4.1 Forces

Chapter 1 | Motion, Forces and Energy

4. Effect of Forces

IGCSE Physics

4.1.1 – Forces Basics

Force:

A push or a pull acting on an object as a result of its interaction with another object.

Force is a vector quantity because it has both magnitude as well as direction.
Unit: Unit of force is Newton (N).
Forces can bring about changes in an object’s motion or shape.
For instance, applying force, such as loading weights onto a spring, can result in the extension of the spring.
Forces can cause alterations in speed, direction of travel, or the overall shape of an object.

Experiment: Measuring Spring Extension:

An experiment is conducted to determine how the extension of springs varies with applied force.

Objective:

To investigate the relationship between the force applied to a spring and the resulting extension.

Procedure:

Secure a spring vertically.
Measure the initial length of the spring.
Apply forces incrementally to the spring, measuring corresponding extensions.
Record force and extension values.
Plot a graph with extension on the y-axis and force on the x-axis.

Experimental setup to see the variation of extension of spring with applied force — Figure: Diagram shows the experimental setup for extension- force graph.

Force Extension Graph

After that, graph is plotted between extension on y-axis and load (force) on x-axis as shown

Force
Linear region
Force-extension
raph
Elastic limit
Extension

Figure: Diagram shows the extension- force graph for elastic material.

Exam Tip

Don’t be confuse with terms proportional and linear.

Proportional Relationship:

A relationship is deemed proportional if its graph is a straight line that passes through the origin (0,0).

Linear Relationship:

A relationship is considered linear if its graph is a straight line but does not necessarily pass through the origin.

Exam Tip

Forces and Motion Simulation

Learning Goals

Understand the term force and how you can calculate the resultant force acting on an object on a straight line?
What will be the effect of friction force on the motion of object?
Hint: Tell either object moves or not on the basis of magnitudes of applied and friction forces.
How much force is required to move light and heavy objects?

Hint: Large force is required to move heavy objects and vice versa.

What will be the effect on speed of object by increasing the mass?
Apply newton’s second law.

Hint: Observe that acceleration of the objects increase with increase in applied force and it decrease by increasing the mass.

Demonstrate first condition of equilibrium.

Hint: Object does not move when resultant force acting on it is equal to zero.

Definitions

Force: It is a push or a pull acting on an object as a result of its interaction with another object.
Balanced forces: Forces are balanced if no resultant force acts on the object.
Unbalanced forces: Forces are unbalanced if resultant force acts on object.
Inertia: Inertia is the tendency of an object to resist a change in its motion.
Newton’s first law of motion: A body at rest remains at rest or moving with uniform velocity continue to move unless acted on by an unbalanced external force.

Friction: A resistive force that act to oppose the relative motion between two surfaces.

Newton’s second law of motion: The acceleration is directly proportional to the applied resultant force and inversely proportional to the mass of body.

Hooke’s Law: The magnitude of restoring force is directly proportional to the extension of spring up to the limit of proportionality. The constant in Hooke’s law is called spring constant.
Limit of proportionality: The point after which extension of an elastic object is not proportional to the force applied to it.
Restoring force: The force that brings the spring back to its original position is spring force (restoring force).
Elastic deformation: This occurs when object returns to its original shape after the force (load) has been removed.
Plastic deformation: This occurs when object does not return to its original shape after the force (load) has been removed.
Circular motion: Movement of an object along a circular path.
Centripetal force: Force that bends normally straight path of an object into circular path.
Centripetal acceleration: Constant acceleration of an object that is always directed towards centre of circle.
Moment of force: Turning effect of force which is equal to product of force and perpendicular distance from pivot point to line of action of force.
Principle of moments: For an object to be in equilibrium, the total clockwise moment about any point is equal to the total anticlockwise moments about same point.
Equilibrium: An object is in equilibrium if the resultant force and resultant moments on that object is equal to zero.
Centre of mass: The single point through which all the mass of an object can be said to act.
Centre of gravity: The point where the weight of an object is considered to be concentrated.
Plane lamina: A body whose mass is concentrated in a single plane.

Exam Tips

1. Effects of Forces:

Understand how forces can cause changes in the motion of an object.
Be familiar with the terms acceleration, deceleration, and resultant force.
Practice answering questions that involve analyzing the effects of forces on the motion of objects.

