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All questions of Forces and Energy for Year 8 Exam
What is the standard unit for measuring speed in scientific contexts?- a)Miles per hour (mph)
- b)Kilometres per hour (km/h)
- c)Metres per second (m/s)
- d)Knots
Correct answer is option 'C'. Can you explain this answer?
What is the standard unit for measuring speed in scientific contexts?
a)
Miles per hour (mph)
b)
Kilometres per hour (km/h)
c)
Metres per second (m/s)
d)
Knots
 | Tutorpedia Coaching answered |
Scientists use metres per second (m/s) as the standard unit for speed globally, as it aligns with the metric system. Miles per hour, kilometres per hour, and knots are used in specific contexts (e.g., road travel, aviation), but they are not the standard scientific unit, making options A, B, and D incorrect.
How does a tennis ball change direction when hit by a racket?- a)The racket exerts an unbalanced force on the ball
- b)The ball’s weight overcomes air resistance
- c)The ball’s speed remains constant
- d)The forces on the ball become balanced
Correct answer is option 'A'. Can you explain this answer?
How does a tennis ball change direction when hit by a racket?
a)
The racket exerts an unbalanced force on the ball
b)
The ball’s weight overcomes air resistance
c)
The ball’s speed remains constant
d)
The forces on the ball become balanced
 | Rajesh Roy answered |
Understanding Direction Change in Tennis Balls
When a tennis ball is struck by a racket, it undergoes a significant change in direction due to specific forces acting upon it. The best explanation for this phenomenon is captured in option 'A'.
Unbalanced Forces at Work
- When the racket hits the tennis ball, it applies a force to the ball.
- This force is unbalanced because it is greater than any other forces acting on the ball at that moment, such as gravity or air resistance.
- An unbalanced force results in a net force that causes the ball to accelerate in the direction of the applied force.
Impact of the Racket's Force
- The racket exerts a force that can change both the speed and direction of the ball.
- This force is crucial for creating the desired shot, whether it's a powerful serve or a precise volley.
- The angle at which the racket meets the ball also influences the ball's trajectory, further demonstrating the role of unbalanced forces.
Why the Other Options are Incorrect
- Option 'B': The ball's weight does not directly overcome air resistance; both forces act simultaneously.
- Option 'C': The ball’s speed does not remain constant; it changes due to the racket's influence.
- Option 'D': The forces on the ball are not balanced when the racket hits it; rather, they become unbalanced, leading to a change in motion.
In summary, the change in direction of a tennis ball when hit by a racket is primarily due to the unbalanced force exerted by the racket. This force alters the ball's speed and direction, facilitating various types of shots in the game.
When a tennis ball is struck by a racket, it undergoes a significant change in direction due to specific forces acting upon it. The best explanation for this phenomenon is captured in option 'A'.
Unbalanced Forces at Work
- When the racket hits the tennis ball, it applies a force to the ball.
- This force is unbalanced because it is greater than any other forces acting on the ball at that moment, such as gravity or air resistance.
- An unbalanced force results in a net force that causes the ball to accelerate in the direction of the applied force.
Impact of the Racket's Force
- The racket exerts a force that can change both the speed and direction of the ball.
- This force is crucial for creating the desired shot, whether it's a powerful serve or a precise volley.
- The angle at which the racket meets the ball also influences the ball's trajectory, further demonstrating the role of unbalanced forces.
Why the Other Options are Incorrect
- Option 'B': The ball's weight does not directly overcome air resistance; both forces act simultaneously.
- Option 'C': The ball’s speed does not remain constant; it changes due to the racket's influence.
- Option 'D': The forces on the ball are not balanced when the racket hits it; rather, they become unbalanced, leading to a change in motion.
In summary, the change in direction of a tennis ball when hit by a racket is primarily due to the unbalanced force exerted by the racket. This force alters the ball's speed and direction, facilitating various types of shots in the game.
What is the unit of measurement for the moment of a force?- a)Newton (N)
- b)Metre (m)
- c)Newton metre (Nm)
- d)Newton per metre (N/m)
Correct answer is option 'C'. Can you explain this answer?
What is the unit of measurement for the moment of a force?
a)
Newton (N)
b)
Metre (m)
c)
Newton metre (Nm)
d)
Newton per metre (N/m)
| | Tutorpedia Coaching answered |
The moment of a force is calculated as Moment = Force (in newtons) × Distance (in metres), resulting in the unit newton metre (Nm). Newton is a unit of force, metre is a unit of distance, and newton per metre is not used for moments, making options A, B, and D incorrect.
What happens to an object when the forces acting on it are balanced?- a)It accelerates
- b)It changes direction
- c)It remains at rest or moves at constant speed
- d)It slows down
Correct answer is option 'C'. Can you explain this answer?
