Download Notes on Friction and Normal Force

III
Everyday forces
A. Weight
1. The weight of an object is dependant of the gravitational field.
2. Weight = mass*gravity (on Earth g = 9.81 m/s2)
a. Example – the Moon has 1/6 the gravity of Earth, calculate the weight of
a 60 kg student on the moon. {98.1 kg or 215.8 lbs}
b. Ex: Fnet = 100 N, two boxes a)20 N and b) 20 kg find a for each
i. A) 49 m/s2
B) 5 m/s2
B. Normal Force
1. The force perpendicular to the surface that opposes gravity.
2. On a flat surface the normal force is equal in magnitude but opposite in direction
of the force due to gravity.
a. find Fn for a 60 kg person (588 N) with a 5 kg fish on them (637 N)
b. find Fn for a 20 N box with a rope pulling up at 5 N (15 N)
3. On an incline the Normal force is a component of gravity. (see drawings)
C. Friction – A force due to irregularities of surfaces (forces change with surfaces)
1. Two Types of Friction
a. Static Friction – the force necessary to cause motion of an object at rest.
Variable force (show examples) has a maximum value
b. Kinetic Friction – the force necessary to keep an object moving at
constant velocity. Fk does not change on a horizontal surface.
2. When at rest with no tendency to move  no friction (no force no friction)
3. When in motion, objects have kinetic friction
4. Forces due to friction are independent of surface area or size or speed.
D. Coefficients of Friction depend on
1. The weight of the object acting perpendicular to the surface (Fn)
2. The two materials in contact (coefficient of friction)
3. Coefficient’s are determined experimentally
4. Ff = Fn 
E. Practice Problems with Friction
1. A 25 kg chair initially at rest on a horizontal floor requires a 365 N horizontal
force to set it in motion. Once the chair is in motion, a 327 N force is required to
keep it moving at constant velocity.
Calculate the coefficient of static friction between the chair and the floor. 1.5
Calculate the coefficient of kinetic friction between the chair and the floor.
1.3
2. What force is required to start moving a 15 N steel box across a steel floor?
11.1N
#6) – A 75 kg box is pulled with a 50 N force directed 30 deg above horizontal. The box
moves at a constant speed. What is the coefficient of kinetic friction of these surfaces.
#7.) A 100 kg box is pushed with a horizontal force of 50 N east on a surface with a
coefficient of kinetic friction = 0.35. What is the acceleration of the box?
F. Inclines
1. Break down the weight (Fg) into x and y components.
2. (sin of angle)(weight) = force in x direction down the incline (equal to force of friction if
in equilibrium/moving at constant velocity
3. (cos of angle)(weight) = normal force and the component of weight in Y direction
4. Force due to Friction always opposes the motion of the object on the plane.
5. Use F=ma and Ff=Fnµ and sometimes your kinematics equations to solve
Ex. 1
1. A 10 kg box slides down a 40 degree incline and accelerates at 2.5 m/s2. What is the coeffiecnt of
kinetic friction on the incline?
2. A 50 kg box is pushed up a 30 degree incline with a force of 100 N directed parallel to the inclines
surface. The coefficient of kinetic friction between the box and the ramp is 0.5. If the box starts
from rest , how fast will it be moving when it reaches the top of a 10 m long incline?
3.
The coefficient of static friction between the 3 kg crate and the 35 degree incline show is 0.3.
What is the magnitude of the minimum force, F, that must be applied to the crate perpendicularly
to the incline to prevent the crate from sliding down the incline?
F