Workshop 7: The Lure of Magnetism
Web Highlights
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. 
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