丁香园AV

A flying car ~30 Minutes

Description: Using different types of forces to balance a car on a desk, and seeing why it rolls off

Objective: To investigate balanced and unbalanced forces.

BC Science Content:

-effects of pushes/pulls on movement (grade K)

-types of forces (grade 2)

-effects of balanced and unbalanced forces in daily physical activities (grade 6)

Materials: medium sized object with wheels(i.e. big toy car, roller skate...), string, two medium sized vessels(i.e. two small buckets, yogurt containers, Styrofoam cups...), many equal sized weights(i.e. pennies, plastic blocks, marbles...), 2 large crayola markers, tape, student's desk, scissors

Doing it:
A1. Modify the vessels so that they can be hung from the string either by punching holes and threading a loop of string though (cups), taping on a loop of sting(yogurt containers), or using the handles(buckets).

B1. The markers will act like pulleys, lessening the friction of the string on the corners of the desk. Tape them to opposite sides of the desk so that when the string is hung over them, the string will not touch the desk(Fig 1.1). Place the object on the middle of the desk and tie one vessel to each end so the cups are hanging about 6 inches over the edge.
B2. place the equivalent of about 15 pennies in each cup so the car is balanced. Ask the students if there is any movement.
B3. Take the scissors and hold them up to one of the strings. ask the students what will happen if you cut the string. Either you cut it or let a student cut it and watch the car roll off the desk. Ask what happened and why. All you did is cut a string, not pull anything. What was pulling something? Why did it only happen once you cut the string?

C. Repeat B1-B3 with three or four times as many weights in each vessel. The car will fly off the desk. Ask them what was different this time. Ask what happened and why. Why did it look the same before the cut but act differently after?

D1. Set up the car again, but do not tie the cup back onto the car. You should have a cup on one side with the large number of weights, and a string on the other side. Get a student to hold the end of the string so that the car stays still. Ask what will happen when you cut their string. Get another student to cut it. Ask what happened and why. Ask what was different between this and trial C.
D2. Set up the car the same way, with the same student holding the string. This time ask what will happen if you cut the other cup. get the student to close his eyes so they can not anticipate it. make sure they aren't pulling hard enough that when the string is cut they falls over. When the string is cut the car should roll off the side of the desk towards him. Ask what happened and why. What was the difference between this and the other trials?

Explanation: For each part, the car begins resting on the desk. There are forces acting on the car, it is just they are balanced(i.e. the same magnitude but acting in opposite directions.). Once you cut a string, the forces are not balanced anymore, and there is just one force acting on the car and that force causes it to move in that direction. Using more weight in part C makes the forces larger, but does not change the fact that they are balanced. The larger force does affect how fast the car travels once it is not balanced however. For part D the student will have to apply enough force to balance the system, therefore playing the exact same role as the cup did. Once the string is cut the car should move in the same fashion because nothing has changed. This is somewhat true for part D2 when the student is the last force to act on the car. The only difference being that the cup will pull until it hits the ground while the student will only pull until he realizes whats happening.