# Rotation of a wheel

1. Mar 18, 2006

### peter86

Why does a wheel move (translate) when it rotates? I mean when you exert a force somewhere on the wheel, it will produce a torque that make it rotate. Now assuming the wheel is in contact with the ground, the wheel will then exert a force on the ground and the ground will exert a counterforce on the wheel. So why doesn't this counterforce counteract on the torque of the wheel and prevent the rotation instead of translation the wheel?

I've been wondering about this for quite some time now but I've found no explanation of it.

2. Mar 18, 2006

### HallsofIvy

Staff Emeritus
Because the force is "unbalanced". If you had an equal (and opposite) force at the top of the wheel, then you would stop the wheel without it translating. A single force acting on an object somewhere other than the center of mass gives both a turning moment and a translation. You have to have a "couple" to get only a turning moment.

3. Mar 18, 2006

### vaishakh

Do you know this? If a wheel rotates, then it is essential that static friction has to act on it. Otherwise it will start to slip or slide.

4. Mar 18, 2006

### thebigcheese

Which is called sliding friction and also explains why cars dont brake very well when they are skidding.

5. Mar 18, 2006

### Staff: Mentor

Consider a car: what is the source of the forces on the wheels? You have the engine, you have friction with the ground, you have internal friction inside the engine and drivetrain. The friction inside the drivetrain might cut 20% from your engine power, a lot going to the ground. The force of friction may include the ground pushing against the wheel and the wheel pushing against the wall, but that doesn't make the forces on the car balanced. Consider standing on a skateboard and pushing against a wall - the wall pushes back with the same force you push on the wall, and you move.

6. Mar 18, 2006

### thebigcheese

Which is called Newtons third law