As a general matter, I would say that money can and does motivate people to work hard. But there are limits to this, and there are so many other important forms of motivation.
In today's world, we all have needs and desires that can be satisfied only with money. A roof over our heads, food on our tables, heat, electricity, and new shoes or a movie must all be paid for. To the degree that more money will help us to meet those needs and desires, most of us are likely to be willing to work harder. And certainly, few of us are willing to work harder without being compensated with more money. Few people are going to volunteer to work overtime for free or agree to produce 25% more in sales without a commission.
On the other hand, there is, I think, a point of diminishing returns. For example, if I work too many hours and cannot sleep enough or be with my family, much less enjoy some leisure activity, offering me even more money is not going to motivate me very well. Similarly, in some jobs, people have physical limits that they will hit, and all the money in the world is not going to persuade them to push beyond their own endurance or strength. No matter how much money you offer me, I can only dig so many ditches in a day, and I have no interest in killing myself or putting myself in the hospital trying to work that much harder.
It is also important to realize that money is not the sole motivation for workers. All of us are highly motivated by recognition, appreciation, a pleasant workplace, and challenging work. Most of us will work harder if these are provided than if they are not, and employers who are capable only of throwing money at their employees, without providing these other motivations, are missing out on the hard and good work they could be getting from their employees.