Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

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Science in Focus: Force and Motion
About the Workshops
1. Making an Impact
2. Drag Races
3. When Rubber Meets the Road
5. Keep on Rolling
6. Force Against Force
7. The Lure of Magnetism
Supplemental Resource List

Workshop 5 Web Highlights


When Katie places the pencil between her hands and presses them together, equal forces are placed on both her palms. If we had used an unsharpened pencil she would probably say they felt about the same. She doesn't feel the forces but rather senses the pressure on the nerve endings in her hand. The point of the pencil has an area about fifty times smaller than that of the eraser end. Since pressure is the force divided by the area over which the force acts, Katie feels fifty times the pressure and some pain on the palm that is pushed against the point of the pencil.

We have seen the effect of pressure before in the series when we dropped a raw egg on sand and hard surfaces. Besides the big difference in the time to bring the egg to rest, when the egg hit the sand, the force was spread over a larger area reducing the pressure on the shell. When we check the air pressure in our tires, we measure the pressure in pounds per square inch (psi). Scientists measure pressure in the unit Pascal (P). A Pascal is equal to a force of one Newton acting on a surface of one square meter.


"Rolling Friction"

We showed a ball rolling along the very flat surface going part way up and back down the wooden ramp and continuing back on the blue surface. The purpose of this demonstration was to show that the ball moves in a straight line unless a force acts to change its motion. It does move in a straight line before it reaches and after it leaves the ramp.

The demonstration also shows something else. There is considerably more friction on the ramp than on the flat blue surface. You can even hear the difference in the sound the ball makes as the ball rolls along. Objects will move in straight lines at constant speed unless a force acts to change their motion. The force acting down the ramp changes the direction of the ball and the frictional force slows the ball down slightly so that it moves more slowly after it leaves the ramp. This is why the spacing of our dots is smaller at the end of the ball's journey.

"Terminal Velocity"

We looked at the forces acting on the coffee filter dropped through air and the marble falling through the dishwashing liquid and saw that when the forces were balanced they fell at a constant speed. They did have to accelerate first to arrive at that final constant speed.

When the objects start to fall gravity acts on them. Frictional force of the fluid also acts on the falling objects, in the opposite direction, slowly at first but increasing with the speed of the objects until the forces are equal. The final speed that falling objects reach is called terminal velocity.

The fluid friction depends on several things including the shape of the object, its frontal area and the properties of the fluid it is moving through. If the fluid flows smoothly over the object as in the case of the marble the force is directly related to the speed but in the case of the filter or a parachute that disturb the air the force increased more quickly.




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