Dynamics - Forces

bonkysleuth SG

A hollow ball is released from the bottom of a pool. After a short time it rises at a constant speed. Which statement best describes the resultant force that is acting on the ball?

A. Upwards and equal to the ball's weight

B.Upwards and greater than the ball's weight

C. Upwards and less than the ball's weight

D. Zero.

My initial prediction was zero because of the phrase 'constant speed'. However under Newton's first law, an object will continue travelling in a straight line at constant velocity when the forces acting on it are balanced or add up to zero. In this case, the ball was in MOTIONLESS state originally. But when released, it moves. This means that there's a change in state and unbalanced force, isnt it? So I was thinking B might be the answer. Please assist. =D

2nd question. In which of the following is a force of friction necessary?

A when a ball is bouncing off a wall

b.when car is accelerating

c.when drum skin is vibrating

d. when a rocket is taking off.

my answer A because the concept is likeable to that of holding a chopsticks, which require friction.

eagle SG

Answer is D

At the point of constant speed, upward force is equal to downward force. Resultant force = 0. Answer is D.

Initially in motionless state, but there was a resultant upward force. The fluid drag force was almost nil. Hence by N2L, the ball accelerates upwards. As the magnitude of the velocity increases, the fluid drag force increases correspondingly, and thus, the resultant force get lesser and lesser till it becomes zero. At this point, which is what the question is asking, the resultant force is zero.

eagle SG

2nd question, Answer is B. Think of how a car move. Draw the forces acting on the wheel, and you will see that friction is required for the car to move

When a ball bounce off the wall, friction is not necessary required. Just think whether you can bounce a ball off an ice floor.

bonkysleuth SG

2nd question, Answer is B. Think of how a car move. Draw the forces acting on the wheel, and you will see that friction is required for the car to move

When a ball bounce off the wall, friction is not necessary required. Just think whether you can bounce a ball off an ice floor.

http://www2.eng.cam.ac.uk/~hemh/kitchen3.htm

this website says friction is somehow needed to get the ball to bounce back. i am not sure. like what you say, if friction is needed to get a car moving, then wouldn't a rocket need friction to operate too? (maybe it doesn't) haha

eagle SG

http://www2.eng.cam.ac.uk/~hemh/kitchen3.htm

That is because the ball is spinning, and that calls for a different type of analysis which I believe is out of both O and A level syllabus. It requires the understanding of the concept of moment of inertia.

Rocket moves partly because of the law of conservation of momentum. Draw the forces out and you see that no friction helps in its movement.

aiglosicicle SG

question one is a trick question. constant speed = no resultant force. dont bother about the explanation they will never ask just remember... CONSTANT SPEED = NO RESULTANT FORCE. this is the same as the concept of terminal velocity... just reversed and i think eagle explain already.