Physics - Friction

Definitions

 

 

Ff = μ * FN

Ff is the friction pulling back on the object (force of friction)

μ is the coefficient of friction. It is either a kinetic (if it is already moving) or static (if it isn�t already moving)

Fs is static friction

Fk is kinetic friction

Ff is the Fsmax when μ is the static coefficient

FN is the Normal force (in our case it is the weight and is measured in newtons (N))

FN =Mg (mass*gravitational pull)

 

Question 1

 

A force of 10N to the right is acting on a wooden crate on a horizontal wooden floor initially moving to the right. The crate weighs 25N.

 

1. Calculate the magnitude of the force of friction on the floor.

Ff = μ * FN

FN = 25N

μ= kinetic(.30)/static(.42)     -its on the table #4

Ff = μ * FN

Ff =(.30)*(25)=7.5

Answer: Ff (or force of friction) =7.5

 

2. What is the magnitude of the net force acting on the crate?

Fy (are the forces acting on it in the y direction (y-axis)) � there is its weight of 25N going down, and the table pushing up with equal force up � so there is no net force in y direction

Fx (are the forces acting on it in the x direction (x-axis)) � there is the force of friction of 7.5N in the �x direction (-7.5) and you are exerting a force of 10N in the +x direction (+10) � the net forces in the x direction is (-7.5+10) =2.5N

Answer: 2.5 N

 

3. Is the crate accelerating?

Answer: yes because Fnet =MA and since you know there is a net force, the acceleration cant be 0 because that would make the net force 0 (Fnet =MA(0), M*0=0= Fnet)

 

 

Question 2

 

A force of 10N is exerted on a steel crate, on a steel horizontal platform, initially moving to the right. The crate weighs 40N. Calculate the magnitude of the force of friction between the crate and the floor.

Ff = μ * FN

Ff = x

FN = 40N

μ = kinetic(.57)/static(.74)

μ * FN

(40)*(.57) =22.8N=Static friction

Answer: 22.8N

 

Question 3: (pg. 107, 23 from textbook)

 

A force of 40N accelerates a 5.0-kg block at 6.0m/s2 along a horizontal surface.

  1. How large is the frictional force?

Fnet =MA

Fnet = (40-x) = MA = (5.0)*(6.0) = 30 = (40-x)

x = 10 = Ff

Answer: Ff =10N

 

  1. What is the coefficient of friction?

 

Ff = μ * FN

FN = Mg (5.0)*(9.8) =49

Ff =10N

10 = (x)*(49)

x = .204

Answer: μ=.204

 

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