Forces 2º ESO

Subject: Physics and Chemistry                                              Teacher: Francisco Sánchez

Title of the Unit: Forces                                         Course: 2^nd ESO

Los objetivos de la unidad son los siguientes:

·         To know what a force is and their different types.
·         To difference between mass and weight.
·         To identify when some forces are balanced.
·         To investigate the relationship between forces acting on an object and that object's motion.
·         To predict the motion of an object.

Con los correspondientes contenidos:

·         What are forces?
·         Types of forces.
·         Mass and weight.
·         Balanced and unbalanced forces.

Vocabulario básico para fuerzas:

Nouns: air resistance, arrow, direction, driving force, force meter, force of attraction, force of repulsion, friction, gravity, mass, motion, unbalanced forces, up thrust, weight

Verbs:  to pull, to push, to slow down, to speed up, to exert

Adjectives:  balanced forces, contact forces, constant speed,  electric force,  non-contact forces,

Prepositions:  on, over, up, down,

Tareas y actividades

·         Complete the activity sheet in small group.
·         Simulation online. Working in pairs.
·         Reading work sheet.
·         Draw diagrams of forces.
·         Fill the gaps.
·         Classify forces.
·         Solve simple numeric problems.
·         Open and work with simulation online.

Organización de las clases:

·         8/9 sessions of 55 minutes (three weeks).
·         While the teacher's explanations. The class will be organized in small groups of 3 or 4 students.
·         Work in pairs with the computer.

Criterios e instrumentos de evaluación.

Evaluation criteria:

·         To know the different types of forces.

·         To know the difference between mass and weight.

·         To know calculate weights

·          To predict the motion of an object. Solve simple problems.

Evaluation instruments:

·         Revision student's work.

·         Written test.

Finalmente he confeccionado un material que conjuga teoría con actividades prácticas. Pienso que lo mejor es adjuntarlo directamente para que  lo podais  utilizar en caso de que os guste.

CONTENTS

1. What are forces?

2. Type of forces

3. Mass and weight

4. Balanced and unbalanced forces

5. Simulation on-line forces and motion.

6. Summary

7. Glossary

1. WHAT ARE FORCES?

Most forces that we use are either pushing (in Catalan "_________ ") or pulling (in Catalan "___________ ") something.

Q1. Indicate, for the following situations, if the force acting is a push or a pull:

a) Opening a drawer.

b) A tennis player hitting a ball with a racket.

c) Moving a trolley in a supermarket.

d) Closing a shutter.

Forces can:

a) Start and stop things from moving

b) Make something speed up or slow down

c) Cause changes in direction

d) Change the shape of an object

e) Change the length of an object.

Forces are acting everywhere. No matter where you look, you will see evidence of forces in use:

Q2. A tennis player applies a force to a ball using a racket. What will happen to the ball?

Forces are measured in Newtons (the symbol is N). A force is drawn as a straight line with an arrow at the end. The arrow shows the direction in which the force is acting.

Q3. Imagine you move a wheelchair.

a) Draw a diagram showing the object and the force applied.

b) Indicate if the force is a push or a pull.

c) Indicate the effect of this force.

Q4. Stretch a spring.

a) Draw a diagram showing the spring and the force applied.

b) Indicate if the force is a push or a pull.

c) Indicate the effect of this force.

2. TYPE OF FORCES

Forces can be classified into two types:

 

Contact forces are not classified as forces of attraction or of repulsion.

Q5. Magnets have a north and a south pole; approach two magnets in two

different ways: a) approaching the opposite poles; b) approaching the like poles.

a) Draw diagrams showing what happens; remember to draw the forces

involved.

b) Are they contact or non-contact forces?

c) What type of forces are they?

d) Are they forces of attraction or of repulsion?

Q6. Rub a balloon with wool or with your own hair for a couple of minutes. Place small bits of paper on the desk and bring the balloon near them.

a) Draw a diagram showing what happens, remember to draw the forces

involved.

b) Are they contact or non-contact forces?

c) What type of forces are they?

d) Are they forces of attraction or of repulsion?

e) What can you say about the charges of the balloon and the ones of the

bits of paper?

Q7. a) Draw a diagram showing the forces acting between the Sun and the

Earth.

b) Are they contact or non-contact forces?

c) What type of forces are they?

d) Are they forces of attraction or of repulsion?

 

Q8. a) Draw the forces acting on the parachute.

 

b) Classify these forces into contact or non-contact forces.

c) Classify them, if suitable, as forces of attraction or of repulsion.

Q9. Draw the forces acting on the car.

 

3. MASS AND WEIGHT

Are mass and weight the same? No.

The weight of an object is calculated using this equation: W=m · g

In this formula mass is expressed in kilograms and the value of gravity depends on the planet you are on (Earth's gravity is 9.8 m/s² and Moon's gravity is 1.6 m/s ²).

Q11. An object has a mass of 3 kg on Earth. Work out:

a) Its weight on Earth.

b) Its mass on the Moon.

c) Its weight on the Moon.

