Module 1
PlanningGuide

Lab 1.3

Lesson 1.1
Lesson 1.2
Lesson 1.3
Lesson 1.4
Lab 1.1
Lab 1.2
Lab 1.3
Lab 1.4
Project 1
Class Test

Module 2

Module 3

Module 4

UNIT 1 - ENERGY & MOTION

Physics Lab 1.3 Forces, Friction & Integrated Forces

This lesson deals with friction forces and forces used to store energy.

The video for this lesson illustrates the following experiments:

• Measure static and dynamic friction between two surfaces
• Measure the elastic potential energy stored in an archers’ bow

Get Started
After watching the video presentation, answer the questions in the exercises below and review the solutions.

Experiments
Carry out the experiments described below.

Record the results using the data tables provided. Enter the experimental results in the appropriate columns and calculate the values needed to complete the data tables.

Answer the questions that are associated with the experiments.

1. When the tension in a spring balance is slowly increased in order to start pulling a block of wood that is attached to it across a smooth surface, the block jumps when it starts to move. Why does the block jump as it starts to move?
2. Spring balances are usually calibrated to measure mass but actually measure the force of gravity on the object being weighed. If the force of gravity on 1 kilogram is 9.81 Newtons, what is the downward force (in Newtons) that is acting on a spring balance if it gives a weight reading of 0.5 kg?
3. If a spring balance gives a mass reading of 100 grams, what is the downward force in Newtons?
4. A block of wood with a mass of 330 grams is resting on a table. What force does it exert on the table top?
5. If a force of 0.5 Newtons is needed to keep the block sliding horizontally across the table, what is the dynamic coefficient of friction between the wooden block and the surface of the table?
   
   Answers to Questions

Experiment 1.3.1 Friction Forces

The purpose of this experiment is to measure frictional forces and to demonstrate the difference between static and dynamic friction.

Materials and Equipment:

• Two block of wood, one with a hook at the end
• Pulley
• Piece of string
• Spring balance

Procedure:

Place the block of wood on the flat surface with one of its largest surfaces in contact with the smooth surface.

Attach the piece of string to the block, thread the string throught the pulley so that an upward movement of the end of the string results in a horizontal movement of the block.

Attach the spring balance to the end of the string.

Gently pull the spring balance upwards until the block just starts to move.

Record the tension force in the string needed to get the block to start moving.

This is an indication of the static friction between the block and the smooth surface.

Keep the block moving by moving the spring balance upwards.

Record the tension force in the string needed to keep the block moving slowly forward.

This is an indication of the dynamic friction between the block and the smooth surface.

Test the effect of pressure and area on the amount of friction between the two surfaces.

Turn the block of wood on its side so that the area in contact with the other surface is smaller.

Turn the block back onto it’s wider side. Place a metal weight with a mass of between 200g and 800g on the block of wood.

Results – Static Friction

Results – Dynamic / Sliding Friction

Questions

1. Does the area in contact between the surfaces affect the amount of static and/or
   dynamic friction?
2. Does the pressure between the two surfaces affect the amount of friction?

Experiment 1.3.2 Bows and Forces

Purpose: The purpose of this experiment is to measure the energy stored in an archer’s bow as the string is pulled away from the bow.

Materials and Equipment:

• • Bow
  • Spring balance: 2 – 5 kg scale
  • Clamp
  • Tape measure: 1 – 2 m

Procedure:

Important Safety Note: Wear safety glasses while working with the spring and bow under tension.

1. Clamp the bow onto a surface or board on which the distance of the string from its normal position can be marked.
2. Use the spring balance to draw the string 2 cm away from the bow.
3. Record the reading on the spring balance.
4. Mark the position of the point where the hook of the balance connects with the string on a piece of paper attached to the board underneath the bow. The spring balance reading can be written next to this mark.
5. Draw the string a further 2 cm and repeat the process of recording the tension and the position of the string.
6. Record the tension in the spring balance another 12 times.

Results

Calculations
The work done on the bow – and hence the energy stored in the bow – is the sum of the products of force x incremental distance. If the variation of force with distance were plotted, this would be equivalent to the area under the graph.
A good approximation could be obtained by taking the average force between two successive readings and multiplying this by 0.02m for each incremental distance of 2 cm moved.

Question
If a line graph is plotted with the force needed to move the string on the y-axis and the distance moved by the string on the x-axis, how would the area under the line be related to the energy stored in the bow?

Answer

Answers to Questions on Friction

1. The block jumps because the force needed to overcome dynamic friction is less than the force needed to overcome static friction. Friction opposes the movement of the block across the surface. Before the block can start to move, the force that is acting to cause movement needs to be greater than the force of friction that opposes movement. When there is no relative movement between the two surfaces that are in contact, the force of friction that opposes movement is known as static friction. When the block starts to move and there is relative movement between the two surfaces, the force of friction that opposes movement is known as dynamic or sliding friction. Static friction is usually larger than dynamic friction. If a spring is used in the process of applying a force to oppose friction, the object being moved jumps because, once movement starts, the force applied is greater than the force needed to overcome dynamic friction.
   
2. 4.905 N: 0.5 kg x 9.81 N/kg = 4.905 N
   
3. 0.981 N: 0.1 kg x 9.81 N/kg = 0.981 N
   
4. 3.24 N: 0.33 kg x 9.81 N/kg = 3.24 N
   
5. 0.154: The coefficient of friction is the (sliding) force needed to cause relative movement of the two surfaces divided by the force that keeps the surfaces in contact. The force keeping the two surfaces in contact is the weight of the block which is 3.24 N. (See question 4 above)
   Coefficient of friction = 0.5 N ¸ 3.24 N = 0.154.

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