# Torque- Vector cross product using both geometric and algebraic methods

1. Nov 30, 2012

### garyd

1. The problem statement, all variables and given/known data
A lever is orientated along the y direction in a Cartesian coordinate system. The length of the lever is 0.5m and one end of it is at the origin of the coordinate system. A (3i-5j)N force applied to the other end of the lever. Calculate the Torque produced by the force acting on the lever about the origin. Do calculation twice, firstly using the geometric definition of the cross product and secondly using the algebraic definition of a cross product

L=(0i+0.5j+0k)m
F=(3i-5j+0k)N

2. Relevant equations

T=LFsin(theta)

T=L × F

3. The attempt at a solution

theta= 90+59= -149°
mag F= √(3^2+-5^2)= 5.83N
mag L= .5m
LFsin(theta)= .5*5.83*sin(-149)=T=-1.5Nm

&

(0*-5) - (.5*3) =T=-1.5k Nm

My lecturer has a solution of -2.5Nm! Help appreciated
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 30, 2012

### Dick

3. Nov 30, 2012

### garyd

I think my lecturer must be putting the lever in the 'i' direction and using theta=-59

4. Nov 30, 2012

### Dick

That's a good guess. There is some kind of typo.

5. Nov 30, 2012