1. Isolate Body
Draw a FBD for a wheelbarrow full of dirt.
The wheelbarrow is three dimensional. For simplicity we will treat the problem as two dimensional by combining the handles together.
Now isolate the body (where body = wheelbarrow + dirt). It is best to draw a silhouette (only showing the outline). This helps to avoid confusion with internal components that should have been included (combined) with the body.
2. Find Force locations
In statics questions like this, look for contact points where forces are applied, and the centre of gravity of the BODY. Both the hand force and Ground force cross the boundary of the body.
3. Line of Action of Forces.
"Wheels: Perpendicular to surface and acting through centre of axle." Since ground is flat, the force must be vertical.Gravity: Always vertical
Hand: Mostly vertical - especially if the ground is flat and we are standing still without friction.
4. Direction of the Forces
Gravity: Gravity is always down. What does GRAVITY do to the body.? It pulls down.Ground: What does the ground do to the body/ It pushes up.
Hand. What does the hand do to the body? It pulls up.
(At this stage there is no difference between pushing or pulling. All the forces below are drawn as if the are pushing because it is clear on the diagram. We will be important later when we go to add the forces up.)
Worked Example 2: The Handle of the Wheelbarrow
1. Isolate Body
2. Find Force locations
Watch out for the boundary here, the force must cross the HANDLE boundary, or outline.
3,4. Line of Action of Forces + Direction of the Forces
Wheel Axle: Vertical, applied through centre of axle. Since ground is flat, the force must be vertical.Handle Gravity: Always vertically down
Load Force: Applied along the whole length of the tray, but take an average force through the centre of mass. It is a gravity force so vertically down (as always).
Hand: Vertical by default (no friction or acceleration) What does the hand do to the body (Handle)? It pulls up.
Free Body Diagram for Handle
