Assignment 25 query

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course Phy 121

12/15 2 pm

025. `query 25

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Question: `qprinciples of physics and gen phy 4.26 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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Your solution:

Would there be a force of momentum causing the ball to fly backwards and the bat to fly backwards to its own path. I suppose there was a gravitational force present as well.

I believe this is the only question of the assignment that Phy 121 students are required to do.

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Given Solution:

`a** Gravity exerts a downward force equal to the weight of the ball.

While in contact with the ball, and only while i contact, 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. **

COMMON STUDENT ERROR: Confusing motion in a direction with force in that direction.

There is no force associated with the motion of the ball. The velocity of the ball in will remain unchanged if there is no net force on the ball. Furthermore, if the is net force has zero component in the x direction, the x velocity remains unchanged; the analogous statement holds for the y direction.

STUDENT QUESTION

I got confused about the motion in the direction with the force in that direction. I think I understand.

INSTRUCTOR RESPONSE

The force tells you the direction of the acceleration, not the direction of the velocity. From the force you can therefore tell the direction of the change in velocity, not the direction of the velocity itself.

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