# Physics- Chapter 4

Total word count: 2052
Pages: 7

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 force any push or pull exerted on an object, measured in newtons ( or pounds in the british system) friction the resistive force that opposed the motion or attempted motion of an object either past another object with which is is in contact or through a fluid mass the quantity of matter in an object. more specifically, it is the measure of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, deflect it, or change in any way its state of motion weight the force upon an object due to gravity, mg ( more generally, the force that an object exerts on a means of support) kilogram the fundamental SI unit of mass. One kilogram (kg) is the mass of one liter ( 1 L) of water at 4 degrees celsius newton the SI unit of force. one newton (N) is the force that will give an object of mass 1 kg an acceleration of 1 m/s volume the quantity of space an object occupies newton’s second law the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force and is inversely proportional to the mass of the object free fall motion under the influence of gravitational pull only terminal speed the speed at which the acceleration of a falling object terminates because air resistance balances gravitational force terminal velocity terminal speed with direction specified Whenever a net force acts on an object, there is a change in the object’s _______. velocity. As you increase your push on a stationary crate, will friction on the crate increase also? The friction force increases and is opposite in direction to your push. When you push horizontally on a crate that doesn’t slide on a level floor, how great is the force of friction on the crate? The friction force is equal and opposite to your push. Once the crate is sliding, how hard do you push to keep it moving at constant velocity? You push with a force equal to and opposite the dynamic friction force. Which varies with location, mass or weight? Weight varies with location, but mass does not. Shake something to and fro and you’re measuring its what? Lift it against gravity and you’re measuring its what? Shaking measures mass, whereas lifting measures weight. What is the weight of a 1-kilogram brick resting on a table? 10 N How does acceleration depend on mass? Acceleration is inversely proportional to mass. The unit of mass is the kilogram, and the unit of weight is the _______. Newton. If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change? The acceleration is reduced to 1/3 of its original value. How does the direction of acceleration compare with the direction of the net force that produces it? The acceleration is in the direction of the net force. State Newton’s second law of motion. Acceleration is proportional to net force and inversely proportional to mass. What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance? When it encounters10 N of air resistance? The net forces are 6 N and 0 N, respectively. What two principal factors affect the force of air resistance on a falling object? The force depends mostly on frontal area and speed. What is the acceleration of a falling object that has reached its terminal velocity? The acceleration is 0 Why does a heavy parachutist fall faster than a lighter parachutist who wears a parachute of the same size? Terminal speed must be higher to make air drag equal the gravity force for a heavier person. If two objects of the same size move through the air at different speeds, which encounters the greater air resistance? The faster object encounters more air resistance. A 1-kg stone and a 10-kg stone have the same acceleration in free fall because _______. the ratio of weight to mass is the same for each. As a skydiver gains speed in falling through the air, air resistance _______. increases. Air resistance on a parachutist at terminal speed _______. is greater for a heavier person 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? No. What is the net force acting on a 1-kg ball in free fall? 10.0 N A grocery bag can withstand 300 N of force before it rips apart. How many kilograms of apples can it safely hold? 30.0 kg Free fall is motion in which gravity is the only force acting. Is a skydiver who has reached terminal speed in free fall? No. Is a satellite above the atmosphere that circles Earth in free fall? Yes. Calculate the weight in newtons of a person having a mass of 60kg . W = 600 N Susie Small finds that she weighs 260N . Calculate her mass. m = 26 kg Calculate the acceleration of a 260000-kg jumbo jet just before takeoff when the thrust on the aircraft is 170000N . a = 0.65 ms2 A 1-kg mass at the Earth’s surface weighs 10 N. The mass of a pet turtle that weighs 10 N is about 1 kg. Compared to the mass of an apple on Earth, the mass of the apple on the Moon is the same. An apple at rest weighs 1 N. The net force on the apple when it is in free fall is 1 N. An apple weighs 1 N. When the apple is held at rest above your head, the net force on the apple is 0 N. A car has a mass of 1000 kg and accelerates at 2 m/s2. What net force is exerted on the car? 2000 N An object released from rest on another planet requires one second to fall a distance of 6 meters. What is the acceleration due to gravity on this planet? 