Force and Motion Module 3

Questions and Answers

What makes bodies at rest to move or stop if they are in motion?

Force

What is inertia?

The tendency of objects to stay at rest or to keep moving with the same velocity.

A brick does not move if equal forces are applied on both sides.

True

What is the relationship defined by Newton's second law of motion?

F = ma Signup and view all the answers

The unit of force is called _____ and its symbol is N.

newton Signup and view all the answers

Which of the following can change the direction of a moving object?

Applying a force Signup and view all the answers

What happens to a ball rolling on the ground after moving a certain distance?

It stops. Signup and view all the answers

What is the SI unit of momentum?

kilogram-metre per second (kg m/s) Signup and view all the answers

Can a balanced force produce any acceleration in a body?

No Signup and view all the answers

Give an example from daily life where the shape of an object changes by applying a force.

Pressing an inflated balloon. Signup and view all the answers

What is an unbalanced force?

Forces that change the state of rest or motion Signup and view all the answers

What is the formula for force according to Newton's second law of motion?

F = ma Signup and view all the answers

What is the force exerted by a mass of 100 g on your palm?

1 N Signup and view all the answers

Why does a fielder move his hands backward while catching a cricket ball?

To decrease the time duration for momentum to become zero Signup and view all the answers

What happens to a person when he falls on a cemented floor?

He gets hurt due to a large force. Signup and view all the answers

Why does a karate player break tiles with a single blow?

Because the force is applied over a short time. Signup and view all the answers

According to the second law of motion, if F = 15 N and m = 3 kg, then a = ______.

5 ms–2 Signup and view all the answers

What is the conservation of momentum?

The total momentum remains constant if no external forces act on a system. Signup and view all the answers

What causes a rocket to move forward?

The ejection of hot gases backward. Signup and view all the answers

Why do we fall forward when a moving bus stops suddenly?

Due to inertia. Signup and view all the answers

What is static friction?

The resistive force before the body starts moving. Signup and view all the answers

Why does a fast moving car slow down when its engine is switched off?

Due to the force of friction acting between the wheels of the car and the ground. Signup and view all the answers

Why do we slip when we step on a banana peel?

Because the friction between the banana peel and the ground is very small. Signup and view all the answers

Why are tyres of automobiles treaded?

Treaded tyres provide better grip with the ground due to increased friction. Signup and view all the answers

Why does a porter carrying a heavy load place a round piece of cloth on his head?

To increase the area of contact, thereby decreasing the pressure on his head. Signup and view all the answers

Why does a nail have a pointed tip?

To increase pressure. Signup and view all the answers

Why are shoulder bags provided with broad straps?

To decrease the pressure exerted on the shoulder. Signup and view all the answers

What is the SI unit of pressure?

Pascal (Pa) or Nm–2. Signup and view all the answers

State Newton’s first law of motion.

Everybody continues in its state of rest or uniform motion in a straight line unless compelled by an unbalanced force to change that state. Signup and view all the answers

Define momentum.

Momentum is the product of mass and velocity. Signup and view all the answers

If a body of mass 10 kg moves with a velocity of 7 m/s, what is the momentum of the body?

70 kg m/s. Signup and view all the answers

If a force of 50 N acts on a body of mass 10 kg, what is the acceleration produced in the body?

5 m/s². Signup and view all the answers

State Newton’s third law of motion.

For every action, there is an equal and opposite reaction. Signup and view all the answers

Why is it difficult for a fireman to hold a hosepipe which ejects a large amount of water at a high speed?

Due to the reaction force exerted in the opposite direction of the water flow. Signup and view all the answers

Study Notes

Friction: Advantages and Disadvantages

Greater normal reaction leads to increased frictional force; friction also depends on normal reaction.
Friction is essential for movement; it helps in walking, writing, and construction.
Treaded tyres enhance grip by increasing friction with the road surface.
Friction converts movement energy into heat, leading to energy loss and wear in machines.
Ball bearings reduce sliding friction by replacing it with rolling friction, increasing machine efficiency.
Friction can wear out shoes and surfaces, leading to maintenance challenges.

Thrust and Pressure

Thrust is the force acting perpendicular to a surface due to an object's weight.
Pressure is defined as thrust per unit area (Pressure = Thrust/Area).
SI unit of pressure is Pascal (Pa), named after Blaise Pascal, a mathematician and scientist.

Key Physical Concepts

Unbalanced forces can change an object's motion or state of rest; balanced forces do not alter these states.
Inertia is the resistance of an object to change its state of motion; more mass equals greater inertia.
Newton’s laws of motion describe fundamental principles of force and motion.
Friction opposes motion and varies with the smoothness of surfaces in contact and normal reaction.

Applications of Thrust and Pressure

Cutting tools like knives are designed with sharp edges to concentrate force on a smaller area, increasing pressure.
Increasing the area under a thrust decreases the pressure, as seen in vehicle tyre designs and the construction of foundations.

