelerating it from rest in .32 s 79 N
What is the braking distance? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 1100 kg (weight = 10800 N) • How much friction does the road provide? b) What is the acceleration of the car at full braking? c) If the car is moving at 25 m/s, how far does it travel before coming to rest?
What is the braking distance? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 1100 kg (weight = 10800 N) • How much friction does the road provide? -3600 N b) What is the acceleration of the car at full braking? -3.26 m/s2 c) If the car is moving at 25 m/s, how far does it travel before coming to rest? 97 m
What is the braking distance? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 2200 kg (weight = 21600 N) • How much friction does the road provide? b) What is the acceleration of the car at full braking? c) If the car is moving at 25 m/s, how far does it travel before coming to rest?
What is the braking distance? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 2200 kg (weight = 21600 N) • How much friction does the road provide? -7200 N b) What is the acceleration of the car at full braking? -3.26 m/s2 c) If the car is moving at 25 m/s, how far does it travel before coming to rest? 97 m
What is the braking distance? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 2200 kg (weight = 21600 N) • How much friction does the road provide? b) What is the acceleration of the car at full braking? c) If the car is moving at 50 m/s, how far does it travel before coming to rest?
What is the braking distance? • The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 2200 kg (weight = 21600 N) • How much friction does the road provide? -7200 N b) What is the acceleration of the car at full braking? -3.26 m/s2 c) If the car is moving at 50 m/s, how far does it travel before coming to rest? 380 m
Vector addition • If two vectors lie in the same direction, add their magnitudes. The sum lies in the same direction. 5N (north) + 10N (north) = 15N (north) 75m/s (east) + 40m/s (east) = 115m/s (east)
Vector addition • If two vectors lie in opposite directions, subtract their magnitudes. The sum lies in the same direction as the longer vector. 5N (south) + 12N (north) = 7N (north) 75m/s (east) + 40m/s (west) = 35m/s (east)
What is the net force? • A 20 N object is lifted with 36 N. . • A 45N object is lifted by a 850 N man. . • A two guys push a desk with 350 N each. . • I pull a desk with 400 N against 180 N friction. .
What is the net force? • A 20 N object is lifted with 36 N. +16 N • A 45N object is lifted by a 850 N man. -895N • A two guys push a desk with 350 N each. +700N • I pull a desk with 400 N against 180 N friction. +220N
Draw two vectors at right angles: • On your graph paper, draw a 5.0 cm vector east and a 7.0 cm vector north. • These vectors might represent x and y components of velocity • vy • vx
Draw two vectors at right angles: • Draw the vectors TIP to TAIL • (re-draw the north vector so its tail starts at the tip of the east vector) • vy • vx
Draw two vectors at right angles: • The RESULTANT vector is the sum of the two vectors. • Measure and record the length of the resultant vector (in cm.) and the angle it makes with vx • vR • vy • vx
Draw two vectors at right angles: • The RESULTANT vector is the sum of the two vectors. • Measure and record the length of the resultant vector (in cm.) and the angle it makes with vx • vR=8.6 cm at 54o • vy • vx
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