12 m/s2 A 1000-kg automobile enters a freeway on-ramp at 20 m/s and accelerates uniformly up to 40 m/s in a time of 10 seconds. How far does the automobile travel during that time? 300 m A 2000-kg car experiences a braking force of 10,000 N and skids to a stop in 6 seconds. The speed of the car just before the brakes were applied was 30 m/s An astronaut on another planet drops a 1-kg rock from rest and finds that it falls a vertical distance of 2.5 meters in one second. On this planet, the rock has a weight of 5 N A ball is thrown vertically into the air. Because of air resistance, its speed when it returns to its starting level compared with its initial speed is less. Suzie Skydiver jumps from a high-flying plane. As her velocity of fall increases, her acceleration decreases. A feather and a coin will have equal accelerations when falling in a vacuum because the ratio of both the feather’s and coin’s weight to mass is the same. Recall Galileo’s Leaning Tower experiment. With negligible air resistance, a heavy and a light object fall to the ground in equal times. with equal accelerations. with the same increases in speed. all of the above A skydiver’s terminal velocity will be greatest if she falls head first. The direction of the force of friction on a sliding crate is _______. opposite to the direction of sliding A crate sits at rest on a factory floor. Friction between the crate and floor occurs _______. when the crate is pushed horizontally, whether sliding or not. What is the condition for an object experiencing free fall? When gravity is the only force acting on an object, it is in free fall. Why doesn’t a heavy object accelerate more than a light object when both are freely falling? The ratio of the weight to mass is the same for all objects in the same locality. Whenever the net force on an object is zero, its acceleration is zero. If an apple experiences a constant net force, it will have a constant speed. acceleration. If you double the net force on an object, you’ll double its acceleration. If the net force on a cart is tripled, the cart’s acceleration is three times as much. A mobile phone is pulled northward by a force of 10 N and at the same time pulled southward by another force of 15 N. The resultant force on the phone is 5 N. The force of friction on a sliding object is 10 N. The applied force needed to maintain a constant velocity is 10 N. A 10-N falling object encounters 10 N of air resistance. The net force on the object is 0 N. An object’s weight is properly expressed in units of newtons. A kilogram is a measure of an object’s mass. Compared to a 1-kg block of solid iron, a 2-kg block of solid iron has twice as much volume. inertia. mass. all of the above Your weight as measured on your bathroom scale is the force due to gravity on you. An object with twice as much mass as another object has twice as much inertia. In which case would you have the largest mass of gold? If your chunk of gold weighed 1 N on the Moon. At equilibrium on a bathroom weighting scale, the downward pull of gravity on you is balanced by an upward support force. When you stand at rest with your left foot on one bathroom scale and your right foot on a similar scale, each of the scales will show readings that when added equal your weight. A tow truck exerts a force of 3000 N on a car, which then accelerates at 2 m/s2. What is the mass of the car? 1500 kg Nellie pulls on a 10-kg wagon with a constant horizontal force of 30 N. If there are no other horizontal forces, what is the wagon’s acceleration? 3.0 m/s2 A force of 1 N accelerates 1-kg box at the rate of 1 m/s2. The acceleration of a 2-kg box by a net force of 2 N is the same. A boulder following a straight-line path at constant velocity has zero acceleration. A skydiver steps from a helicopter and falls for a few seconds until terminal velocity is reached. Thereafter, until he opens his parachute, his acceleration is zero. A 500-N parachutist opens his chute and experiences an air resistance force of 800 N. The net force on the parachutist is then 300 N upward. Two tennis balls fall through the air from a tall building. One of them is filled with lead pellets. The ball to reach the ground first is the lead-filled ball. Consider a business jet of mass 40000kg in takeoff when the thrust for each of two engines is 28000N .Find its acceleration. a = 1.4 ms2 One pound is the same as 4.45 newtons.What is the weight in pounds of 1 newton? W = 0.220 lb A heavy block at rest is suspended by a vertical rope. When the block accelerates upward by the rope, the rope tension is greater than its weight. A single brick falls with acceleration g. The reason a double brick falls with the same acceleration is its ratio of force to mass is the same. A bowling ball and a baseball accelerate equally when falling in a vacuum because the ratio of their weights to mass is the same. A light woman and a heavy man jump from an airplane at the same time and open their same-size parachutes at the same time. Which person will get to a state of zero acceleration first? The light woman A light woman and a heavy man jump from an airplane at the same time and open their same-size parachutes at the same time. Which person will get to the ground first? The heavy man A 1-kg rock that weighs 10 N is thrown straight upward at 20 m/s. Neglecting air resistance, the net force that acts on it when it is half way to the top of its path is 10 N. Scotty Skydiver of mass 100 kg experiences air resistance of 500 N, and an acceleration of about 0.5 g. A 10-kilogram block is pushed across a horizontal surface with a horizontal force of 20 N against a friction force of 10 N. The acceleration of the block is 1 m/s2. An astronaut on another planet drops a 1-kg rock from rest. The astronaut notices that the rock falls 2 meters straight down in one second. On this planet, how much does the rock weigh? 4 N

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