Practical Examples

Examples of useful friction in daily life include walking, nail fixing, and braking in vehicles.
Examples of undesirable friction include the wear of components in machinery and the need for lubricants.

Inferences from Activities

The depth of penetration when pushing objects into sand depends on the area of contact; smaller areas penetrate deeper.
The efficiency of force application is directly related to how broadly or narrowly force is applied.

Key Formulas and Units

Pressure formula: Pressure = Thrust / Area
SI units: force is measured in Newtons (N), pressure in Pascals (Pa).### Forces and Motion
Opposite forces of different magnitudes acting on an object result in the object moving in the direction of the greater force, referred to as unbalanced forces.
Unbalanced forces can alter an object's state of rest or motion, while balanced forces do not produce any acceleration.

Inertia

Inertia is the tendency of an object to remain at rest or in uniform motion unless acted upon by an external force.
Observable examples include dust particles falling from a carpet when beaten or leaves falling from a shaken tree.

Examples of Inertia in Daily Life

A sprinter continues running after the finish line due to inertia.
Difficulty in extracting tomato sauce from a bottle relates to inertia; a sudden jerk allows the sauce to move while the bottle stops.

Inertia and Mass

Heavier objects exhibit more resistance to changes in their state of motion due to greater inertia, demonstrated by pushing an empty box versus a box filled with books.
Objects like a cricket ball require more force to stop than lighter objects like a table tennis ball due to differences in mass.

Newton’s First Law of Motion

Newton’s First Law states that an object remains in its state of rest or motion unless acted upon by an unbalanced force.
This law is also known as the law of inertia and explains why passengers in a bus fall backward when the bus accelerates suddenly.

Momentum

Momentum (p) is defined as the product of an object's mass (m) and its velocity (v): ( p = mv ).
The unit of momentum is kg·m/s, having both magnitude and direction, aligned with velocity.

Newton’s Second Law of Motion

The second law defines the relationship between force, mass, and acceleration using the formula: ( F = ma ).
Force affects the rate of change of momentum and is proportional to it.
Applications in real life explain how a fielder catches a fast ball by moving their hands backward to reduce the force felt.

Examples of Second Law of Motion

Falling on a hard surface results in a high impact due to a rapid change in momentum compared to a softer surface, which extends the time to stop.
In karate, a fast strike generates significant force over a short period, maximizing impact on the target.

Newton’s Third Law of Motion

This law states that for every action, there is an equal and opposite reaction.
In practical scenarios, a balloon launches in the opposite direction to the escaping air, and a boat moves backward when a person jumps out.

Key Features of Action and Reaction

Action and reaction forces act on different bodies and are equal in magnitude but opposite in direction.
Such forces can be non-contact, exemplified by magnetic interactions.

Conservation of Momentum

The law of conservation of momentum postulates that in the absence of external forces, the total momentum of a system remains constant before and after a collision.### Newton's Laws of Motion and Momentum
Change of momentum relates directly to the applied force: ( F = \frac{p_2 - p_1}{t} )
When ( F = 0 ), momentum remains constant, indicating the conservation of momentum.
An experiment with marbles demonstrates conservation: removing one marble and hitting it shows that the last marble moves while the struck marble stops.

Examples of Collision

Example 10.3: A bullet of mass 0.03 kg fired at 100 m/s results in a rifle recoil velocity of −1.0 m/s.
- Calculation of recoil uses conservation of momentum: ( m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2 ).
- Negative sign indicates opposite movement direction of the rifle compared to the bullet.
Example 10.4: A 5 kg rifle firing a bullet at 250 m/s has a recoil velocity of −1 m/s.
- Solving for bullet mass gives ( m = 0.02 ) kg, illustrating how conservation principles apply in real-life scenarios.

Friction and Motion

Moving objects stop due to friction, which acts in the opposite direction to their motion.
Static friction occurs before an object starts moving, while sliding or kinetic friction takes over during movement.
Sliding friction is typically less than static friction.

Factors Affecting Friction

Surface texture plays a crucial role in friction: smooth surfaces (e.g., glass) exhibit less friction than rough surfaces (e.g., sand).
An activity with a pencil cell on different surfaces illustrates variations in distance traveled due to frictional differences.

Intext Questions

Investigative questions explore concepts such as why water exits a wet cloth when shaken, why passengers lurch forward in a bus stop, momentum differences between loaded and unloaded trucks, calculating momentum, and the technique of boxers moving heads back during punches, providing insights into practical applications of momentum and forces.

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Description

This quiz covers the concepts of force and motion, focusing on the principles of moving objects. It builds on previous lessons about linear motion and introduces new ideas related to the dynamics of movement. Test your understanding and retention of Module 3 material.