Chapter 4: The Effects of Forces

By the end of this chapter, you will be able to:

a) know that a force is a push or a pull and that the unit of a force is newton (N).

b) know the effects of balanced and unbalanced forces on objects.

c) understand the existence of the force of gravity and distinguish between mass and weight.

d) appreciate that the weight of a body depends on the size of the force of gravity acting upon it.

e) understand the concept of friction in everyday life contexts.

f) understand the meaning of adhesion and cohesion as forms of molecular forces.

g) explain surface tension and capillarity in terms of adhesion and cohesion and their application.

Keywords

• adhesion
• mass
• balanced force
• meniscus
• capillary action
• newton
• cohesion
• contact forces
• non-contact forces
• resultant force

4.1: Introduction

When you push a wheelbarrow or kick a ball, it moves. Also, when you release objects from your hands, they fall to the ground. Have you ever wondered why such things happen? To understand this, think about forces and their effects.

In this chapter, you will be able to explore the nature and types of force and describe how forces move or change the shape of objects and understand some common applications of forces.

4.2: The Meaning of Force

Activity 4.1 Understanding the meaning of a force

Perform the following activities in pairs:

a) Pull each other slowly.

b) Throw a pen gently and observe what happens.

c) Lift your book in space.

d) Stretch a rubber slowly and gently. Do not break it.

Questions

1. From the above activities performed, state what was observed in each case.

2. State other scenarios where force is experienced.

Figure 4.1: A man pulling a goat

Now, let us look at Activity 4.2 to understand more about forces.

Activity 4.2 Understanding more about forces

What you need

Two bricks, two thin long sticks, three empty soda bottles (plastic) and your desk (or table).

What to do In groups:

a) Let one member of the group place one brick on the floor.

b) Carefully, note the direction in which you apply force on the object

c) Then, let another person place this brick on the table.

What did your friend do to the brick in order to put it on the table. Did he (she) push it, or pull it?

In groups, discuss and record your findings.

Likewise, select two members and let each one of them pick one stick. b) c)

Let each of these two members break the stick, one after the other, as you watch carefully. What do you notice?

In order to break the sticks, did your friends push or pull the sticks? In groups, discuss and record your findings.

Let each of the three members in your group pick an empty soda bottle.

a) Ask each one of them to crush her (his) bottle as you watch carefully. What do you observe?

b) In order to crush the bottle, did your friends push or pull the bottle?

c) In groups, discuss and record your findings.

d) Finally, let one of your group members move your desk a little to the front without lifting it. What do you notice?

e) In order to shift the position of the desk, did your friend push the desk, pull the desk or “something” the desk?

Discuss your answers and let the secretary record your group’s answer.

Write a report and present your group’s findings to the rest of the class. We notice that in all our investigations, we used effort or energy to cause a change in position or shape of the objects that we had.

Figure 4.2: Effect of force on the state of a body

A force is a push or pull upon an object resulting from the object’s interaction with another object. The SI units for a force is a newton (N)

Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force.

4.3: Effects of Forces

Activity 4.3 Investigating the effects of force

What you need The Effects of Forces Rubber bands, forcemeters, springs and small blocks of wood/ brick.

What to do In pairs, use the materials as instructed:

1. Push a small block/brick on a table or on the ground. What happens to its position?

Using your hands, pull the rubber band as shown in Figure 4.3 below: 3.

Figure 4.3

State what causes the changes in the shape of the rubber band.

What would happen to the rubber band if much force is applied to it? Explain your reasoning.

4. Also, pull the other side of a forcemetre. Observe and note down what ha Is there change in displacement of the spring inside it

In summary, the effects of forces are:

Forces:

cause a stationary object to move.

cause a moving object to change its speed and direction.

• change the size and shape of an object.

4.4: Categories of Force

All forces (interactions) in the universe between objects can be placed into two broad categories:

1. Contact forces

2. Non-contact forces

Contact forces

These are types of forces that result when two interacting objects are physically in contact with each other.

Examples of contact forces include frictional forces, tensional forces, reaction forces and air resistance forces.

Non-contact forces

A non-contact force is a force which acts on an object without it coming into physical contact with the object.

Examples are gravitational forces, magnetic forces (of attraction and repulsion) and electric forces, among others.

Two magnets will exert a magnetic force on each other (as a pull or a push) even when they are largely separated from each other in terms of metres (or kilometres) as a measure of displacement.

Activity 4.4 Discussing and analysing categories of force

While in pairs, discuss:

a) different varieties of forces and how they occur.

b) categories of forces and their example. Make a report and present your findings to the whole class.

