Why is no work done by the force of gravity when a body moves in a horizontal surface?

it does but it remains constant. only if you move in a vertical plane are you pulling or pushing against gravity. if you hold a heavy object in your hand and try move it from left to right it seems to pull your hand down doesn't it. thats because gravity always pulls but the force remains constant. You have to exert energy to hold it there equivelant to gravity. if you slide an object across a table gravity holds it to the table but the table exeerts a force to hold the object up(the tables strength) Does that make sense? sound s stupid now i read it he he he good luck

It is, all actions have an opposite and equal reaction. Gravity is trying to push the object through the surface, however the molecular force between the object and the surface is counteracting the force of gravity.

Newtons third law of motion. To every action (force applied) there is an equal and opposite reaction (or force) The gravity is exerting a force on the object, but their is an equal and opposite force pushing up from the surface of the ground onto the object, giving a net result of zero for force in the vertical plane.

Work is force times distance. The gravitational force is down. Therefore, if there is no down movement, no work is done by the gravitational force.

If i understand ur question correctly: because the force of gravity is constant and the same across the horizontal surface- there is no potential energy to convert to work. i think (-:

it is the force of gravity that keeps the body moving horizontally (unless it is trapped between two layers) - without gravity the body would be randomly placed.

But surely gravity is keeping the body on the horizonatal surface? Also the body is not moving vertically, hence why gravity does not come into the forces at work, it is kinetic energy that moves the body.

err.... it is. but the overiding force is one that sends it along a horizontal plane. I'm assuming you mean if it is on a table or something. Gravity does not have enough force to pull it through the surface so it can olny move sideways. If it runs out if surface it will fall. If it is a projected object like a trown ball or a bullet, gravity will bring it down to the nearest surface the object cannot pass through.

w(work done)=F.d where F=force ,d=displacement of body. This is not a simple product but it is scalar product(if 10 you will learn in higher classes).which means, W=Fd*cos(angle between F and d) So when body is moving in horizontal direction ,angle between gravity and dispacement is 90 degree. And,cos90=0 Hence ,W=0.