Presentation on theme: "Consider a horse pulling a buggy. Is the following statement true?"— Presentation transcript:
1 Consider a horse pulling a buggy. Is the following statement true? The weight of the horse and the normal force exerted by the ground on the horse constitute an interaction pair that are always equal and opposite according to Newton’s third law.1. yes2. noThey are not always equal, on an incline for example.Answer: 2. The normal force is a repulsive contact force between theground and the horse. The weight is the gravitational force exerted byEarth on the horse. These two forces are of different origin and do notconstitute an interaction pair.The normal force is a repulsive contact force between theground and the horse. The weight is the gravitational force exerted byEarth on the horse. These two forces are of different origin and do notconstitute an interaction pair.
2 1. the two forces form an interaction pair. Consider a car at rest. We can conclude that the downward gravitational pull of Earth on the car and the upward contact force of Earth on it are equal and opposite because1. the two forces form an interaction pair.2. the net force on the car is zero.3. neither of the aboveAnswer: 2.These two forces cannot be an interaction pair because they acton the same object. Because the car is at rest, however, its momentum isconstant (and zero). Because net force equals the time rate of change inmomentum, the net force on the car must be zero. This means that thetwo forces must be equal and opposite.Choice 1 is not true because both forces act on the same object!
3 The setup photographed below, known as a horizontal Atwood machine, consists of a heavy cart connected by a string passing over a pulley to a lighter mass. In this case the mass of the cart M = 875 grams and the mass hanging on the end of the string m = 200 grams. When the system is held in place the spring scale indicates that the force (in grams) pulling on the cart is equal to the mass m hanging on the string, as seen in the lower photograph.Assume that because this is physics the pulley is frictionless.After the system is released and begins to move the spring scale will read:(1) 0 grams.(2) greater than 0 but less than 200.(3) 200 grams.(4) greater than 200 grams.
4 Determine the exact amount of tension for the Horizontal Atwood Machine. yNWe are going to compare the tension to the amount of mass required to cause an equivalent gravitational force. (The tension is not equal to a weight!!)MmTWMM = 875 gm = 200 gTxyFor M:WmFor m:
5 A person pulls a box across the floor A person pulls a box across the floor. Which is the correct analysis of the situation?The box moves forward because the person pulls forward slightly harder on the box than the box pulls backward on the person.2. Because action always equals reaction, the person cannot pull the box- the box pulls backward just as hard as the person pulls forward, so there is no motion.3. The person gets the box to move by giving it a tug during which the force on the box is momentarily greater than the force exerted by the box on the person.4. The person’s force on the box is as strong as the force of the box on the person, but the frictional force on the person is forward and large while the backward frictional force on the box is small.5. The person can pull the box forward only if he or she weighs more than the box.Answer: 4.The force exerted by the person on the box is equal to that exertedby the box on the person. The person moves forward because of aforward frictional force exerted by the floor. The frictional force exertedby the floor on the box is much smaller.