4.5: Other Forces

Weight (W)

This is a force due to gravity acting on an object because of its mass. An object’s weight is directed down, towards the centre of gravity of a body.

Tension (T)

Figure 4.4: Forces acting on a body at rest

The normal force is directed perpendicular to the surface.

Friction

It is the force, between rough solids that are in physical contact, that oppose each other.

Figure 4.5: Direction of frictional force on a moving body

Friction is directed opposite to the direction of relative motion or the intended direction of motion of either of the surfaces.

Tension force is defined as the force that is transmitted through a rope, string or wire when pulled by forces acting from opposite sides. The tension force is directed over the length of the wire and pulls energy equally on the bodies at the ends. This can be visualised in a tug of war.

Figure 4.6: Tension in a rope

However, it is not the opposing team that supplies the force of tension. There is a tension in the rope in opposition to each team’s pull.

Elastic force

This is a force exerted by an object that is under deformation (typically tension or compression) and which will return to its original shape when released, for example, a spring or rubber band.

A catapult uses the elasticity of rubber to “fire” a stone to a target as shown in Figure 4.7.

Buoyancy

This is a force exerted on an object which is immersed in a fluid. Buoyancy is usually directed up.

Drag

This is a force that resists the motion of an object through a fluid. Drag is directed opposite to the direction of motion of the object relative to the fluid.

Thrust

This is a force that a fluid exerts when expelled by a propeller, turbine, rocket, etc. It is also a force exerted on a swimmer when his/her hand pushes the water backward.

Figure 4.8: Weight Direction of drag on a floating body

Thrust is directed opposite to the direction the fluid is expelled.

Electrostatic force

This is the attraction or repulsion between charged bodies. This is experienced in everyday life through the static cling of small papers on plastic material.

Figure 4.9: Attraction or repulsion between charged bodies

Magnetic force

The attraction or repulsion between magnets and other magnetic materials.

Figure 4.10: Magnetic force nolone Smugh

Activity 4.5 Understanding forces in day-to-day life

In groups, discuss scenerios where each of the forces stated above are experienced in your day-to-day interactions.

Balanced and unbalanced forces

Consider the figure below:

The elephants in Figure 4.11 are pulling a log in opposite directions by exerting equal but opposite forces. In which direction will they move? Explain your answer.

A boy whose weight is 350N acting downwards is exactly balanced by the normal reaction of the ground/floor he is standing on. Similarly, the weight of a book acting downwards is exactly balanced by the reaction force of the table on which it is placed

When balanced forces act on an object at rest, the object will not move.

NOTE:

Unbalanced forces usually cause motion in the direction of the bigger force.

Unbalanced forces cause an effect called a resultant force.

Look at the following illustrations:

Activity 4.6 Analysing the effect of resultant force

Key Question: What is the effect of resultant force? The Effects of Forces In pairs, analyse Figure 4.16 below (tug of war) and answer the questions that follow:

Explain what you think would happen if:

group A exerts the same force as group B.

group B exerts a larger force than group A.

State scientific description that explains cases in (a) and (b).

Share your findings in class.

NOTE: When more than one force acts on an object, it is the net/resultant force that is important. Now, suppose the forces are parallel to each other and all in the same direction. Can you determine the resultant

NOTE: In this case, the net force is simply the sum of all the vectors. This is because they are in the same direction

Resultant force using Pythagoras theorem

We can obtain the resultant force of two forces that are at right angles using Pythagoras theorem. For example: If two forces 4N and 3N act at right angles shown as below:

4.6: The Gravitational Force

When a ball is thrown upwards, it returns to the ground; when a mango detaches from the tree, it falls to the ground.

The force that makes objects fall to the ground on Earth is referred to as gravitational force. Gravitational force surrounds us. It is what decides how much we weigh. It also determines how far a football will travel when kicked before it returns to the ground!

NOTE: Gravitational force is what causes objects to have weight.

Gravity is the force that attracts bodies towards each other; the force that causes mangoes to fall towards the ground and the planets to orbit the sun.

The gravitational force on Earth is equal to the force the Earth exerts on your mass.

When you are at rest, on or near the surface of the Earth, the gravitational force is equal to your weight. When you weigh yourself, the scale tells you how much gravity is acting on your body mass.

On Earth, the value of gravity (g) is 10 metres per second squared, (s2).

We therefore calculate the weight of a body from, w = m x g, where, w is weight, m is mass and g is gravitational force (g = 10m/s2).

