Topic 1: Forces and Motion

Lessons:

Syllabus Statements:

use the following units: kilogram (kg), metre (m), metre/second (m/s), metre/second2 (m/s2), newton (N), second (s) (1.1), 
understand and use distance - time graphs (1.2), 
recall and use the relationship between average speed, distance moved and time (1.3), 
recall and use the relationship between acceleration, velocity and time (1.4), 
interpret velocity - time graphs (1.5), 
determine acceleration from the gradient of a velocity - time graph and the distance travelled from the area between the graph and the time axis (1.6), 
express a force as a push or pull of one body on another (1.7), 
identify various types of force (e.g. gravitational, electrostatic etc) (1.8), 
distinguish between vector and scalar quantities (1.9), 
appreciate the vector nature of a force (1.10), 
add forces that act along a line (1.11), 
understand that friction is a force that opposes motion (1.12), 
recall and use the relationship between unbalanced force, mass and acceleration (1.13), 
recall and use the relationship between weight, mass and g (1.14), 
describe the forces acting on falling objects and explain why falling objects reach a terminal velocity (1.15), 
describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time (1.16), 
recall and use the relationship between the moment of a force and its distance from the pivot (1.22), 
recall that the weight of a body acts through its centre of gravity (1.23), 
recall and use the principle of moments for a simple system of parallel forces acting in one plane (1.24), 
understand that the upward forces on a light beam supported at its ends vary with the position of a heavy object placed on the beam (1.25), 
describe how extension varies with applied force for helical springs, metal wires and rubber bands (1.26), 
recall that the initial linear region of a force - extension graph is associated with Hooke’s law (1.27), 
associate elastic behaviour with the ability of a material to recover its original shape after the forces causing deformation have been removed (1.28).

Speed, Distance and Time

Speed is the distance that something moves in a given time.

20 metres per second means that something moves 20 metres in one second.

What about 50 miles per hour? Or 600 kilometers per minute? - They're all speeds.

In school we use metres per second

 

Experiment: How to display speeds

Using a datalogger draw graphs of distance against time for...

1) Acceleration, 2) Constant Speed, 3) Rest, 4) Deceleration.

Save each of these graphs, and then import them into a single word document. Next to each graph, describe what you see.

 

How to interpret distance time graphs

  • The velocity (or speed) is the gradient of the distance-time graph
  • The steeper the graph, the faster the speed
  • A negative gradient means speed in the opposite direction

Task: Draw and label the graph with the following...

  • Diagonal line= Constant speed
  • Diagonal line downwards = Constant speed backwards
  • Horizontal line = Stationary
  • Steeper Diagonal line = Higher constant speed

Questions:

1) What is the speed during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the speed during the last 40 seconds?
4) When was the object travelling the fastest?

 

Task: work through the online exercises

Cars moving at the same/ different speeds

Runners in a 400m race

Homework/ Extension:

Complete the worksheets on Speed and Acceleration

Physics for You pp134

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Acceleration, Velocity, Time

Task: Define the following terms: Distance, Time, Speed, Acceleration

What does the distance graph of an accelerating object look like?

 

We need to define some new ideas before we can work with this.

Vectors

Vectors are physical quantities with size and direction. Scalars are quantities with size only.

Examples

Vector Scalar
Displacement Distance
Velocity Speed
Acceleration Time???

Displacement: Displacement is distance with direction. Eg: 100m SW

Velocity: Velocity is speed with a direction. Eg: 30m/s NNE.

Acceleration: Because acceleration is a vector it has size and direction.

Speeding up is positive acceleration, slowing down is negative acceleration.

 

 

Experiment: Drawing velocity-time graphs with ticker tapes to show acceleration.

Worksheet for students

So we can use distance time graphs to show velocity and velocity time graphs to show acceleration.

Challenge: Can you work out how to find the distance travelled from a velocity-time graph?

 

Extension: Playing with s-t, v-t and a-t graphs

Questions on speed and acceleration (2)

Physics for You pp130-133

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Forces

Questions:

What is a force? Can you name 5 different forces?

How do forces affect motion?

So how do things move if there are no forces acting on them?

What will happen to the box when the following forces are applied?

Newton's Laws of Motion tell us about the way objects behave when forces act.

Newton 1

"Every body continues at rest or constant speed in a straight line unless acted upon by an external force."

Explain these using Newton's 1st Law:

 

 

Don't get drunk at weddings and try and demonstrate this!