What happens to an object when the forces acting on it are balanced?
a)
It accelerates
b)
It changes direction
c)
It remains at rest or moves at constant speed
d)
It slows down
| | Tutorpedia Coaching answered |
Balanced forces are equal in size and opposite in direction, resulting in a net force of zero. According to Newton’s First Law, an object with no net force will either remain at rest or continue moving at a constant speed in a straight line. Options A, B, and D describe scenarios with unbalanced forces.
How does a sharp knife increase pressure when cutting?- a)By increasing the force applied
- b)By reducing the contact area
- c)By increasing the contact area
- d)By reducing the force applied
Correct answer is option 'B'. Can you explain this answer?
How does a sharp knife increase pressure when cutting?
a)
By increasing the force applied
b)
By reducing the contact area
c)
By increasing the contact area
d)
By reducing the force applied
| | Tutorpedia Coaching answered |
Pressure is calculated as Pressure = Force ÷ Area. A sharp knife has a smaller contact area, which increases pressure for the same force, making cutting easier. Increasing force also raises pressure but is not the primary role of sharpness, and increasing area reduces pressure, making options A, C, and D incorrect.
Why is the base of a dam wider than the top?- a)To reduce water flow
- b)To balance the dam’s weight
- c)To increase atmospheric pressure
- d)To withstand higher pressure at greater depth
Correct answer is option 'D'. Can you explain this answer?
Why is the base of a dam wider than the top?
a)
To reduce water flow
b)
To balance the dam’s weight
c)
To increase atmospheric pressure
d)
To withstand higher pressure at greater depth
 | Rahul Kumar answered |
Pressure in liquids increases with depth due to the weight of the liquid above. The dam’s base is wider to provide greater structural strength to resist the higher water pressure at the bottom. It does not reduce flow, affect atmospheric pressure, or balance weight in this context, making options A, C, and B incorrect.
A car travels 200 metres in 40 seconds. What is its speed?- a)5 m/s
- b)8 m/s
- c)10 m/s
- d)2 m/s
Correct answer is option 'A'. Can you explain this answer?
A car travels 200 metres in 40 seconds. What is its speed?
a)
5 m/s
b)
8 m/s
c)
10 m/s
d)
2 m/s
 | Keystone Instructors answered |
Speed is calculated as Speed = Distance ÷ Time. Here, 200 metres ÷ 40 seconds = 5 m/s. Option B (200 ÷ 25 = 8), option C (200 ÷ 20 = 10), and option D (200 ÷ 100 = 2) result from incorrect calculations or assumptions.
What factor increases the rate of diffusion in gases?- a)Lower temperature
- b)Smaller concentration difference
- c)Higher temperature
- d)Reduced particle movement
Correct answer is option 'C'. Can you explain this answer?
What factor increases the rate of diffusion in gases?
a)
Lower temperature
b)
Smaller concentration difference
c)
Higher temperature
d)
Reduced particle movement
| | Rahul Kumar answered |
Higher temperatures increase the speed of gas particles, accelerating diffusion as particles move faster from areas of high to low concentration. Lower temperatures slow diffusion, smaller concentration differences reduce the rate, and reduced particle movement is not a factor, making options A, B, and D incorrect.
What does a horizontal line on a distance-time graph represent?- a)Constant speed
- b)Increasing speed
- c)Stationary object
- d)Decreasing speed
Correct answer is option 'C'. Can you explain this answer?
What does a horizontal line on a distance-time graph represent?
a)
Constant speed
b)
Increasing speed
c)
Stationary object
d)
Decreasing speed
 | Subset Academy answered |
A horizontal line on a distance-time graph indicates that the distance remains constant while time progresses, meaning the object is stationary (speed = 0). A straight upward line represents constant speed, a curved line may indicate changing speed, and a downward line shows movement back toward the starting point, making options A, B, and D incorrect.
What causes a falling object with a parachute to reach terminal velocity?- a)The parachute increases the object’s weight
- b)Air resistance becomes equal to the object’s weight
- c)The parachute reduces atmospheric pressure
- d)The object’s speed increases indefinitely
Correct answer is option 'B'. Can you explain this answer?
What causes a falling object with a parachute to reach terminal velocity?
a)
The parachute increases the object’s weight
b)
Air resistance becomes equal to the object’s weight
c)
The parachute reduces atmospheric pressure
d)
The object’s speed increases indefinitely
| | Tutorpedia Coaching answered |
Terminal velocity occurs when the upward air resistance equals the downward weight of the falling object, balancing the forces. This results in a constant speed with no further acceleration. The parachute does not increase weight, reduce atmospheric pressure, or cause indefinite speed increase, making options A, C, and D incorrect.
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