Studying Force of Friction

A1 Studying force of friction

Objective

To investigate the effect on friction of the following factors: 1. normal force that presses the two surfaces together 2. materials that the two surfaces are made of 3. area in contact of the two surfaces

Apparatus

Rectangular wooden block
Wooden plank
Glass plate
Plastic plate
String
Scissors
Jack
Spring balance
Electronic balance
Half-meter rule
Retort stand and clamp
G-clamp
Trolley[pic]2
Standard weights (100g)[pic]5

Theory

Frictional forces act along the common surfaces between two bodies in contact so as to resist the relative motion of the two bodies. The frictions involved form an action-reaction pair.

A horizontal force F is applied to the wooden plank. As the plank is at rest, the friction force[pic] is equal to F and directs to the opposite side to resist the motion. The friction is static as there is no motion.

If F is increased, [pic]will also increase until it reaches the limiting static friction[pic]. Once the body starts to move, the frictional force would fall to a smaller value[pic], which remains constant even the applied force F is increased further.

In this experiment, the effects of the normal force, the materials of the surfaces and the contacting areas on [pic] and [pic] are to be investigated.

Procedure

1. The mass and the areas of the three faces of the rectangular wooden block were measured by using the electronic balance and half-meter rule respectively.

Effect of the normal force on friction

2. The apparatus was set up as shown below.

3. The wooden plank (together with the two trolleys underneath) was slowly pushed away from the spring balance. When the block started to slide, the spring balance was read quickly. The maximum value was the limiting static friction[pic]. The wooden plank was keep pushing to maintain a steady sliding and then the spring balance that indicated the kinetic friction[pic] was read again. 4. The measurement of [pic] and [pic] was repeated two more times to get the average values. 5. A 100g standard weight was added on the wooden block each times to increase the normal force. Then the measurement was repeated.

Effect of materials on friction

6. The wooden plank was replaced with the glass plate. The same face of the wooden block and two 100g standard weights were used. The measurement was repeated two more times. 7. The glass plate was replaced with the plastic plate and the measurement was repeated two more times.

Effect of area on friction

8. Different faces of the wooden block were used. The wooden plank and two 100g standard weights were used.

Data and Data Analysis

Wooden block:
Mass = 359.2g [pic] 0.05g
Area of face 1 = 20 cm[pic]7 cm = 140 [pic]
Area of face 2 = 7 cm[pic]3.6 cm = 25.2 [pic]
Area of face 3 = 20 cm[pic]3.6 cm = 72 [pic]

Effect of the normal force on friction

Surfaces in contact: wood on wood
Area of the surface in contact A = 140 [pic]
| |Normal force|Limiting static friction [pic]/N |Kinetic friction [pic]/N |
| |R/N | | |
| | |Trial 1 |
| |Trial 1 |Trial 2 |Trial 3 |Mean |Trial 1 |Trial 2 |Trial 3 |Mean |
|Wood on wood |2.2 |2.3 |2.4 |2.3 |1.1 |1.1 |1.2 |1.1333 |
|Wood on glass |3.3 |3.5 |3.4 |3.4 |2.2 |2.1 |1.9 |2.0667 |
|Wood on plastic |2.8 |3.2 |2.9 |2.9667 |2.1 |2.1 |2.3 |2.1667 |

By [pic], we have
[pic]=[pic] and [pic]=[pic] ➢ For wood on wood:
[pic]=[pic]=0.412 and [pic]=[pic]=0.203 ➢ For wood on glass:
[pic]=[pic]=0.610 and [pic]=[pic]=0.371 ➢ For wood and plastic:
[pic]=[pic]=0.532 and [pic]=[pic]=0.389

➢ Order of the friction: wood on glass[pic] wood and plastic[pic] wood on wood ➢ Force of friction varies with different surfaces in contact if all other factors are kept constant.

Effect of area on friction

Surfaces in contact: wood on wood
Normal force R =
|Area in contact A/[pic] |Limiting static friction [pic]/N |Kinetic friction [pic]/N |
| |Trial 1 |Trial 2 |Trial 3 |Mean |Trial 1 |Trial 2 |Trial 3 |Mean |
|140 |2.2 |2.3 |2.4 |2.3 |1.1 |1.1 |1.2 |1.1333 |
|72 |1.9 |2.1 |2.1 |2.0333 |1.2 |1.2 |1.2 |1.2 |
|25.2 |1.9 |2.0 |2.0 |1.9667 |1.0 |1.0 |1.1 |1.0333 |

By [pic], we have
[pic]=[pic] and [pic]=[pic] ➢ For area in contact = 140[pic]
[pic]=[pic]=0.412 and [pic]=[pic]=0.203 ➢ For area in contact = 72[pic]
[pic]=[pic]=0.365 and [pic]=[pic]=0.215 ➢ For area in contact = 25.2[pic]:
[pic]=[pic]=0.353 and [pic]=[pic]=0.185

➢ Both [pic] and [pic] for the three faces of the wooden block are similar. ➢ The area of contact of sliding surface does not influence the force of friction if all other factors are kept constant.

Errors

| | |Systematic Error |Random Error |
|1 |The applied forces on the wooden plank may not be constant. | |√ |
|2 |Human error in reading the scale of the spring balance |√ | |
|3 |The string is not horizontal under tension. | |√ |
|4 |The wooden block and the plank are not completely in contact. | |√ |
|5 |The surfaces of the wooden, plastic and glass planks may not be | |√ |
| |perfectly even. | | |

Discussion

Advantage of pulling the wooden plank, instead of pulling the wooden block with the spring balance.
If we pull the wooden block with the spring balance, the applied force F will be measured. However, we can only read [pic] when the block starts to move. As [pic] is independent of F, [pic] will remain constant even [pic] is increased further. Therefore, we cannot read [pic] from the spring balance but the applied force which is lager than the [pic]. However, [pic] and [pic] can be measured directly by pulling the wooden plank instead of pulling the wooden block with the spring balance.

Pushing a car

Both [pic] and [pic] are independent of the surface area of the body and [pic] is independent of the relative velocity of the body.
Since F=[pic]N and [pic][pic][pic], the force needed to start the motion is lager than the force needed to keep the object in uniform motion. Therefore, it is more difficult to start the motion but once the car is moving, it is relatively easier to maintain its motion in pushing a car.

Design of the braking system in bicycles

In the design of the braking system in bicycles, the weight of the bicycles and the coefficient of friction of material of the wheels should be maximized to achieve a larger frictional force. By F=[pic]N, both [pic] and N increase, the frictional force will increase.

Conclusion

The greater the normal force, the greater the friction if all other factors are kept constant. Also, force of friction varies with different surfaces in contact if all other factors are kept constant. However, the area of contact of sliding surface does not influence the force of friction if all other factors are kept constant.
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