Motion Under Gravity (Vertical Motion)
1. Situation in Figure
A ball is thrown vertically upward from the top of a building.
- Height of building = 25 m
- Initial velocity of ball = 20 m/s upward
- Acceleration due to gravity acts downward
- Take upward direction as positive
Therefore:
- Initial velocity → u = +20 m/s
- Acceleration → a = -10 m/s²
PART 1: Motion from A to B (Upward Motion)
At highest point B:
- Final velocity becomes zero
- Because the ball stops for a moment before coming down
Use first equation of motion:
Substitute values:
Conclusion
Time taken to go upward = 2 seconds
PART 2: Motion from B to C (Falling Down)
Now the ball falls from highest point to ground.
At point B:
- Initial velocity = 0
- Acceleration = -10 m/s²
Maximum height above ground:
(25 m building + 20 m rise)
Use second equation of motion:
Substitute values:
Conclusion
Time to fall downward = 3 seconds
Total Time of Flight
Final Answer
Total time taken by the ball to hit ground.
SECOND METHOD (Shortcut Method)
Use whole motion directly.
Given:
- Initial height = 25 m
- Final height = 0
- Initial velocity = 20 m/s
- Acceleration = -10 m/s²
Use:
Substitute:
Solve quadratic equation:
Important NEET Concepts
1. Sign Convention
If upward is positive:
- Upward velocity → positive
- Downward velocity → negative
- Gravity → negative
2. At Maximum Height
Velocity becomes zero only for an instant.
3. Acceleration Due to Gravity
Near Earth:
For NEET numericals often take:
Free Fall Notes (Example 2.4)
Definition of Free Fall
When an object moves only under gravity (air resistance neglected), it is called free fall.
Important Equations of Free Fall
Velocity-Time Relation
If object released from rest:
Then:
Position-Time Relation
If released from rest and origin at starting point:
Velocity-Position Relation
If released from rest:
NEET Quick Tips
- Gravity always acts downward
- At highest point velocity becomes zero
- Acceleration never becomes zero
- Time going up ≠ time coming down if starting and ending heights are different
Most Important Formula Sheet
| Equation | Formula |
|---|---|
| First Equation | v = u + at |
| Second Equation | s = ut + 1/2 at² |
| Third Equation | v² = u² + 2as |
NEET Memory Trick
“UVS”
Remember:
- (u) → initial velocity
- (v) → final velocity
- (s) → displacement
- (a) → acceleration
- (t) → time
Motion Under Gravity Class 11 Physics Notes for NEET & JEE
 |
| Motion under gravity showing a ball thrown upward from a building with velocity, height, and time calculations. |
CBSE Class 11 Physics: Motion Under Gravity Important Questions and Answers
These exam-oriented questions and answers cover MCQs, very short answer questions, short answer questions, long answer questions, assertion-reason questions, fill in the blanks, case study questions, statement-based questions, and match the columns.
Multiple Choice Questions (MCQs)
Question 1
A ball is thrown vertically upward with an initial velocity of 20 m/s. What is its velocity at the highest point?
Options:
- A) 20 m/s
- B) 10 m/s
- C) 0 m/s
- D) -20 m/s
Answer: C) 0 m/s
Question 2
What is the value of acceleration due to gravity near the Earth's surface?
Options:
- A) 0 m/s²
- B) 9.8 m/s² upward
- C) 9.8 m/s² downward
- D) 20 m/s² downward
Answer: C) 9.8 m/s² downward
Question 3
At the highest point of vertical motion, the acceleration of the body is:
Options:
- A) Zero
- B) 9.8 m/s² upward
- C) 9.8 m/s² downward
- D) Infinite
Answer: C) 9.8 m/s² downward
Very Short Answer Questions
Question 1
What is free fall?
Answer: The motion of an object under the influence of gravity alone is called free fall.
Question 2
What is the velocity of a body at the highest point of its motion?
Answer: The velocity becomes zero at the highest point.
Question 3
What is the SI unit of acceleration?
Answer: metre per second squared (m/s²).
Question 4
Does acceleration become zero at the highest point?
Answer: No. Acceleration remains equal to acceleration due to gravity and acts downward.
