Question

14. Students Norma Lee and Rangh are discussing force and motion as discussed in class.

(The question didn't specify if the speed is constant or not, please list both possibilities.) Norma is describing the motion of a cart on a level frictionless track and identifying the

forces acting on the cart. She has listed gravity, the track pushing upward and a push in

the direction of motion as acting when the cart is coasting down the track.

Lee is puzzling over a case of circular motion and how a ball moving in a circle with constant

speed can have a net force acting when the speed is not changing.

  Rangh replies that it might help if they consider what object is exerting each force.

Analyze the three students’ statements, identifying what is wrong or right BASED ON YOUR LAB EXPERIENCE AND DATA and knowledge of physics principles.

Answer 1

1) Norma is right about gravity and the track pushing upward. She is however wrong about a push in the direction of motion since the cart is coasting. A push will give rise to an accelerated motion and not uniform speed.

2) Lee has a valid doubt. The fact is that force not only changes the velocity in magnitude. It also changes the direction of the velocity.

Newton's laws describing the motion of objects states that the object continues to be in its state of rest or straight line motion if no force is applied.

But in uniform circular motion, the direction is changing all the time. The velocity is not always along the x direction or the y direction. It is continuously hanging and this is the result of the force.

3) Rangh is right . Imagine a ball on the end of a string which is being spun around the head by a boy. There is uniform circular motion. Now, if the string is cut, the ball flies away, no more in circular motion. Also, the string slacks and falls down and the hand does not feel any force even if the boy keeps spinning the string around.

Thus, the conclusion is that there was some force that was required for circular motion.

This is actually called the centripetal force.