9. define normal force.\n10. describe the direction of the normal force in relation to the surface it acts…

Brief Explanations:

9. The normal force is the force exerted by a surface on an object perpendicular to the surface.
10. The normal force acts perpendicular to the surface it acts upon.
11. When an object is on an inclined plane, the normal force is less than the object's weight and is perpendicular to the inclined - plane surface. It is equal to the component of the object's weight perpendicular to the inclined plane ($N = mg\cos\theta$, where $m$ is mass, $g$ is gravitational acceleration, and $\theta$ is the angle of the incline).
12. Friction is a force that resists the relative motion or tendency of relative motion between two surfaces in contact.
13. Static friction acts when two surfaces are not moving relative to each other and opposes the initiation of motion. Kinetic friction acts when two surfaces are in relative motion and opposes the ongoing motion. Static friction has a maximum value given by $f_s=\mu_sN$ and kinetic friction is given by $f_k = \mu_kN$, where $\mu_s$ and $\mu_k$ are coefficients of static and kinetic friction respectively and $N$ is the normal force.
14. Rougher surface textures increase friction as there are more irregularities to interlock. Heavier objects result in a greater normal force, and since $f=\mu N$, a greater normal force leads to a greater frictional force (assuming the coefficient of friction remains constant).
15. Tension force is a pulling force transmitted through a rope, string, cable or similar object when it is taut.
16. When two people pull on opposite ends of a rope, tension acts along the rope, trying to stretch it, and has the same magnitude at all points in the rope (assuming the rope is mass - less).
17. When the load increases, the tension in the rope increases proportionally as the rope has to support the additional weight or resist the additional pulling force.
18. Spring force is the force exerted by a spring when it is compressed or stretched. According to Hooke's law, $F=-kx$, where $F$ is the spring force, $k$ is the spring constant, and $x$ is the displacement from the spring's equilibrium position.

Answer:

9. The force exerted by a surface on an object perpendicular to the surface.
10. Perpendicular to the surface.
11. It is less than the object's weight and is perpendicular to the inclined - plane surface, equal to $mg\cos\theta$.
12. A force that resists relative motion or tendency of relative motion between two surfaces in contact.
13. Static friction resists initiation of motion between non - moving surfaces, kinetic friction resists ongoing motion between moving surfaces.
14. Rougher textures increase friction, heavier objects (greater normal force) increase friction.
15. A pulling force transmitted through a taut rope, string, etc.
16. Tension acts along the rope, trying to stretch it, and has the same magnitude at all points (assuming mass - less rope).
17. The tension increases proportionally.
18. The force exerted by a spring when compressed or stretched, $F = - kx$.