Newton 2

"The resultant force on an object is equal to the objects mass times its acceleration"

Task:

Work through the online exercise to explain how Newton's 2nd Law allows us to do this...

 

 

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Unbalanced Forces

If an unbalanced force acts on a body then it experiences an acceleration.

The acceleration is related to the unbalanced force by Newton's 2nd Law.

Experiment: How is the force on the trolley related to it's acceleration?

Marking rubric

Questions

  1. What force is needed to accelerate a mass of 12kg at 5m/s2 ?
  2. The same force acts on another mass and it accelerates at 6m/s2. What is its mass?

Extension:

Resultant forces worksheet

Homework:

Complete the worksheets on Forces

Physics for You pp 139 q14-17 and pp153 q37 and 38

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Unbalanced Forces 2

Friction:

There are two types of friction:

  • Static friction where the frictional force exactly opposes any other forces. The body doesn't move.
  • Kinetic friction which is greater than static and happens when the body is moving.

Air Resistance

Air resistance is the bouncing of air molecules off the side of a barrier. The faster something is moving, the more molecules bounce off it every second, so the bigger the force of air resistance.

Task: Draw a diagram to illustrate this

 

Terminal Velocity

When an object falls it accelerates because of the force of gravity.

As its speed increases the force of air resistance pushing upwards also increases.

When the upward force of air resistance is equal to the downward force of weight, then the object will stop accelerating and fall with constant speed. This is called Terminal Velocity .

Watch film and record the speed of a parachutist at different times in her descent.

Task:

Plot the speed-time graph for the parachutist and label it to show the different parts of the descent.

Questions

  1. What happens to the air resistance as her speed increases?
  2. When these forces are the same what happens to her speed?
  3. When the forces are balanced then an object travels at a constant speed, what is the name given to this constant speed?
  4. Apart from the skydiving region, there are two other regions on the graph when she travels at a constant speed - what are they?

More on speed-time graphs

Task:

Using the simulation experiment to answer these questions:

  1. What happens to the slope of the v-t graph when you increase the breaking force? What does this show?
  2. What happens to the area under the graph when you increase the reaction time? What does this show?
  3. What happens to the area under the graph when you increase the initial velocity? What does this show?
  4. What guidance can you give to drivers based on these observations?

Look at Physics for you pp98 for help with this.

Homework:

Complete the worksheets on Falling Freely and Car Safety

Physics for You pp98

 

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Moments

Task:

You have an unknown mass, a 20g mass, a pivot and a metre ruler.

Find the unknown mass.

 

So what do you remember about moments?

and they are calculated from...

 

Experiment:

1. Work through the online experiment, and construct a table showing the clockwise and anticlockwise moments for 10 different situations.

NB: Make sure the spring balance vertical, beam horizontal box is ticked.

Clockwise Anticlockwise
Force (N) Distance from Pivot (m) Moment (Nm) Force (N) Distance from Pivot (m) Moment (Nm)
           

2. Now repeat the experiment for real, and draw a second table.

3. Compare the results for both of the experiments.

  • Which is more accurate? Why.
  • What are the limitations of this experiment?
  • Is the law of moments demonstrated?

Homework:

Complete the worksheet on Balancing

Physics for You pp100-101

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Hooke's Law

When forces are applied to materials, they deform. Hooke's Law describes how materials behave when the are deformed.

Spring demo:

What is the difference between length and extension?

What do we mean by the term elastic deformation?

What is the elastic limit?

We write an equation for a straight line graph like this, as...

y a x

In this experiment, y= F, and x= e,

F a e

F = ke

Where k is a constant that relates Force to extension, which is called the spring constant. The gradient of our graph tells us this.

k changes from one material to another.

Experiment:

Do the same experiment, but this time to destruction with an elastic band.

Draw a diagram, and complete the table of results for your experiment

Force (N) extension (cm)
   

What do you notice about the graph?

Tasks:

Mark on your graph the following features...

  • Elastic deformation
  • Plastic deformation
  • Elastic limit
  • Breaking point

Now work out the spring constant (N/cm) for the elastic band during the elastic part of the deformation.

Extension: Demo of Hooke's Law applied to stretchig a wire.

Exercise:

Complete these problems using the ideas of Hooke's Law.

Homework:

Complete the worksheet on changing shape

Physics for You pp79

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anrophysics 2009