Casper drags a packing crate at a steady slide over a wooden floor. The floor presses on the crate with a normal force of 200 N, and the surface pair has μ_k = 0.30.
How large is the kinetic friction force on the crate?
Well, you already have the equation. The kinetic friction law and the static maximum share one shape: a coefficient multiplied by the normal force.
f = μN
The coefficient μ has no unit, so μN comes out in newtons, the unit of the normal force itself.
In f = μN, the symbol N is the normal force. The unit newton is also written N, but the unit sign always follows a number.
The case tells you which coefficient to use.
If the surfaces are already sliding over each other, the kinetic coefficient applies: f_k = μ_k N.
If the object is at rest and on the verge of sliding, friction sits at its maximum, so the static coefficient applies: f_s,max = μ_s N.
Read the case, pick the coefficient, then multiply.
Casper's crate is sliding, so f_k = 0.30 × 200 = 60. The kinetic friction force is 60 N.
This lesson is practice. Given the coefficients and the normal force, you read the case, pick the coefficient, and multiply, quickly and the same way every time.
A sled slides over wet grass. μ_k = 0.20 and the normal force is 350 N. Calculate the kinetic friction force.
Write down the values in the question:
μ_k = 0.20
N = 350 N
f_k = ?
Write down the equation:
f_k = μ_k N
Substitute in the values:
f_k = 0.20 × 350
Simplify:
f_k = 70 N
The kinetic friction force on the sled is 70 N.
Movers lean on a bookcase, and it sits on the verge of sliding. μ_s = 0.50 and the normal force is 190 N. Calculate the friction force on the bookcase.
Write down the values in the question:
μ_s = 0.50
N = 190 N
f_s,max = ?
The bookcase is at rest and on the verge of sliding, so friction is at its maximum.
Write down the equation:
f_s,max = μ_s N
Substitute in the values:
f_s,max = 0.50 × 190
Simplify:
f_s,max = 95 N
The friction force on the bookcase is 95 N.
A sled slides over packed snow. μ_k = 0.30 and N = 150 N. Enter the kinetic friction force in N.
A dresser sits at rest, on the verge of sliding. μ_s = 0.60 and N = 220 N. Enter the friction force in N.
A toolbox slides over a steel bench. μ_k = 0.25 and N = 320 N. Enter the kinetic friction force in N.
A crate sits at rest, on the verge of sliding. μ_s = 0.40, μ_k = 0.30, and N = 130 N. Enter the friction force in N.
A box is already sliding over a floor. μ_s = 0.45, μ_k = 0.35, and N = 240 N. Enter the friction force in N.
A trunk sits at rest, on the verge of sliding. μ_s = 0.45, μ_k = 0.38, and N = 400 N. Enter the friction force in N.
A filing cabinet slides over an office floor. μ_s = 0.30, μ_k = 0.22, and N = 300 N. Enter the friction force on the cabinet in N.
Movers lean on a vending machine, and it sits at rest on the verge of sliding. μ_s = 0.55, μ_k = 0.42, and N = 160 N. Enter the friction force on the machine in N.
A shipping box slides over a warehouse floor. The surface pair has μ_k = 0.18, and the normal force on the box is 450 N. Enter the kinetic friction force in N.