Speed and Force

Motion and force are important concepts in understanding the physical world around us. In Grade 7 Physics, these topics help us understand how objects move and interact with each other. Let's take a deeper look at these concepts.

What is speed?

Motion refers to the change in position of an object over time. Everything from a car moving on the road to a ball rolling on the ground involves momentum. To describe motion, we need to consider the starting point, the distance traveled, and the direction.

Types of motion

• Linear motion: This is the most direct type of motion and occurs when an object moves in a straight line. An example of this is a train moving on straight tracks.
• Circular motion: When an object moves in a circular motion, like the Earth orbits the Sun, it undergoes circular motion.
• Rotational motion: This occurs when an object rotates about an internal axis, such as a spinning top.
• Oscillatory motion: This is when an object moves back and forth in a regular pattern, such as a swing moving back and forth.

Measuring speed

To measure speed we focus on distance, speed and velocity:

• Distance: It is the distance an object has travelled. It is measured in units like metres, kilometres, etc.
• Speed: It tells us how fast something is moving. It is calculated by dividing the distance by the time taken.
• Velocity: Unlike speed, velocity includes the direction of motion. It is a vector quantity, which means it has both magnitude and direction.

For example, if a car travels 90 kilometers north in 2 hours, its speed is:

Speed = Distance / Time = 90 km / 2 h = 45 km/h

However, its velocity can be written as 45 km/hr in north direction.

Graphical representation of motion

Graphs can be useful for showing motion. Below is an example of a straight line graph showing time versus distance.

This graph shows that as time passes, the distance increases, which indicates constant speed.

What are forces?

Forces are pushes or pulls that cause objects to move, stop, or change direction. Forces can be classified based on their effect. There are many types of forces.

Types of forces

• Gravitational force: It is the force of attraction between objects due to their mass. This is the reason why things fall down when dropped.
• Friction force: This force opposes motion. For example, when you slide a book across a table, friction slows it down.
• Magnetic force: It is seen in the way magnets and electric charges attract or repel each other.
• Normal force: It is the supporting force applied to an object that is in contact with another stationary object. For example, a book placed on a table.

Newton's laws of motion

Newton's first law: The law of inertia

This law states that an object remains at rest or in uniform motion in a straight line unless a force is applied to it. Simply put, things do not naturally change their state of motion.

Example: A ball placed on a level ground stays there unless it is kicked.

Newton's second law: The law of acceleration

This law tells how the velocity of an object changes when an external force is applied to it. The equation of this law is:

F = ma

Where F is the applied force, m is the mass of the object, and a is the acceleration.

Example: If you push two different weights with the same force the lighter weight will move faster.

Newton's third law: Action and reaction

This law states that for every action there is an equal and opposite reaction. This means that forces always come in pairs.

Example: When you jump, your feet apply a force to the ground, and the ground exerts an equal and opposite force pushing you up.

Friction: A special force

Friction is a force that resists motion between two surfaces in contact. It plays an important role in everyday life.

Factors affecting friction

The amount of friction depends on two main factors:

• Type of surface: Smooth surfaces have low friction, while rough surfaces have high friction.
• Normal force: Friction increases when surfaces are pressed together by a greater weight or force.

Friction is beneficial while moving, but it can also cause wear and tear on machinery.

Balanced and unbalanced forces

Balanced forces

When the forces acting on an object are equal in size but opposite in direction, we say that the forces are balanced. Balanced forces do not change the motion of an object.

Unbalanced force

If the forces acting on an object do not cancel each other out, resulting in a net force, they are unbalanced. Unbalanced forces cause a change in motion.

Example: Consider a tug of war game:

• If both teams pull equally, the forces balance out, and the rope doesn't move.
• If one team pulls harder, the forces become unbalanced, and the rope moves in the direction of the greater force.

Discovery of simple machines

Simple machines make work easier by allowing humans to apply less force. These include levers, pulleys, screws, inclined planes, wedges, wheels, and axles.

Lever: A rigid bar that pivots on a point called a fulcrum. Example: seesaw.

Pulley: A wheel with grooves for a rope or cable. Example: Flagpoles use pulleys to raise a flag.

Inclined plane: A flat surface that is tilted at an angle to help lift heavier loads with less force. Example: A ramp.

Conclusion

Motion and force are fundamental concepts in physics that explain how the world around us works. By understanding these concepts, students can better understand the activities they see and do every day. Whether it's cars on the highway or children playing on a seesaw, motion and force are involved. The interplay between motion and forces enables us to analyze the mechanics of nearly all physical interactions. As students study and experiment, they develop a deep intuition for these principles, which lays the foundation for more advanced study in physics.

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