Mechanics question

Question:
Eddy cycles up a hill. His mass, together with his bicycle, is 92 kg. His driving force is 125 N and resistance from friction is 45 N. Eddy travels 350 metres along the road which rises through a vertical height of 32 metres. His starting speed is 8.2 m s−1. Find his final speed.

I know the formulae but I cant get the right answer.

OP

help

Original post by Rico07

Question:
Eddy cycles up a hill. His mass, together with his bicycle, is 92 kg. His driving force is 125 N and resistance from friction is 45 N. Eddy travels 350 metres along the road which rises through a vertical height of 32 metres. His starting speed is 8.2 m s−1. Find his final speed.

I know the formulae but I cant get the right answer.

What have you tried?

Find the gain in KE by considering energy and work done
Hence find final speed

Reply 3

marniedavies03

12

what’s the answer (don’t wanna give u the wrong method)?

Reply 4

LLmed

9

You’ll need to use a mix of SUVAT and F=MA. What is the answer, I’ll do it in a sec and see if I can get the correct answer for you.

Reply 5

Rico07

OP

3

Original post by BobbJo

What have you tried?

Find the gain in KE by considering energy and work done
Hence find final speed

Thats where im struggling. Is it rght to say Final Kinetic + Final GPE = Initial Kinetic - Energy lost ?

Reply 6

username3249896

12

Original post by Rico07

Thats where im struggling. Is it rght to say Final Kinetic + Final GPE = Initial Kinetic - Energy lost ?

No
Final KE = Initial KE + Work done by driving force - Work done against friction - gain in GPE

Reply 7

Rico07

OP

3

Original post by BobbJo

No
Final KE = Initial KE + Work done by driving force - Work done against friction - gain in GPE

Ah that makes sense. Thank you thats all I needed.

Original post by Rico07

Question:
Eddy cycles up a hill. His mass, together with his bicycle, is 92 kg. His driving force is 125 N and resistance from friction is 45 N. Eddy travels 350 metres along the road which rises through a vertical height of 32 metres. His starting speed is 8.2 m s−1. Find his final speed.

I know the formulae but I cant get the right answer.

Original post by Rico07

Ah that makes sense. Thank you thats all I needed.

The content of energy concept in A level is usually easy but it is a well-known fact that many teachers are not teaching it properly for students to approach energy or work problems systematically.

I understand that you had already worked out the answer to the question. But I suggest that you try to approach such question in a systematic way in future.

The general approach is:
1. Choose a system based on the problem
2. Write down the conservation of energy equation for the system (avoid writing initial total energy is equal to final total energy as this tends to cause some confusion. Students who write such equation usually don’t know what is the system that they have chosen.)

Original post by Rico07

Thats where im struggling. Is it rght to say Final Kinetic + Final GPE = Initial Kinetic - Energy lost ?