2. Resultant Force, Balanced and Unbalanced Forces:

Know the definition of resultant force and how to calculate it when forces act in the same or opposite directions.
Understand the concepts of balanced forces (resultant force is zero) and unbalanced forces (resultant force is nonzero).
Be able to recognize situations where forces are balanced or unbalanced.
Practice solving problems related to calculating resultant forces and determining the resulting motion.

3. Newton’s First Law of Motion:

Understand Newton’s first law, which states that an object at rest will remain at rest, and an object in motion will remain in motion with constant velocity unless acted upon by a net external force.
Be able to apply this law to explain various scenarios involving the motion of objects.
Practice with questions that require you to use Newton’s first law to analyze situations.

4. Newton’s Second Law of Motion:

Understand Newton’s second law, which relates the force acting on an object, its mass, and its acceleration (F = ma).
Know how to calculate acceleration, force, or mass using this formula.
Practice solving problems involving the application of Newton’s second law.

5. Hooke’s Law:

Understand Hooke’s law, which describes the relationship between the force applied to a spring and the resulting extension or compression.
Be able to use the formula F = kx, where F is the force, k is the spring constant, and x is the displacement.
Practice solving problems related to Hooke’s law and the behavior of springs.

6. Circular Motion:

Understand the forces involved in circular motion, including centripetal force.
Be familiar with the concept of centripetal acceleration and its relation to the direction of motion.
Practice answering questions related to circular motion, including the forces acting on objects moving in circular paths.

7. Friction Force:

Understand the nature of friction as a force that opposes motion.
Be familiar with the factors affecting friction, including the nature of the surfaces in contact and the normal force.
Practice solving problems that involve calculating frictional forces and understanding their effects.

8. Turning Effect of Forces:

Understand the concept of torque or the turning effect of forces.
Know the factors that affect the torque, including the force applied and the distance from the pivot point.
Be able to calculate torque using the formula τ = rF, where τ is torque, r is the lever arm, and F is the force.
Practice solving problems involving the turning effect of forces.

Summary:

Force is a vector quantity that is caused by pull or push on an object. It is measured in newton.
Resultant force has the same effect as combined effect of all the forces to be added.
If all the forces acting on an body in same direction, then the resultant force is obtained by adding all forces.
If all the forces acting on an body in opposite direction, then the resultant force is obtained by subtracting these forces.
Forces are said to be balanced when all the forces are combined in such a way that they cancel each other and no resultant force acts on the body.
When all of the forces combine in such a way that they do not cancel out and a resultant force acts on the object, then the forces are said to be unbalanced.
Newton’s First Law states that an object with no net force acting on it remains at rest or moves with constant velocity in a straight line.
Newton’s Second Law states that the acceleration of an object is directly proportional to the net force on the object and inversely proportional to the mass of the object. Mathematically, F=ma.
Hooke’s Law states that for a spring F=kx, where k is spring constant.
Stiff springs have large value of spring constant and soft springs have small value of spring constant.
On extension-force graph,

Linear region shows elastic deformation where object obeys Hooke’s law
Non-linear where shows plastic deformation where Hooke’s law does not obey and object changes permanently.
Gradient of force-extension graph gives spring constant.
Centripetal acceleration is always acting towards centre of circle and is perpendicular to its velocity.
Speed is constant but the direction of velocity is always changing in circular motion.
Force on an object moving in circular path depends on its mass, its velocity and radius of circle.
The moment of force is the measure of its turning effect having unit Nm.
If the force is applied in the same line as that pivot, then the object cannot rotate and remains stationary.
If the force is applied in a different line to that of pivot, then the object rotates.
The total clockwise moment about any point is equal to the total anticlockwise moments about same point for an object to be in equilibrium.
Centre of gravity is the point where the weight of an object is considered to be act.
The stability of an object depends on the location of centre of gravity.

When centre of gravity is below the point of suspension, then the object is in stable equilibrium.
When centre of gravity is above the point of suspension, then the object is in unstable equilibrium.
If the line of action of weight of object moves outside the base, there will be resultant moment and object will topple.

