Studying Force of Friction

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A1 Studying force of friction


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


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


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.


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...
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