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Lecture 4

PHY 2020 Lecture 4: Chapter 4-5

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PHY 2020

Chapter 4- Newton’s Second Law of Motion - Acceleration depends on net force o a= F/m o F= ma - Why does stuff accelerate? o An outside force is acting upon it. - (1)A 1-kg rock is thrown at 10 m/s straight upward. Neglecting air resistance, what is the net force that acts on it when it is halfway to the top of its path? - (2) What would be the net force halfway up, if you were to throw it at 100 m/s ? o (1) Gravity is the net force when halfway to the top o (2) Gravity- after leaving your hand the only force acting upon the ball is gravity. - Three identical blocks are pulled, as show, on a horizontal frictionless surface. If tension in the rope held by the hands is 30 N, what is the tension in the other ropes? - Free fall o Only one fact acts on object- gravity - Friction o Comes in two “flavors” static and kinetic (sliding) ▪ Static, when stuff at rest and there is friction - Why do we have groves in car tires and none in skateboard tires? o Tires with grooves to displace and redirect water from between the road surface and underside of the tire. - Friction is always opposed to the motion - When Sanjay pushed a refrigerator across a kitchen floor at a constant speed the force of friction between the refrigerator and floor is o Equal and opposite to Sanjay’s push - When Sanjay pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the fridge and floor is o Less than Sanjay’s push - Suppose in a high-flying airplane the captain announces over the cabin public address system that the plane is flying at a constant 900 km/h and the thrust of the engines is a constant 80,000 Newton’s o The acceleration is zero because the velocity is constant because the acceleration is zero, the net force is zero. ▪ Air resistance is 80,000 - A 400-kg bear grasping a vertical tree slides down a constant velocity. What is the friction force that acts on the bear? o 400 kg * 10 m/s^2 (gravity)= 4,000 N - Can you distinguish between mass and weight? o Mass: matter in an object o Weight: force due to gravity. - Since an object weighs less on the surface of the Moon than on Earth’s surface does it have less inertia on the Moon’s surface? o Inertia is the same but the weight is different (less on the Moon) - What relationship does mass have with inertia o As mass increases so does inertia - What relationship does mass have with weight? What’s the more fundamental mass or weight? o Mass in an intrinsic factor - Checkpoint 6: Nellie Newton skydives from a high-flying helicopter. As she falls faster and faster through the air, does her acceleration increase, decrease, or remain the same? o Acceleration decreases because net force decreases. - Why is it that a cat that accidently falls from the top of a 50-story building hits a safety net below no faster than if it fell from the 20-th story? o The cat reaches terminal velocity at or before falling 20 stories and therefore falling from a higher point doesn't cause the cat to go faster - A falling skydiver of mass 100 kg experiences 500 N air resistances. The acceleration of the skydiver is o Acceleration= net force/ mass= 500 N/ 100 kg= 0.5 g Clicker and Homework Questions 1. Does a 2-kg iron brick have twice as much inertia as a 1-kg iron brick? Yes 2. Compare to a 1-kg block of solid iron, a 2-kg block of solid iron has the same None of these 3. Consider a cart pushes along a rack with a certain force. If a force remains the same while the mass of the cart decreases to half, the acceleration of the cart Double 4. Push a cart along a track so twice as much net force acts on it. If the acceleration remains the same, what is a reasonable explanation? A mass of the cart doubled 5. A skydiver steps
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