EXERCISE 4.2

Calculate the weight of a body if it is mass is 90 kg. a)

b) What is the mass of a ball whose weight is 200N?

Does the weight of a body depend on the size of the force of gravity acting on it?

4.7: Friction

What happens when a block is pushed on the surface? What would happen when the block is made large?

The motion of the block is resisted. This resistance is due to friction. Friction is the resistance to motion of one object slidding over another. It is the force exerted by a surface as an object slides over it. Therefore, friction is the force that opposes the motion.

Activity 4.7 Investigating friction on surfaces

Key Question: How can you realise frictional force?

What you need

An adjustable inclined plane, a smooth metallic or plastic cup (without grooves or ridges), a piece of clean cloth (enough to cover the inclined plane), a newspaper, a plastic sheet (enough to cover the inclined plane), a geometrical set and a 1 kg block (smooth) of wood.

What to do

With the items provided to you, design an experiment to investigate frictional force on different surfaces.

What conclusion can you draw?

Activity 4.8 1. 2. Discussing the factors that affect friction

In your groups, discuss the factors that affect friction between two surfaces.

Present your information to other classmates.

Types of friction

There are two very common types of friction force. These are sliding and static friction.

Sliding friction results when an object slides across a surface. As an example, consider pushing a box across a floor. State other scenarios where sliding friction is experienced.

Figure 4.18: Work done against frictional force

The floor surface offers resistance to the movement of the box. We often say that the floor exerts a friction force upon the box.

This is an example of a sliding friction force since it results from the sliding motion of the box.

If car tyres slide after brakes have been applied to bring the car to a stop, there is a sliding friction force exerted upon the car tyres by the roadway surface.

This friction force is also a sliding friction force because the car is sliding across the road surface. The force that stops / prevents objects from moving in the direction of the force is refered to as static friction. This frictional force is always highly experienced by the opposing object.

How can we reduce friction?

There are a number of ways to reduce friction. Can you name them? Let us do assignment 4.1 below:

Assignment 4.1 Discussing ways of reducing friction

In pairs, discuss some of the ways to reduce friction.

Present your findings to the class.

Activity 4.9 Applications of friction

Read the passage below and answer the questions that follow: Ibrahim was driving, under a light drizzle, towards Jinja at 45 kilometres per hour. Two kilometres after leaving Mukono town, a motorcycle rider (a boda-boda) suddenly skidded off the road 85 metres in front of the car. Ibrahim immediately applied on his brakes but it was a useless move. There was an accident but fortunately enough, no one sustained injuries.

The car slammed on to the motorcycle despite the vigorous application of brakes.

The Police, soon after, arrived on the accident scene and carried out their own observations and measurements.

By simply looking at the tyres, why did the Police officers conclude that the car was in bad mechanical condition, yet the car was fairly new, as it was only two years old?

a) What was the problem between the tyres and the road?

b) What should the tyres have looked like?

c) What should have been the case between the tyres and the road for a car in good mechanical condition?

d) Considering the passage, how is friction useful? Friction plays an important part in many everyday processes. For instance, when two objects rub together, friction causes some of the energy of motion to be converted into heat.

Friction is also useful in walking; it makes us not to slip. It is also useful in the motion (and braking) of bicycles, cars, buses etc. The tyres are I so as to increase the grip (friction between the tyre and the road) of the tyre on the road.

1. the factors that affect frictional force between two surfaces in contact.
2. the ways of reducing frictional force. why heavy objects can easily slide across a smooth surface rather than a rough surface.
3. why car tyres become smoother and thinner with time and why the tyre tread is important.
4. why it is easier to write with a pencil on paper than on glossy plastic.
5. In your groups, discuss disadvantages of friction in your day- to-day life.

4.8: Intermolecular Forces

After eating beef soup on a plastic plate, it is always hard to wash it with cold water only. What is normally done is to apply hot water with soap. This process makes soap molecules break down fat (beef soup) molecules which are eventually removed from the plate.

The forces that break down molecules of different materials (fats and plate) for them to interact is called Intermolecular forces.

Activity 4.10 Performing an activity on adhesion and cohesion forces

What you need

A glass, water, oil, dropper

What to do

a) Using a dropper, put about (5) five drops of water on a glass.

b) Gently and slowly tilt the glass.

Observe carefully how water spreads on the glass.

Write your observations between drops of water and the glass.

e) Repeat procedure (a) for oil.

f) Write your observation on how oil drops behave on a glass.

The tendency by water molecules to spread over glass (glass molecules) is because adhesive forces are weaker than cohesive forces.