Q12. a) What would the weight of a 100 g rat be?

b) And its mass?

c) What would its weight and mass be on the Moon?

Q13. a) Measure the mass of an object using a balance. Record your result in the table below.

b) Work out the weight of the object on Earth using the formula and record

your result in the table below.

c) Measure the weight of the object on Earth using a force meter and record

your result in the table below.

4. BALANCED AND UNBALANCED FORCES

Q14. For the following situations: a) work out and draw the net force or

resultant force; b) indicate if the forces acting on each object are balanced or unbalanced forces.

 

If two equal forces are applied to an object in opposite directions the two forces cancel each other out and are called balanced forces. When the forces acting on an object are balanced the net force acting on the object is 0. We can also say that the object is in equilibrium.

When all the forces acting on an object do not cancel out, we say they are not balanced and they are called unbalanced forces. When the forces acting on an object are not balanced the net force is not 0.

How does the state of motion of an object change when the forces acting on it are balanced or not?

Let us see some examples of objects that, initially, were still:

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If two equal forces are applied to an object in opposite directions the two forces cancel each other out and are called balanced forces. When the forces acting on an object are balanced the net force acting on the object is 0. We can also say that the object is in equilibrium.

When all the forces acting on an object do not cancel out, we say they are not balanced and they are called unbalanced forces. When the forces acting on an object are not balanced the net force is not 0.

How does the state of motion of an object change when the forces acting on it are balanced or not?

Let's see now some examples of objects that are moving:

 

The bicycle is moving at a constant speed, because the forces acting on the bicycle are balanced:

This bicycle is accelerating (speeding up) because it is moving and the net force acts in the direction of movement

Q16. For the following situations: a) write, on the diagram, the forces acting on the object; b) work out and draw the net or resultant force; c) indicate the state of motion of the object.

 

Q17. For the following situations: a) write, on the diagram, the forces acting on the object; b) work out and draw the net or resultant force; c) indicate the state of motion of the object.

 

6. SIMULATION ON-LINE FORCES AND MOTION.

You and some friends are at the park. You find some rope and decide you'd like to play a game of tug-of-war.  Unfortunately, there are 5 people so you can't have an equal amount of people on each side.  One of your friends suggests that the two biggest people should be on one side, while the three smaller people should be on the other side.   Do you think this is a fair way to split up teams?   Why or why not? 

Question:  What causes objects to move or stay still?

Open up PhET simulation "Forces and Motion."

http://phet.colorado.edu/en/simulation/forces-and-motion-basics

TASK 1

a.     Place 2 people that are the same size the same distance away from the cart. 
b.    Make a prediction about the movement of the cart. 

c.    AFTER you have observed the actual movement, click on the sum of the forces box at the top right hand corner of the simulation.  Record the number in the data chart. TASK 2

a.    Place 2 people that are the same size different distances away from the cart. 
b.    Make a prediction about the movement of the cart. 
c.    AFTER you have observed the actual movement, click on the sum of the forces box at the top right hand corner of the simulation.  Record the number in the data chart. 

	Predicted Movement	Actual Movement (none, left, right)	Sum of Forces (0, x-left, x-right)
Same size, different placement on rope.

TASK  3

a.    Place 2 people that are different sizes the same distance away from the cart. 
b.    Make a prediction about the movement of the cart. 
c.    AFTER you have observed the actual movement, click on the sum of the forces box at the top right hand corner.  Record the number in the data chart.

	Predicted Movement	Actual Movement (none, left, right)	Sum of Forces (0, x-left, x-right)
Different size, same placement on rope.

TASK 4

a.    Place 2 people that are the different sizes different distances away from the cart. 
b.    Make a prediction about the movement of the cart. 
c.    AFTER you have observed the actual movement, click on the sum of the forces box at the top right hand corner of the simulation.  Record the number in the data chart. 

	Predicted Movement	Actual Movement (none, left, right)	Sum of Forces (0, x-left, x-right)
Different size, different placement on rope.

Q18. What causes objects to move or stay still?

Analysis Questions/Reasoning:

Give an example of a balanced force.

Give an example of an unbalanced force.

True or false?  Balanced forces cause a change in motion.  T/F

How do you know this?

True or False?  Unbalanced forces cause a change in motion T/F

How do you know this?

Q19.Now that you have had a chance to experiment with the simulation, go back to the question at the beginning of the investigation.  What do you think would be the best way to divide up your friends for the game of tug-of-war? Be sure to explain your reasoning. 

Q20.What questions/investigations do you have after using this simulation? 

Devise an experiment of your own using the simulation that helps to answer your question. 

6. SUMMARY

 

7. GLOSSARY

air resistance

arrow

balanced forces

constant speed

contact forces

direction

drawer

driving force

electric force

force

force meter

force of attraction

force of repulsion

friction

gravitational force

gravity

magnetic force

mass

motion

non-contact forces

shape

spring

still

to pull

to push

to slow down

to speed up

unbalanced forces

upthrust

weight

Francisco Sanchez