Exam Tip

Results:

Before Cross Sign:
- Extension is directly proportional to applied force.
- Hooke’s law applies in this region.
- Elastic deformation occurs as force increases.
At the Cross Sign (Elastic Limit):
- Elastic limit, or limit of proportionality, is reached.
- Elastic deformation continues within this limit.
After Cross Sign:
- Extension is not directly proportional to applied force.
- Hooke’s law does not apply.
- Plastic deformation occurs beyond the elastic limit.

Masses and Spring Simulation

Learning Goals

The apparatus consists of a spring, ruler, masses and pointer. Arrange the apparatus as shown in figure.
Initially, note the reading from ruler when no mass is attach with spring. This reading is the initial length of spring.
Now attach one mass with spring, with the pointer attached to the bottom. The length of spring changes. With the help of ruler, again determine the length of the spring.
Start increasing the attached masses with the spring and note down the corresponding lengths of spring until total of ten masses have been added.
In a table, record the readings of mass (in kg) and length (in cm).
Repeat the experiment three to four times and average the results.

After completing the experiment, the spring force (load) can be calculated by multiplication of spring mass with the acceleration due to gravity, which is 9.81Nkg. And the spring’s extension can be calculated by subtracting the original length of the spring from each new length of spring.

Exam Tip

Don’t be confuse with terms proportional and linear.

Proportional Relationship:

A relationship is deemed proportional if its graph is a straight line that passes through the origin (0,0).

Linear Relationship:

A relationship is considered linear if its graph is a straight line but does not necessarily pass through the origin.

Exam Tip

Forces and Motion Simulation

Learning Goals

Understand the term force and how you can calculate the resultant force acting on an object on a straight line?
What will be the effect of friction force on the motion of object?
Hint: Tell either object moves or not on the basis of magnitudes of applied and friction forces.
How much force is required to move light and heavy objects?

Hint: Large force is required to move heavy objects and vice versa.

What will be the effect on speed of object by increasing the mass?
Apply newton’s second law.

Hint: Observe that acceleration of the objects increase with increase in applied force and it decrease by increasing the mass.

Demonstrate first condition of equilibrium.

Hint: Object does not move when resultant force acting on it is equal to zero.

Definitions

Force: It is a push or a pull acting on an object as a result of its interaction with another object.
Balanced forces: Forces are balanced if no resultant force acts on the object.
Unbalanced forces: Forces are unbalanced if resultant force acts on object.
Inertia: Inertia is the tendency of an object to resist a change in its motion.
Newton’s first law of motion: A body at rest remains at rest or moving with uniform velocity continue to move unless acted on by an unbalanced external force.

Friction: A resistive force that act to oppose the relative motion between two surfaces.

Newton’s second law of motion: The acceleration is directly proportional to the applied resultant force and inversely proportional to the mass of body.

Hooke’s Law: The magnitude of restoring force is directly proportional to the extension of spring up to the limit of proportionality. The constant in Hooke’s law is called spring constant.
Limit of proportionality: The point after which extension of an elastic object is not proportional to the force applied to it.
Restoring force: The force that brings the spring back to its original position is spring force (restoring force).
Elastic deformation: This occurs when object returns to its original shape after the force (load) has been removed.
Plastic deformation: This occurs when object does not return to its original shape after the force (load) has been removed.
Circular motion: Movement of an object along a circular path.
Centripetal force: Force that bends normally straight path of an object into circular path.
Centripetal acceleration: Constant acceleration of an object that is always directed towards centre of circle.
Moment of force: Turning effect of force which is equal to product of force and perpendicular distance from pivot point to line of action of force.
Principle of moments: For an object to be in equilibrium, the total clockwise moment about any point is equal to the total anticlockwise moments about same point.
Equilibrium: An object is in equilibrium if the resultant force and resultant moments on that object is equal to zero.
Centre of mass: The single point through which all the mass of an object can be said to act.
Centre of gravity: The point where the weight of an object is considered to be concentrated.
Plane lamina: A body whose mass is concentrated in a single plane.

Exam Tips

1. Effects of Forces:

Understand how forces can cause changes in the motion of an object.
Be familiar with the terms acceleration, deceleration, and resultant force.
Practice answering questions that involve analyzing the effects of forces on the motion of objects.

2. Resultant Force, Balanced and Unbalanced Forces:

Know the definition of resultant force and how to calculate it when forces act in the same or opposite directions.
Understand the concepts of balanced forces (resultant force is zero) and unbalanced forces (resultant force is nonzero).
Be able to recognize situations where forces are balanced or unbalanced.
Practice solving problems related to calculating resultant forces and determining the resulting motion.