DID YOU KNOW

Adhesive forces are forces of attraction between the same molecules while cohesive forces are forces of attraction between two or more different molecules.

EXERCISE 4.3

In groups, state and explain real life scenarios where cohesive and adhesive forces are experienced.

For example, if cohesion forces between the water molecules is stronger than that of the adhesion forces between them, then the individual molecules will attract each other, thus resulting in settling. In case the adhesion forces of the water surfaces are stronger than those of the cohesion forces of the water molecules, then the water tends to disperse.

Figure 4.23: Adhesion of water on a plant

4.9: Surface Tension

Activity 4.11 Observing surface tension

Key Question: How can you observe surface tension in water?

What you need

A clean sheet of writing paper, three or four small pieces of paper, a 250 ml beaker, 1000 ml of clean water, a dropper, a pair of tweezers, soap solution and a needle (or paper clip).

What to do

Part 1

a) By observation, which substance is denser, water or paper?

b) Fill the 250 ml beaker with water but it should not overflow.

c) Put one small piece of paper onto the water in the beaker.

d) Carefully, observe and record what is happening.

e) Using a dropper, place a drop of water onto soap solution.

f) Gently place another small piece of paper onto the droplet of water using tweezers.

Note and record your observations.

As a group, discuss what you have observed and record your conclusions.

Part 2

a) Which material is denser, water or the needle (or paper clip)?

b) Fill the 250 ml beaker with water but the water should not overflow.

c) Place the sewing needle (or paper clip) on the little slip of paper (or a small piece of tissue paper) and very gently rest the paper on the surface of the water. Paper clips on water

d) Carefully, observe what happens.

e) Record your observations and suggest reasons for what happened.

Present your conclusions for both experiments (part 1 and part 2) to the rest of the class.

The stronger attraction between the water molecules as opposed to the attraction of the water molecules to the air makes it more difficult to move an object through the surface than to move it when it is completely submersed.

The forces between liquid molecules are responsible for the phenomenon known as surface tension.

“The property of the surface of a liquid that allows it to behave as an elastic skin.” Surface tension allows insects (e.g. water striders), usually denser than water, to float and slide on a water surface.

Question

State other purposeful activities which use the effect of surface tension.

Insect walking on water

Figure 4.24: Consequences of surface tension

Surface tension is responsible for the shape of liquid droplets.

Activity 4.12 Reducing existence of surface tension in water

Key Question How can surface tension in water be reduced?

What you need Water, a pin, oil and a basin.

What to do Perform this activity in groups.

a) Pour water in a basin, almost half-full and let the water come to stand still/settle.

b) Carefully, place a pin on top of the water. State your observation and explain it.

c) Now pour oil into the water.

d) Note all the observations in cases b) and c)

What conclusions can you draw from your observations?

Share your findings with other groups and finally with the whole class.

4.10: Capillary Action

Capillary action (sometimes capillarity or capillary effect) is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity.

It results from the forces of adhesion, cohesion, and surface tension.

Activity 4.13 Performing an activity to understand capillarity

What you need Water, paraffin, glass tubes of different diametres and large beakers/basins.

What to do

Part 1

First brainstorm these questions while you are your groups.

Is there a relationship between the height to which a liquid rises and the size of the pore (or width of the glass tube)?

State your hypothesis!

Part 2

a) Pour water into a wide container like a beaker.

b) Dip one of the glass tubes completely into the water in the beaker.

d) Measure and record how high the water rises up the tube in a suitable table of results.

e) Repeat procedures (b) and (c) for the remaining glass tubes.

g) Plot a graph of height of the water column against internal diametre of the tube.

h) Discuss what the graph is telling you.

Repeat procedures (a) to (f) using paraffin instead of water.

Compare your findings for the two liquids (water and paraffin).

Was your hypothesis true?

Present your findings to the rest of the class.

Figure 4.25: Capillary action

Without capillary action, the water level in all tubes would be the same. Capillary action is important to us since it helps us naturally by pumping out tear fluid in the eye.

Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upwards.

Figure 4.26: Meniscus turned upwards and downwards

The surface tension acts to hold the surface intact. walls is stronger than the cohesive forces between the liquid Capillary action (with water) occurs when the adhesion to the molecules

Figure 4.28: Capillary action in tubes of different sizes

If the liquid molecules are strongly attracted to the tube molecules, the liquid creeps up the inside of the tube until the weight of the liquid and the adhesive forces are in balance. The smaller the diameter of the tube, the higher the liquid rises.

It is through capillary action occurring in plant cells that water and dissolved nutrients are brought from the soil up through the roots and into a plant.

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