Force Diagrams

Remember the toppling towers? Why do the blocks stay still but plastic cups fall over? See if you can explain it using the words mass and inertia. Turn and talk to the others on the table.

Remember the Marble Ramps? Why do more marbles push the cup farther? See if you can explain it using the words mass and inertia. Turn and talk to the others on the table.

Remember the Marble Ramps? Why doesn’t the speed change? See if you can explain it using the words force and gravity. Turn and talk to the others on the table.

Remember the Rocket balloons? What is the pattern between the amount of air and the travel distance? What caused the forward motion? See if you can explain it using forces. Turn and talk to the others on the table.

In all of these examples, motion was caused by UNBALANCED forces on the SYSTEM.

What’s this about unbalanced forces? If forces are exactly balanced, things will stay still = no motion. If forces are unbalanced, things will move.

Force Diagrams – Free Body Diagrams When we draw the forces acting on a single object, we call it a “Free Body Diagram or Force Diagram ” A Free Body Diagram of an object only shows the forces acting on that object. Free Body Diagrams do not include the forces that the object itself applies on other things. Ask yourself, ‘If I were this object, which forces would I feel acting on me?

You try it. Make this simple sketch, then add two force arrows: The size of the force corresponds to the length of the arrow. What would it mean if the arrow pointing up was longer? What would it mean if the arrow pointing down was longer?

You try it. Make this simple sketch, then add two force arrows: What would it mean if the arrow pointing up was longer? There would be more air resistance than gravity, and the parachute would go up. What would it mean if the arrow pointing down was longer? There would be more gravitational pull than air resistance, and the girl would go down.

What if the arrows were the same? If the arrows are the same length, it means that the forces are equal, but in opposite directions. (Write this down) This means that NO MOTION would happen, and the parachute would just be suspended in air.

You try it. Make this simple sketch, then add two force arrows: The force of gravity always pulls straight down. The “normal” force is the upward force applied by a surface and it is always perpendicular to the surface. If something is standing still, gravity and normal force are equal, but in the opposite direction.

You try it. Make this simple sketch, then add two force arrows: Will this refrigerator move up or down? Will this refrigerator move side- to-side? Which force arrow represents gravity? Which force arrow represents the normal force?

Things only change their motion if an UNBALANCED force acts on them.

Now you try it… What is the “net” force? Will this object move? How do you know? If so, what direction will it move? Draw this in your notes.

Now you try it… What is the “net” force? Will this object move? How do you know? If so, what direction will it move? Draw this in your notes.

Now you try it… What is the “net” force? Will this object move? How do you know? If so, what direction will it move? Draw this in your notes.

Let’s diagram the toppling towers BEFORE a card was pulled out… Are the forces balanced? How do you know? Draw this and add force arrows.

Let’s diagram the marble ramps when the marble was half-way down… Are the forces balanced? How do you know? Draw this in your notes, then insert 3 force arrows.

Let’s diagram the balloon rockets when the balloon was about 0 Let’s diagram the balloon rockets when the balloon was about 0.5 meters from the start… Are the forces balanced? How do you know? Draw this and insert 4 force arrows.