3. Newton’s First Law of Motion:

Understand Newton’s first law, which states that an object at rest will remain at rest, and an object in motion will remain in motion with constant velocity unless acted upon by a net external force.
Be able to apply this law to explain various scenarios involving the motion of objects.
Practice with questions that require you to use Newton’s first law to analyze situations.

4. Newton’s Second Law of Motion:

Understand Newton’s second law, which relates the force acting on an object, its mass, and its acceleration (F = ma).
Know how to calculate acceleration, force, or mass using this formula.
Practice solving problems involving the application of Newton’s second law.

5. Hooke’s Law:

Understand Hooke’s law, which describes the relationship between the force applied to a spring and the resulting extension or compression.
Be able to use the formula F = kx, where F is the force, k is the spring constant, and x is the displacement.
Practice solving problems related to Hooke’s law and the behavior of springs.

6. Circular Motion:

Understand the forces involved in circular motion, including centripetal force.
Be familiar with the concept of centripetal acceleration and its relation to the direction of motion.
Practice answering questions related to circular motion, including the forces acting on objects moving in circular paths.

7. Friction Force:

Understand the nature of friction as a force that opposes motion.
Be familiar with the factors affecting friction, including the nature of the surfaces in contact and the normal force.
Practice solving problems that involve calculating frictional forces and understanding their effects.

8. Turning Effect of Forces:

Understand the concept of torque or the turning effect of forces.
Know the factors that affect the torque, including the force applied and the distance from the pivot point.
Be able to calculate torque using the formula τ = rF, where τ is torque, r is the lever arm, and F is the force.
Practice solving problems involving the turning effect of forces.

Summary:

Force is a vector quantity that is caused by pull or push on an object. It is measured in newton.
Resultant force has the same effect as combined effect of all the forces to be added.
If all the forces acting on an body in same direction, then the resultant force is obtained by adding all forces.
If all the forces acting on an body in opposite direction, then the resultant force is obtained by subtracting these forces.
Forces are said to be balanced when all the forces are combined in such a way that they cancel each other and no resultant force acts on the body.
When all of the forces combine in such a way that they do not cancel out and a resultant force acts on the object, then the forces are said to be unbalanced.
Newton’s First Law states that an object with no net force acting on it remains at rest or moves with constant velocity in a straight line.
Newton’s Second Law states that the acceleration of an object is directly proportional to the net force on the object and inversely proportional to the mass of the object. Mathematically, F=ma.
Hooke’s Law states that for a spring F=kx, where k is spring constant.
Stiff springs have large value of spring constant and soft springs have small value of spring constant.
On extension-force graph,

Linear region shows elastic deformation where object obeys Hooke’s law
Non-linear where shows plastic deformation where Hooke’s law does not obey and object changes permanently.
Gradient of force-extension graph gives spring constant.
Centripetal acceleration is always acting towards centre of circle and is perpendicular to its velocity.
Speed is constant but the direction of velocity is always changing in circular motion.
Force on an object moving in circular path depends on its mass, its velocity and radius of circle.
The moment of force is the measure of its turning effect having unit Nm.
If the force is applied in the same line as that pivot, then the object cannot rotate and remains stationary.
If the force is applied in a different line to that of pivot, then the object rotates.
The total clockwise moment about any point is equal to the total anticlockwise moments about same point for an object to be in equilibrium.
Centre of gravity is the point where the weight of an object is considered to be act.
The stability of an object depends on the location of centre of gravity.

When centre of gravity is below the point of suspension, then the object is in stable equilibrium.
When centre of gravity is above the point of suspension, then the object is in unstable equilibrium.
If the line of action of weight of object moves outside the base, there will be resultant moment and object will topple.

Exam Tip

Curriculum

4.1 Forces

Chapter 1 | Motion, Forces and Energy

4.1.1 – Forces Basics

Force:

Experiment: Measuring Spring Extension:

Force Extension Graph

Exam Tip

Forces and Motion Simulation

Definitions

Exam Tips

Summary:

Results:

Masses and Spring Simulation

Exam Tip

Forces and Motion Simulation

Definitions

Exam Tips

Summary:

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