Chapter+2

=Section 7 - Frictional Forces=

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What Do You See?
In one picture a girl is pulling a shoe over ice very easily using very little force. In the picture the same girl is trying to pull a shoe over a rough sandy surface and is having a really hard time even though she's using a lot more force.

What Do You Think?
Different sports take place on different materials and could require to be made of certain materials. These features would help by increasing and decreasing friction for specialized purposes in the sports.

Read and Summarize Physics Talk
Newton's second law states that constant velocity only occurs when acceleration is 0. For acceleration to be 0 net forces must balance out to equal 0. The force of friction acting on the shoe was equal to the tension force from you pulling on it. Since they were equal this made the net forces equal to zero. This same thing applies to the net vertical forces of the shoe. The Normal force of the floor acting on the shoe is the same as the weight of the shoe pushing down on the floor. Once again the forces balance out since they are equal but point in opposite directions. The coefficient of sliding friction is a defined ratio: the force required to slide an object on a surface with constant speed divided by the perpendicular force exerted by the surface on the object, AKA the normal force. Since it is a force divided by a force, it has no units.

=Section 8=

Physics To Go
1.) The person rotates their body and winds up their shoulder to build up potential energy before throwing the ball. There is no work being done as the person rotates before throwing the ball.When they do throw it they are transferring their energy into the ball giving the ball kinetic energy and getting it moving.

2.) A person first winds up the golf club to build up potential energy and then releases it with the swing, transferring it into the ball giving the golf ball kinetic energy. There is no work being done as the person rotates before swinging the club.

3.) 1/2mv^2 = mgh 1/2(12^2) = 9.8h 72/9.8 = h 7.3 meters = height

4.) Because it is merely a tool used to propel himself forward into the air. He isn't using the stick as a ladder of any kind. He is just using the elasticity of it to transfer his energy into it so he can be propelled upward.

5.) If someone is running fast enough they might be able to break or melt the pole when they take their vault because too much energy will be transferred into the pole.

6.) 1/2mv^2 = mgh (v^2) = 2(9.8)(4.55) v = sqrt(89.18) v = 9.44 m/s

7.) 1/2mv^2 = mgh (v^2) = 2(9.8)(6.14) v = sqrt(120.34) v = 10.97 m/s He was running 1.53 m/s faster than her.

8a.) 1/2mv^2 = mgh (v^2) = 2(9.8)(100) v = sqrt(1,860) v = 43.13 m/s

8b.) Yes. All objects fall at the same speeds when friction is ignored.

What Do You Think Now?
The length of the pole is a very minor factor when it comes to contributing to the maximum height attainable from vaulting. The major factor is speed which is not as easy for a human to increase on their own. They would need to run much faster in order to gain more height. The vaulters transfer their kinetic energy from running into work done on the pole which is then transferred back to the vaulter as kinetic energy that sends them flying into the air.

=Section 9=

What Do You Think?
It is not possible for anyone or anything to defy gravity and remain in the air longer than something else. Gravity is a constant force and affects all things at a continuous rate.

What Do You Think Now?
No, nobody is except to the rule of gravity. It affects everyone. What is really going on during "hang time" is just an illusion, they are still moving both vertically and horizontally. The velocity near max height is simply very low and gives the appearance of hanging.