course Phy 121
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22:36:35 gen phy 4.21 free-body diagram of baseball at moment hit, flying toward outfield gen phy list the forces on the ball while in contact with the bat, and describe the directions of these forces
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RESPONSE --> My diagram show the Force from the velocity of the ball moving toward the negative x axis. The Force of the bat acting up the ball toward the postive x axis, and gravity acting upon the ball towards the negative y axis
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22:37:01 ** Gravity exerts a downward force equal to the weight of the ball. The bat exerts a normal force, which pushes outward along a line originating from the central axis of the bat. This force is perpendicular to the surface of the bat at the point of contact. Unless the direction of the ball is directly toward the center of the bat, which will not be the case if the ball is hit at an upward angle by a nearly level swing, there will also be a frictional force between bat and ball. This frictional force will be parallel to the surface of the bat and will act on the ball in the 'forward' direction. COMMON STUDENT ERROR: The gravitational force and the force exerted by the ball on the bat are equal and opposite. The force of the bat on the ball and the gravitational force are not equal and opposite, since this is not an equilibrium situation--the ball is definitely being accelerated by the net force, so the net force is not zero. **
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RESPONSE --> ok
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22:38:53 gen phy list the forces on the ball while flying toward the outfield, and describe the directions of these forces
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RESPONSE --> I have two foces shown; force towards in a motion in line with the point of contact from the bat; which is the velocity creating force from the bat and gravity acting downward on the ball.
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22:39:50 ** after impact the forces are gravity, which is constant and in the y direction, and air resistance. The direction of the force of air resistance is opposite to the direction of motion. The direction of motion is of course constantly changing, and the magnitude of the force of air resistance depends on the speed of the ball with respect to the air, which is also changing. **
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RESPONSE --> I did not take into account air resistance in either case... but I guess I should have since it is there :-)
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22:45:25 gen phy give the source of each force you have described
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RESPONSE --> The force of the ball heading toward home plate the force is created by the pitcher placing the ball in motion with some velocity; Air resistence in the opposite direction of the motion and gravity acting downward on the ball as it reaches home plate. At home plate the batter placed a force on the ball by hitting with a greater force than created by the pitcher and sets the ball off in the direction as hit by the bat surface. (gravity is of course constance in acting up on the ball). Heading to the outfield the ball has a force from the bat that is negated by two other forces which are a downward force from gravity and a force from air resistance in the opposite direction.
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22:46:02 ** The gravitational force is the result of the gravitational attraction between the ball and the Earth. The normal force is the result of the elastic compression of bat and ball. The frictional force is due to a variety of phenomena related to the tendency of the surfaces to interlock (electromagnetic forces are involved) and to encounter small 'bumps' in the surfaces. **
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RESPONSE --> ok
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22:47:12 gen phy what is the direction of the net force on the ball while in contact with the bat?
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RESPONSE --> While in contact with the bat the direction of the net force will be towards the bat
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22:48:45 ** The normal force will be the greatest force, much greater than friction or gravity. The frictional force will likely also exceed the gravitational force. The y component of the normal force will overwhelm the y components of the frictional force and the gravitational force, both of which are downward, giving us a net y component slightly less than the y component of the normal force. The x component of the normal force will be reinforced by the x component of the frictional force, making the x component of the net force a bit greater than the x component of the normal force. This will result in a net force that 'tilts' forward and slightly down from the normal force. Note that the frictional force will tend to 'spin' the baseball but won't contribute much to the translational acceleration of the ball. This part is a topic for another chapter. **
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RESPONSE --> I guess I am think in terms of contact with the bat before the swing.... but ""in contact with the bat"" would be the follow through for the bat swing
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22:52:30 gen phy what is the net force on the ball while flying toward the outfield?
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RESPONSE --> The ball will have a gravitation force and a force created by the bat swing and air resistance. The force created by the bat swing will be much greater than the air resistance thus will only slow the velocity of the ball as it travels. gravity will act up on the object until it rests into the outfielders hand (hopefully the outfielder is earning his money out there) So the net forces acting on the ball will cause the ball to have forces pulling it toward the outfield and in a downward motion.
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22:53:50 ** The net force will consist of the downward gravitational force and the force of air resistance opposing the motion. If the ball is rising the y component of the air resistance will be in the downward direction, reinforcing the gravitational force and giving a net downward y component slightly exceeding that of gravity. If the ball is falling the y component will be in the upward direction, opposing the gravitational force and giving a net downward y component slightly less than that of gravity. In either case the x component will be in the direction opposite to the motion of the ball, so the net force will be directed mostly downward but also a bit 'backward'. The are also unbalanced air pressure forces related to the spinning of the ball but these won't be considered here. **
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RESPONSE --> I did not discuss the angle of the ball off the bat...depending on the angle the ball and the peak of the projectile motion.. But I think I covered the basics of the air resistance and gravity. :-)
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22:53:56 Univ. 5.88 (5.84 10th edition). Elevator accel upward 1.90 m/s^2; 28 kg box; coeff kin frict 0.32. How much force to push at const speed?
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RESPONSE --> Not Required
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22:53:59 STUDENT SOLUTION AND INSTRUCTOR COMMENT: The magnitude of kinetic friction force is fk = mu-sub k * N. First we add the 1.9 to 9.8 and get 11.7 as the acceleration and times that by the 28 kg and get 327.6 as the force so plugging in we get fk = 0.32 * 327.6 = 104.8 N. ** Good. The net force Fnet on the box is Fnet = m a = 1.90 m/s^2 * 28 kg. The net force is equal to the sum of the forces acting on the box, which include the weight mg acting downward and the force of the floor on the box acting upward. So we have Fnet = Ffloor - m g = m a. Thus Ffloor = m g + m a = 28 kg * 9.8 m/s^2 + 28 kg * 1.90 m/s^2 = 28 kg * 11.7 m/s^2 = 330 N, approx. Being pushed at constant speed the frictional force is f = `mu * N, where N is the normal force between the box and the floor. So we have f = .32 * 330 Newtons = 100 N, approx. **
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RESPONSE --> ok
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