Friction

Davidson Students Volunteer for ScienceTraining Presentation

Spring, 20117th Grade

Definition of Friction

• Friction- Friction is a force that opposes the motion of an object. It always acts in the opposite direction to the attempted motion. Its size depends on how rough the surface is and on the weight pushing down on the surface.

Experimenting With Friction• Address the following problem to the class:

– When we lift an object, we have to overcome the force of gravity. Unless we can pull upwards with a larger force than that of gravity, we will not be able to lift that object. When we pull an object across a table top, we have to overcome the force of friction. Unless we can pull sideways with a force larger than that of friction, we will not be able to move that object.

– How can we measure friction? – On what factors does friction depend on?– How can friction be reduced?

• Attach the large, wooden block to the spring balance.

• Slowly pull on the block and measure how hard you can pull without the block moving (repeat this 2 or 3 times to ensure an accurate reading of the spring balance). Write this value on the board.

• Ask the class the following questions:– Why did the block not move as soon as any force was applied to it?– What prevented the block from moving, although it was being pulled?

– Answer: The force of friction prevented the block from moving. The force of friction is equal to the largest force that can be applied to the block before it begins to move, but it acts in the opposite direction.

Measuring Friction

Measuring Friction

• Refer back to the force value you wrote on the board and ask the following questions:– If the force exerted on the block equaled: (place

your value here), what did the force of friction equal and in what direction was it exerted?

– What factors affect the size of the force of friction?

Effect Of Weight On Force Of Friction

• Draw the following chart on the board:

• Place the large,wooden block on the surface board:

Place Here (smooth surface)

Effect Of Weight On Force Of Friction

• Add two of the lead weights to the surface board:

• Pull with as much force on the spring balance without moving the wood. (repeat 3 times and average results).

• Record each trial and the average of the three in the chart on the board.

• Repeat the previous procedure with 4,6, and 8 weights on the wooden block.

Effect Of Weight On Force Of Friction

• After recording the data in the chart, have the class determine the relationship between the number of weights added to the block and the size of the force of friction.

• The results should show that as the weight increased, the force of friction increased.

Effect Of Different Surfaces On Force Of Friction

• Ask the class what type of surface on the surface board will provide the greatest force of friction: cardboard, cork, sandpaper, or rubber.

• Draw the following chart on the board:

Effect Of Different Surfaces On Force Of Friction

• Place the large wooden block with eight weights on the cardboard surface of the board:

• Pull on the spring balance as hard as possible without moving the board and record the force of friction in the chart (repeat 3 times).

• Average the three trials.• Repeat process on each surface on the surface board.• Be sure not to change the weights on the board during this

procedure!

8 lead weights

Effect Of Different Surfaces On Force Of Friction

• After recording all the data in the chart, have the class determine the characteristics of a surface that will provide a low force of friction.

Forces That Turn A Beam• Consider a beam that is able to turn about a point called its fulcrum. Force F1

will attempt to turn the beam downwards to the left, in a counter-clockwise direction. Force F2 will attempt to turn the beam downwards to the right, in a clockwise direction. – When will the forces balance each other?– Feel free to draw this diagram on the board to illustrate the concept above.

– See next slide for apparatus assembly

F2F1

Apparatus Assembly• This assembly requires two screws, a screwdriver, a large, wooden block

with 2 holes, and a smaller wooden block with a fulcrum.

Partially insert screws into the two large holes

Sit the vertical piece on top of the large wooden block and finish screwing in the screws (there are 2 holes in the bottom of vertical piece).

Finished assembly

Fulcrum

Forces That Turn A Beam

• Place two weights on a paper clip hook in the sixth hole on the left hand side of the ruler.

• Place a second hook in the fourth hole on the right hand side of the ruler.

• Add weight to the right hand side of the hook until the ruler balances.

• See diagram below for numbering of holes on the ruler.

Forces That Turn A Beam

• Draw the following chart on the board:

• The diagram tells you where to hang the weights and the number of weights to hang on the left side.

• Fill in the blanks for the right side by experimenting at different positions or with different weights.

Forces That Turn A Beam• Answers to the previous chart:

• Ask the following question:– Did you need the same number of weights on each side of the

beam (ruler) in order to obtain a balance?• No.

• Have the class try to formulate a rule that will tell you whether the beam will balance or not.– Formula: number of weights x position of hook = number of

weights x position of hook

Conclusion

• Recap force and friction• Discuss the importance of friction• Discuss possible ways to reduce friction based

off of the results in the second part of this lab• Discuss different ways of applying force to an

object• Answer any questions the kids may have or

discuss other things that you feel are relevant to this lab