# Skier down a hill- f=ma (is the QUESTION correct?)

#### VidsEpic

3rd time I'm posting in a day.

1. The problem statement, all variables and given/known data

A skier (68 kg) starts from rest but then begins to move
downhill with a net force of 92 N for 8.2 s. The hill levels
out for 3.5 s. On this part of the hill, the net force on the
skier is 22 N [backwards]. T/I
(a) Calculate the speed of the skier after 8.2 s.
(b) Calculate the speed of the skier at the end of the
section where the hill levels out.
(c) Calculate the total distance travelled by the skier before
coming to rest.

Question 8 on the book in the link above ^

2. Relevant equations

f = ma

displacement = .5 * a * t^2

v^2 = u^2 + 2as

3. The attempt at a solution

a) a = f/m = 1.35 m/s/s * 8.2s = 11m/s (Correct according to answer key)

b) same procedure as above, simple physics, answer is 9.86 m/s, rounds to 10 m/s (Correct according to key)

c) For segment A .5*a*t^2 gives 45.5m and for segment B, v^2 = u^2 + 2as gives 32.45m,
but wait, I don't see what happens after that? The question doesn't even tell me when or if the skier comes to rest.

I have 78m so far, am I missing something in the question or is there something wrong with it.

Last edited by a moderator:
Related Introductory Physics Homework News on Phys.org

#### cepheid

Staff Emeritus
Gold Member
3rd time I'm posting in a day.

1. The problem statement, all variables and given/known data

A skier (68 kg) starts from rest but then begins to move
downhill with a net force of 92 N for 8.2 s. The hill levels
out for 3.5 s. On this part of the hill, the net force on the
skier is 22 N [backwards]. T/I
(a) Calculate the speed of the skier after 8.2 s.
(b) Calculate the speed of the skier at the end of the
section where the hill levels out.
(c) Calculate the total distance travelled by the skier before
coming to rest.

Question 8 on the book in the link above ^

2. Relevant equations

f = ma

displacement = .5 * a * t^2

v^2 = u^2 + 2as

3. The attempt at a solution

a) a = f/m = 1.35 m/s/s * 8.2s = 11m/s (Correct according to answer key)

b) same procedure as above, simple physics, answer is 9.86 m/s, rounds to 10 m/s (Correct according to key)

c) For segment A .5*a*t^2 gives 45.5m and for segment B, v^2 = u^2 + 2as gives 32.45m,
but wait, I don't see what happens after that? The question doesn't even tell me when or if the skier comes to rest.

I have 78m so far, am I missing something in the question or is there something wrong with it.
Yeah, this question is a little weird, in the sense that 3.5 s is not enough time for the skier to come to rest from 11 m/s, at that acceleration (on the straightaway). Furthermore, the problem doesn't tell you what happens after the level section, so on the surface it seems like you don't have enough info to answer the question.

If you ignore the bit in part c about "coming to rest" as being a mistake, and just calculate the total distance travelled on the downhill (8.2 s) and level (3.5 s) sections, you DO get 82 m. You have made a mistake in calculating the distance of the second portion (the level part).

Last edited by a moderator:

"Skier down a hill- f=ma (is the QUESTION correct?)"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving