Transcript of Consider a horse pulling a buggy. Is the following statement true? The weight of the horse and the...
- Slide 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 Newtons third law. 1. yes 2. no
They are not always equal, on an incline for example. The normal
force is a repulsive contact force between the ground and the
horse. The weight is the gravitational force exerted by Earth on
the horse. These two forces are of different origin and do not
constitute an interaction pair.
- Slide 2
- 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 because 1. the two forces
form an interaction pair. 2. the net force on the car is zero. 3.
neither of the above Choice 1 is not true because both forces act
on the same object!
- Slide 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.
- Slide 4
- Determine the exact amount of tension for the Horizontal Atwood
Machine. M m M = 875 g m = 200 g x y x y N WMWM WmWm T T For M: For
m: We 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!!)
- Slide 5
- A person pulls a box across the floor. Which is the correct
analysis of the situation? 1.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 persons 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.