Class 9 (Science assignment)
Force and Motion
Force and motion are important concepts in our everyday life. A force is a push or pull that can change the state of rest or motion of an object. It can also change the direction, speed, or shape of an object. The SI unit of force is newton (N). Force is a vector quantity because it has both magnitude and direction.
The motion of an object is described in terms of distance, displacement, speed, velocity, and acceleration. Distance is the total path travelled by an object, whereas displacement is the shortest distance between its initial and final positions in a particular direction. Speed is the distance travelled per unit time, while velocity is displacement per unit time. Acceleration is the rate of change of velocity with time.
According to Newton's laws of motion, an object remains at rest or continues to move with uniform velocity unless an external force acts on it. This property of an object is called inertia. Newton's second law states that the force acting on an object is directly proportional to the product of its mass and acceleration, expressed as F = ma. Newton's third law states that for every action, there is an equal and opposite reaction. These principles can be observed in many situations. For example, when a person pushes a wall, the wall exerts an equal and opposite force on the person.
Forces may be balanced or unbalanced. Balanced forces do not change the state of motion of an object, while unbalanced forces can change its motion. Friction is a force that opposes the relative motion between two surfaces in contact. Although friction can cause wear and energy loss, it is also useful for walking, writing, braking vehicles, and gripping objects.
Understanding force and motion helps us explain the movement of vehicles, falling objects, sports activities, and many other physical phenomena in our daily lives.
A. Short-Answer Questions
1. Conceptual
What is force? Write its SI unit and mention any two effects of force.
2. Conceptual
Differentiate between distance and displacement with one suitable example.
3. Conceptual
What is inertia? Explain with an example from everyday life.
4. Conceptual
State Newton's three laws of motion in brief.
5. Numerical Question
A car travels a distance of 150 m in 10 seconds. Calculate its average speed.
6. Numerical Question
A force of 40 N is applied to an object of mass 8 kg. Calculate the acceleration produced in the object.
7. Application/Reasoning
Why is friction necessary for walking and stopping vehicles? Explain with suitable examples.
B. Multiple-Choice Questions
1. Which of the following is the SI unit of force?
A. Joule
B. Newton
C. Watt
D. Pascal
2. Which of the following is a vector quantity?
A. Distance
B. Speed
C. Mass
D. Velocity
3. The tendency of an object to resist a change in its state of rest or motion is called:
A. Momentum
B. Acceleration
C. Inertia
D. Friction
4. According to Newton's second law of motion:
A. F = m/a
B. F = ma
C. F = a/m
D. F = m + a
5. A body travels 100 m in 20 seconds. Its average speed is
A. 2 m/s
B. 5 m/s
C. 20 m/s
D. 2000 m/s
6. Which law of motion is commonly known as the law of action and reaction?
A. Newton's first law
B. Newton's second law
C. Newton's third law
D. Law of gravitation
7. Which force opposes the relative motion between two surfaces in contact?
A. Gravitational force
B. Magnetic force
C. Frictional force
D. Electrical force
8. If the velocity of an object changes with time, the object is said to have:
A. Mass
B. Acceleration
C. Inertia
D. Density
9. Which situation best demonstrates Newton's third law?
A. A book lying on a table
B. A ball slowing down due to friction
C. A swimmer pushing water backward and moving forward
D. A stationary object remaining at rest
10. Two equal and opposite forces acting on an object are called:
A. Unbalanced forces
B. Balanced forces
C. Frictional forces
D. Magnetic forces
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