Chapter 9: Forces

Forces Introduction

In this chapter, you will explore the nature and various types of forces together with the way they affect objects.
We often use the term force to describe many situations in our day to day activities.
In fact, we may sometimes fail to know that we are actually using it.
Opening a door, riding a bicycle and lifting an object are some of the most common activities that we perform in life. All these basically require an application of a force.
9.1: Meaning of a Force
Consider a ball at rest and that player A wants to pass it to B. In this case, the player will kick the ball in the direction of his team mate.
We say that a force has been applied on the ball. Similarly, a force is always applied to displace a table from one position to another. Can you now identify other instances where we apply forces? How can we then define a force?

Activity 9.1: Explaining the meaning of a force
What you need: notebooks, braille paper, stylus, slate, Perkins brailler.
What to do;
I. In groups, analyse the picture in Figure 9.1 and identify the activities being carried out in each case.

2. Classify the above mentioned activities as being either a push or a pull on an object.
3. Suggest a suitable definition of a force and state its Sl unit.
4. Basing on the above, identify other examples of forces and state instances where they are applied.
   A force is a vector physical quantity. Implying that it has both the magnitude and direction. There are different types of forces and these have a number of effects on a body to which they are applied.
   For instance, forces may cause objects to move in a circular path, rotate about a
   fixed point, just to mention but a few.

However, the extent of the effect of these forces depends on the resultant force acting on an object. By the resultant force, we simply mean that one force which represents a number of forces acting on a system.
9.2: Balanced and Unbalanced Forces
Study Figure 9.2 and state why the person at side A is falling down? Which side do you think has more force than the other?
In this section, you will understand the meaning of balanced, unbalanced forces and state their effects in everyday life.

Activity 9.2: Understanding balanced and unbalanced forces
What you need: metre rule, thread, knife edge, 100 g masses, stone, notebooks,braille paper, stylus, slate, Perkins brailler.
What to do:
In groups:
I. Place a metre rule provided on a knife edge.

2. Use a thread provided to suspend a stone and a 100 g mass at opposite ends of the metre rule.
3. Adjust the position of the mass (m) until the balance point is attained as shown in the diagram.

4. What do you think will happen to the system when the stone is moved 2 cm towards the pivot?
5. Identify and name systems with:
   (a) Balanced force
   (b) Unbalanced forces
6. State effects of unbalanced forces.

The effect on a body depends on the magnitude and direction of the resultant unbalanced force acting on a body.
For forces acting in the same direction, we obtain the resultant force by getting their sum. The resultant of forces acting in opposite direction but along the same line is determined by subtracting their magnitudes.
For forces acting at right angles, we use the Pythagoras theorem to obtain their resultant force. Suppose that you have forces ^Fl and ^{F 2} acting on a body at right angles as shown below.

We use Pythagoras theorem to obtain the resultant. It states that

Where F_R is the resultant force.

9.3: Effect of forces
As previously discussed, a force acting on the body may cause it to start moving,stop moving, change shape, size, accelerate and so on.
However, our main focus in this section is to learn more about particular force which causes an object to stop moving.

Friction
Have you ever wondered how moving objects such as a ball eventually comes to rest even without applying any force on it? What do you think exactly happens when an object is moving on the surface of another or just in air?

Which motion in Figure. 9.3 do you think will be smoother than the other. Ourinterest in this section is to understand friction, analyze its effects, investigate the factors which affect friction and devise ways of reducing or increasing friction.
Activity 9.3: Understanding the term friction
What you need: notebooks, braille paper, stylus, slate, Perkins brailler.
What to do

1. State which force enables the bottle in Figure.9.4 to just not slip through the hand and falls down.
2. Identify the direction of the force mentioned in (1) if the bottle wasto move down wards.
3. From the observations above, suggesta suitable definition of frictional force

Friction is every where around us. Bodies experience friction when at rest, in motion and so on. In fact with out friction, an object pushed into motion would continue to move forever. There are basically two types of friction and these include: Static and dynamic friction. An object at rest is experiencing static friction. Therefore, a force required to change its state should be greater than the friction. In addition,
dynamic friction only occurs between surfaces in relative motion.
Factors affecting friction

You have discovered that friction opposes relative motion between two surfaces in contact. This opposition depends on a number of factors which range from the nature of surfaces in contact to the magnitude of force. Let us investigate the factors affecting friction in Activity 9.4.

Activity 9.4: Investigating the factors that affect friction
What you need: notebooks, braille paper, stylus, slate, Perkins brailler.
Suppose you have three different cases of motion as shown in the Figure 9.5 .

What to do
1.In which situation do you think will the force of friction be the:
(b)lowest
(a)highest
Arrange the situations in order of increasing friction.
Suppose that you have two bricks with the same nature of surfaces but different masses placed on a table.
2.Suppose you push the bricks with the same amount of force on the table, state which brick will move at a higher speed.
3.From the above, identify the factors affecting friction in this case.
4.Discuss about the effects of friction.
5.State some applications of this concept of friction.
6.Suggest some possible ways of reducing the difficulty with which one surface moves over the other.

9.4: Gravity
Suppose you have a piece of paper and a ball held at the same height above the earth’s surface. Which of the two objects do you think will take less time to reach the ground when all are released at the same time? What do you think brings about this difference in the interval time? In this section, you will learn about the concept of gravity, weight of a body and how this varies with its mass and distance from the earth’s surface.
Weight of the Body
Weight is derived physical quantity which tells us about how much force is experienced by a body placed in a gravitational field. What matters in this case is investigating what determines the magnitude of Weight and probably state its direction.

Activity 9.5: Determining the weight acting on objects
What you need : pendulum bob, spring balance, beaker, weighing scale (electronic balance), metallic rod, notebooks, braille paper, stylus, slate, Perkins brailler.
What to do
In groups

1. Use an electronic balance and a spring balance to determine the mass of each object above.
2. Record your results in a table below.

Weight

3. Compare the value of mass and weight to generate the relationship between mass and weight of a body in the earth’s gravitational field.
   In a mathematical point of view, weight is written as w = mg where; m is the mass of the body, and g is the acceleration due to gravity. The value of g keeps changing as you move from one place to another. Scientific discoveries tells us the fact that g has a value of 10 me and 1.6 mse on the surface of the earth and moon respectively.

Acceleration due to gravity varies inversely with distance outside the earth and directly with distance inside the earth. The direction of this attraction force is always towards the body of higher mass.
For instance, in the solar system, this force is directed towards the sun whose mass is higher. This gravitational force provides the centripetal force which keeps the earth and other planets in motion around the sun.