Short Answer Questions
Question 1
Why does a ball thrown upward slow down as it rises?
Answer: A ball thrown upward slows down because gravity acts downward, opposite to the direction of motion. This downward acceleration continuously decreases the velocity until it becomes zero at the highest point.
Question 2
Write the equations of motion under gravity.
Answer:
For upward positive direction:
v = u − gt
s = ut − ½gt²
v² = u² − 2gs
where:
- u = initial velocity
- v = final velocity
- g = acceleration due to gravity
- s = displacement
- t = time
Question 3
Differentiate between velocity and acceleration at the highest point.
Answer:
| Velocity | Acceleration |
|---|---|
| Velocity becomes zero. | Acceleration remains 9.8 m/s² downward. |
| Changes direction after reaching the top. | Direction remains downward throughout the motion. |
Long Answer Questions
Question 1
A ball is thrown vertically upward from the top of a building 25 m high with an initial velocity of 20 m/s. Calculate the total time taken to reach the ground. Take g = 10 m/s².
Answer:
For upward motion:
v = u − gt
0 = 20 − 10t
t = 2 s
Therefore, time to reach the highest point = 2 seconds.
Maximum height gained above the building:
h = u² / 2g
h = (20)² / (2 × 10)
h = 400 / 20
h = 20 m
Total height above ground:
25 + 20 = 45 m
For downward motion:
s = ½gt²
45 = 5t²
t² = 9
t = 3 s
Time of descent = 3 seconds.
Total time of flight:
T = 2 + 3
T = 5 s
Final Answer: The ball reaches the ground after 5 seconds.
Assertion and Reason Questions
Question 1
Assertion (A): Velocity becomes zero at the highest point.
Reason (R): Gravity stops acting at the highest point.
Answer: Assertion is true, but Reason is false because gravity acts continuously throughout the motion.
Question 2
Assertion (A): Acceleration due to gravity remains constant during vertical motion.
Reason (R): Gravity acts continuously downward.
Answer: Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
Fill in the Blanks
Question 1: The acceleration due to gravity always acts __________.
Answer: downward
Question 2: At the highest point of motion, velocity becomes __________.
Answer: zero
Question 3: Motion under gravity alone is called __________.
Answer: free fall
Question 4: The SI unit of acceleration is __________.
Answer: m/s²
Case Study Questions
A student throws a ball vertically upward from the top of a building with an initial velocity of 20 m/s. The height of the building is 25 m. Assume g = 10 m/s².
Question 1
How much time does the ball take to reach the highest point?
Answer: 2 seconds.
Question 2
What is the maximum height gained above the building?
Answer: 20 metres.
Question 3
What is the total height above the ground at the highest point?
Answer: 45 metres.
Question 4
What is the velocity at the highest point?
Answer: 0 m/s.
Question 5
What is the total time of flight?
Answer: 5 seconds.
Match the Columns
| Column A | Column B |
|---|---|
| Highest Point | Velocity becomes zero |
| Free Fall | Motion under gravity alone |
| Acceleration Due to Gravity | 9.8 m/s² |
| SI Unit of Acceleration | m/s² |
Answers:
- Highest Point → Velocity becomes zero
- Free Fall → Motion under gravity alone
- Acceleration Due to Gravity → 9.8 m/s²
- SI Unit of Acceleration → m/s²
Important Exam Tip
Remember that at the highest point of vertical motion, velocity becomes zero but acceleration due to gravity remains constant and acts downward. This is one of the most frequently asked concepts in CBSE Class 11 Physics examinations.
Internal Links
Motion in a Straight Line Notes
Kinematics Complete Revision Notes
Difference Between Speed and Velocity
Free Fall Numerical Problems
Acceleration Due to Gravity Explained
Vector Basics for NEET
Projectile Motion Notes
NCERT Solutions – Motion in a Straight Line
Important Kinematics Formula Sheet
Previous Year NEET Kinematics Questions
Example 3.4 Solution Explained for Beginners
Block and Trolley System NEET Solution
Newton's Laws of Motion Notes
Work, Energy and Power Notes
Fundamental Forces in Nature Guide
No comments:
Post a Comment