 Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum                    Workshop 7: The Lure of Magnetism

Web Highlights  1. Magnetism 2. All Objects Fall at the Same Rate 3. Mass and Inertia 4. Automobile Physics

Workshop 7's Featured Classroom

Harwich Elementary School, Harwich, Massachusetts, Grade 4,
Teacher: Janet Smithers

Answer to Workshop 7's Going Further
How Many Forces Can You Find?

In example #1 ,#4 and #6 the forces are balanced. In each case the ball is either at rest or moving with a constant speed.

In example #2 and #3 where the ball is dropped or thrown upward, only gravity acts on the ball so you should have one vector, representing the ball's weight. In #2 the ball keeps speeding up until it hits the ground while in #3 it slows down, stops, and then falls to the ground just like the one that is dropped.

In #4, if frictional effects are small the ball will move at a constant speed, because there are no forces acting in its direction of motion.

#5 is a little tricky. If you thought the ball would slide you would show two unbalanced forces. The weight of the ball acting straight down and a smaller "normal " force provided by the ramp that acts perpendicular to the surface. If you thought the ball would roll you would also include a small force at the point of contact between the ball and the ramp. This force causes the ball to "topple" and roll in response to the net force acting down the ramp. Remember the net force is the sum of the weight and normal force. It is zero if the ramp is horizontal and equal to the weight if the ramp is vertical.

1. A ball sits on a table. 2. A ball is dropped. 3. A ball is thrown up in the air. 4. A ball rolls across a table. 5. A ball rolls down a ramp. 6. A ball hangs from a string. 