So i have this line of code:

if(input.isKeyDown(Input.KEY_W)&&canJump())
    {
        body.applyLinearImpulse(new Vec2(0, 30), cam.screenToWorld(body.getPosition()));
        System.out.println("I can jump!");
    }

My problem is that the console display I can jump! but the body doesn't do that. Can you explain to me if i do something wrong?

Some more code. This function creates my 'hero' the one supposed to jump.

private Body setDynamic(float width, float height, float x, float y)
{
    PolygonShape shape = new PolygonShape();
    shape.setAsBox(width/2, height/2);

    BodyDef bd = new BodyDef();
    bd.allowSleep = true;
    bd.position = new Vec2(cam.screenToWorld(new Vec2(x + width / 2, y + height / 2)));
    bd.type = BodyType.DYNAMIC;
    bd.userData = new BodyInfo(width, height);

    Body body = world.createBody(bd);
    body.createFixture(shape, 10);

    return body;
}

And this is the main update loop:

if(input.isKeyDown(Input.KEY_A))
    {
        body.setLinearVelocity(new Vec2(-10*delta, body.getLinearVelocity().y));
    }
    else if (input.isKeyDown(Input.KEY_D))
    {
        body.setLinearVelocity(new Vec2(10*delta, body.getLinearVelocity().y));
    }
    else
    {
        body.setLinearVelocity(new Vec2(0, body.getLinearVelocity().y));
    }

    if(input.isKeyDown(Input.KEY_W)&&canJump())
    {
        body.applyLinearImpulse(new Vec2(0, 30), body.getPosition());
        System.out.println("I can jump!");
    }
    world.step(delta * 0.001f, 10, 5);

}