# Omg Read This I Dun Understand Help

1. Sep 28, 2004

### Faiza

Omg Read This I Dun Understand!! Help

A man pushes a heavy load across a horizontal floor. What is the work done by gravity on the load?
a. The work depends on the weight of the load.
b. The work done by gravity is negative.
c. The work done by gravity is zero.
d. The work done by gravity is positive.

2. Sep 28, 2004

### Pyrrhus

Remember Work is the scalar product of displacement and Force, so what's the angle between the displacement and the Force of Gravity (weight)?

Last edited: Sep 28, 2004
3. Sep 28, 2004

### Faiza

no i dun think so

4. Sep 28, 2004

### Faiza

the work done by a constant force F on a particle is defined as the product of the component of the force in the direction of the particles displacement and the magnitude of the displacement. If F makes an angle theta with the displacement then the work done by f is W=Fdcos thetha
if thetha is 90 degress its zero?

5. Sep 28, 2004

### Faiza

i checked the answer and its zero

6. Sep 28, 2004

### Faiza

:{ lol i dun get it

7. Sep 28, 2004

### Pyrrhus

what you don't think?

$$\frac{dW}{d \vec{x}} = \vec{F}$$

$$dW = \vec{F} \cdot d \vec{x}$$

Scalar Product or Point Product

$$\vec{A} \cdot \vec{B} = | \vec{A}||\vec{B}|cos\theta$$

where $$\theta$$ is the angle between the two vectors.

8. Sep 28, 2004

### Pyrrhus

What's the cosine of 90 degrees?

9. Sep 28, 2004

### Faiza

Cos(90)=0??????????

10. Sep 28, 2004

### Pyrrhus

Yes, so the work done by Gravity is???

11. Sep 28, 2004

### Faiza

Zero................... Lol