Question

A skater glides to the right (positive direction) in a routine. Which of the following is true about the skater’s horizontal motion during the glide, ignoring the effects of ice friction and air resistance? Explain your answer. a) the skater has negative displacement, b) the skater’s velocity increases, c) the skater experiences a small negative acceleration, d) the skater’s velocity remains constant, e) the skater has zero velocity.

2. How much vertical force (in Newtons) must a patient apply to a 5 kg dumbbell to accelerate it upward at 4.9 m/s?
3. An athletic trainer pushes horizontally on the ankle of the extended left lower leg of a supine athlete to flex the hip and stretch the hamstrings. If the trainer performs another repetition pushing with the same force and direction but on the middle of the lower leg, what effect does this have? (Hint – think about how this is changing the distance between the axis of rotation and the application of force)
4. An athlete doing isometric leg raises must make what hip flexor torque (in Nm) to counter the gravitational torque created by the weight of his legs (-200 N) acting at the center of gravity of the lower extremity (0.5 m distal from hips). Assume the legs are elevated above the floor and oriented horizontally. Show your work. (See sample problem 14.3 for an example similar to this)

Answer 1

CONCEPT REQUIRED- The question requires the concept of motion in a straight line, lets see this in our subsequent steps based on the part of question

SOLUTION -

Q1 - a) Displacement is defined as the net distance covered between final and initial point. Now the skater is moving towards right. So his final position will be ahead of the initial position and hence we can say that the displacment is positive. Mathematically -
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So as the position is ahead of the initial position, the displacement will be positive.

b) We can not say if there will be a net increase or decrease of the velocity because no data is provided as to if he is going up a ramp ( in which case velocity will decrease ) or down a ramp ( in which case velocity will increase). Although its given that we have to ignore air resistance is same and ice friction is to be ignored, so we can safely say that no net fhorizontal force will be exerted, and hence the velocity will remain positive.

c) Again in the absence of a frictional force or resistive force, the net force is nil. And we know that , and so the acceleration will be 0 as oer the given data.

d) The velocity of the skater, as discussed in part c will remain constant. Hence this statement is true.

e)A zero velocity means that the the skater is not moving at all, naturally this is not the case, hence we can safely say that this statement is false.

And that's how we get the asnwer :)