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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!**Does anything remain in motion?**• So what causes objects to come to rest?**Does anything remain in motion?**• So what causes objects to come to rest? Friction Happens**Friction opposes motion**V F The surfaces in contact cause friction**Decrease friction**• Wheels • Skates/skis • Sleds • Lubricants • Magnetic levitation Increase friction • Brakes • Rubber tires • Tread on tires/shoes • Spikes**Even without a surface…**V Air resistance …air resistance will slow moving objects**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.**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:**• So, what direction is the object going? • vy • vx**Draw two vectors at right angles:**• So, what direction is the object going? • Up and to the right —right? • 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. • 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 • 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