# Frictional Force

What is the difference between tractive force and frictional force? Which one drives the vehicle? What are the directions of the forces with respect to the wheel?

#### bhargavmech Profile Answers by bhargavmech

• Aug 24th, 2009

The Tractive force is the pulling force exerted by a vehicle, or machine.

Friction is the force resisting the relative lateral (tangential) motion of solid surfaces, fluid layers, or material elements in contact.

The drive wheels of an accelerating car experience a  pointing forward

#### bhargavmech Profile Answers by bhargavmech

• Aug 24th, 2009

The Tractive force is the pulling force exerted by a vehicle or machine.

Friction is the force resisting the relative lateral (tangential) motion of solid surfaces fluid layers or material elements in contact.

The drive wheels of an accelerating car experience a pointing forward. It is not the direction of movement of the vehicle they oppose, it is the direction of (potential) sliding between tire and road.

The force to drive a vehicle is composed of the sum of (1) road resistance, (2) force necessary to climb a grade, (3) force needed to accelerate to final velocity in the allowable time, (4) force to overcome air resistance, on fast moving vehicles.

Wheel is under the influence of engine torque, brake torque, tire tractive force, wheel friction force, normal reaction force from the ground, and gravity force.

Assume that wheel is moving in the forward direction and wheel is rotating in clockwise direction.
Both tire tractive force and wheel frictional force act in the direction of motion.
Normal reaction force from the ground acts in upward direction, gravity force acts in downward direction. Engine torque acts in clockwise direction while braking torque acts in anti-clockwise direction.

#### NSK7777 Profile Answers by NSK7777 Questions by NSK7777

• Aug 25th, 2009

Now consider a drive wheel
rotating in a dry surface and other one on a icy surface.
Slip do not occur
in the first one because traction force is less than frictional force.
In the second one traction force is more than frictional force so slip occurs (since mu is less)
then how could both traction and frictional force act in a same direction?

#### bhargavmech Profile Answers by bhargavmech

• Aug 29th, 2009

Yes you are right.
But as you know that traction force is applied by the car wheels (tyres) against
the ground.
As the wheel is moving forward, the traction force is in the forward direction
and coming to the friction, It is not the direction of movement of the vehicle
they oppose it is the direction of (potential) sliding between tire and road.

So even friction acts in the forward direction.