Name _________________________________
Sliding Friction in Real Life
Background: In almost all situations, if an object is moving, there will be a frictional force
that acts in the opposite direction of the motion. The magnitude of the frictional force is due to
several things, including:
 The material of the two surfaces in contact
 The weight of the object
 Whether the object is at rest, or already in motion.
In this activity, we are going to analyze a vehicle that hits it brakes, and begins to skid. We
will look at both the final velocity and skidding distance for vehicles of varying masses, as well
as tire and road conditions.
Calculate normal force for 3 vehicles:
We will assume that our road is flat and straight. Therefore the normal force of each vehicle
will equal the Fgravity which is the weight of the car.
𝐹𝑤𝑒𝑖𝑔ℎ𝑡 = 𝑚 𝑥 9.8
Choose your favorite make and model vechicle for each category and look up the mass in
kilograms. Record in the table below:
Category
Your choice make/model
Mass in
kg
Fnormal
Small car: sports car
or motorcycle
Medium car: (sedan)
Large: (SUV or truck)
Calculating Ffriction:
Ffriction results from both the normal force of the object, as well as the types of materials in
contact (tire and road). A value known as the coefficient of kinetic friction (k) measures the
amount of resistance between objects. The force of friction can then be calculated using the
equation:
𝐹𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 𝜇𝑘 𝐹𝑛𝑜𝑟𝑚𝑎𝑙
Below are the given k values for the combinations or tire/road surface we will analyze today.
Use these values to compete the table that follows:
Surfaces
New Tires/Dry Roads
Surfaces
k
New Tire / Dry Road
0.7
New Tire / Wet Road
0.4
Worn Tire / Dry Road
0.62
Worn Tire / Wet Road
0.37
Small Vehicle
Ffriction
Medium car
Ffriction
Truck/SUV
Ffriction
New Tires/Wet Roads
Worn Tires/Dry Roads
Worn Tires/Wet Roads
Calculating acceleration:
In our situation, we are assuming that the vehicle was moving forward and then began to skid.
Once the skidding began, the driver removed his/her foot from the gas pedal, hence there is no
applied force in the forward direction any longer. The only force that determines the net force
is the force of friction. Use Newton’s 2nd law, F = ma, to calculate the acceleration of each
vehicle on each road condition.
Surfaces
New Tires/Dry Roads
New Tires/Wet Roads
Worn Tires/Dry Roads
Worn Tires/Wet Roads
Small Vehicle
a
Medium car
a
Truck/SUV
a
Calculating skidding distance and speed:
Use the following kinematic equation to look at the different skidding distances for each
vehicle. We will also look at the increase in skidding distance when a car is going over the
speed limit.
𝑣𝑓 2 = 𝑣𝑖 2 + 2𝑎∆𝑥
1)
Assume that each vechicle was traveling at the posted highway speed limit of 105 km/hr
(equivalent to 65 mi/hr) when it went into a skid and then came to rest. Determine the
skidding distance for each vehicle:
Small vehicle:
New Tire/Dry Road:
New Tire/Wet Road:
Medium car:
New Tire / Dry Road:
Worn Tire/ Dry Road
SUV/truck:
New Tire / Wet Road:
Worn Tire / Wet Road:
2)
Assume that each vechicle was speeding at 135 km/hr (equivalent to 84 mi/hr) when it
went into a skid and then came to rest. Determine the skidding distance for each
vehicle:
Small vehicle:
New Tire/Dry Road:
New Tire/Wet Road:
Medium car:
New Tire / Dry Road:
Worn Tire/ Dry Road
SUV/truck:
New Tire / Wet Road:
Worn Tire / Wet Road: