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# M2 Work/Energy/Power watch

1. The total mass of a man and his cike is 80kg. The man freewheels down a slope inclinded at an angle x to the horizontal, where sinx = 1/7, with constant acceleration of magnitude 0.9ms^-2

a) Prove that the total magnitude of the resistive forces opposing the motion is 40N.

I get 184(although if I change the sign of one of them I get the correct answer). Also, if it has a constant acceleration, does it have a constant driving force?
2. If the mass is constant, then the resultant is constant, and if the slope is constant then yes, constant driving force.
3. (Original post by imasillynarb)
The total mass of a man and his cike is 80kg. The man freewheels down a slope inclinded at an angle x to the horizontal, where sinx = 1/7, with constant acceleration of magnitude 0.9ms^-2

a) Prove that the total magnitude of the resistive forces opposing the motion is 40N.

I get 184(although if I change the sign of one of them I get the correct answer). Also, if it has a constant acceleration, does it have a constant driving force?
Ok i can do this questions!!!

Right - as the man freewheels, the driving force is the man's weight, as this is helping him go down the slope

So

Resultant force = ma
Df - R = ma
(80 x 9.8 x sin(sin-1 1/7) - R = (80 x 0.9)
112 - R = 72
R=112 - 72
R = 40

In answer to the second part of your question regarding whether the driving force remains constant if the acceleration remains constant - i would assume so (-though not entirely sure!!!) - but in this question it is not relevant as like i said vefore the driving force helps the man down the hill

Hope this helps
4. (Original post by Silly Sally)
Ok i can do this questions!!!

Right - as the man freewheels, the driving force is the man's weight, as this is helping him go down the slope

So

Resultant force = ma
Df - R = ma
(80 x 9.8 x sin(sin-1 1/7) - R = (80 x 0.9)
112 - R = 72
R=112 - 72
R = 40

In answer to the second part of your question regarding whether the driving force remains constant if the acceleration remains constant - i would assume so (-though not entirely sure!!!) - but in this question it is not relevant as like i said vefore the driving force helps the man down the hill

Hope this helps
Cheers
5. b) Find the time required for the man to cover 180m from rest.

The man cycles up the same slope at a constant speed of 6 m/s, the resistive forces remain unchanged.
c) Find, in W, the power that must be exerted by the man.
d) If the man suddenly increased his work rate by 240W, fin the magnitude of his acceleration up the slope at this instant

Done b and c, cant do d. Help please!
6. (Original post by imasillynarb)
b) Find the time required for the man to cover 180m from rest.

The man cycles up the same slope at a constant speed of 6 m/s, the resistive forces remain unchanged.
c) Find, in W, the power that must be exerted by the man.
d) If the man suddenly increased his work rate by 240W, fin the magnitude of his acceleration up the slope at this instant

Done b and c, cant do d. Help please!
What was his initial power, iue. part c's answer? that would save me working that out! then it would be (well assuming the intial resistances do not include force due to gravity):

Power = driving force x speed
find D

then use F = MA to find a!
7. (Original post by Hoofbeat)
What was his initial power, iue. part c's answer? that would save me working that out! then it would be (well assuming the intial resistances do not include force due to gravity):

Power = driving force x speed
find D

then use F = MA to find a!
62kw sorry, whats D again?
8. (Original post by imasillynarb)
62kw sorry, whats D again?
Oops, thats a different question, its 912W sorry again!
9. (Original post by imasillynarb)
b) Find the time required for the man to cover 180m from rest.

The man cycles up the same slope at a constant speed of 6 m/s, the resistive forces remain unchanged.
c) Find, in W, the power that must be exerted by the man.
d) If the man suddenly increased his work rate by 240W, fin the magnitude of his acceleration up the slope at this instant

Done b and c, cant do d. Help please!
Do you have an ans? Cos i have an ans myself, but not sure is it is right

Also is the ans to part b 20 sec and the ans to part c 912W?
10. (Original post by Silly Sally)
Do you have an ans? Cos i have an ans myself, but not sure is it is right

Also is the ans to part b 20 sec and the ans to part c 912W?
yeh thats what I got for b and c, what did you get for d?
11. (Original post by imasillynarb)
b) Find the time required for the man to cover 180m from rest.

The man cycles up the same slope at a constant speed of 6 m/s, the resistive forces remain unchanged.
c) Find, in W, the power that must be exerted by the man.
d) If the man suddenly increased his work rate by 240W, fin the magnitude of his acceleration up the slope at this instant

Done b and c, cant do d. Help please!
Ok for part d i ot 0.5 ms^-2

How i did it:

Well from part c, the ans is 912 W

As the power has increased by 240W, new power= 912+240 = 1152

You know the speed is 6 so you find driving force usinf power = Df x speed

I got the driving force as 192 N

Total resistance is 152N

so: Df - R = ma
192 - 152 = 80a
40 = 80a
a = 0.5

can some one tell me wther it is right ans?
12. (Original post by Silly Sally)
Ok for part d i ot 0.5 ms^-2

How i did it:

Well from part c, the ans is 912 W

As the power has increased by 240W, new power= 912+240 = 1152

You know the speed is 6 so you find driving force usinf power = Df x speed

I got the driving force as 192 N

Total resistance is 152N

so: Df - R = ma
192 - 152 = 80a
40 = 80a
a = 0.5

can some one tell me wther it is right ans?
Its right, cheers, although, how is the total resitance 152 ??
13. (Original post by imasillynarb)
Its right, cheers, although, how is the total resitance 152 ??
Well, you know the resistance due to motion is 40 N as you worked it out in the part a, then you add to this the resistance due to weight which is
(80 x 9.8 x sin(sin-1 1/7) = 112N

Therefore total resistance = 40 + 112
= 152N

14. (Original post by Silly Sally)
Well, you know the resistance due to motion is 40 N as you worked it out in the part a, then you add to this the resistance due to weight which is
(80 x 9.8 x sin(sin-1 1/7) = 112N

Therefore total resistance = 40 + 112
= 152N

Thanks alot, are you doing further maths?
15. (Original post by imasillynarb)
Thanks alot, are you doing further maths?
NOOOOOOOOOOOOOOOOOO

NEVER!!!!

I have much of a problem with "proper" maths as it is!!!
16. (Original post by imasillynarb)
Thanks alot, are you doing further maths?
17. (Original post by Silly Sally)
NOOOOOOOOOOOOOOOOOO

NEVER!!!!

I have much of a problem with "proper" maths as it is!!!
So youve got your M2 exam on the 25th as well then?
18. (Original post by imasillynarb)
So youve got your M2 exam on the 25th as well then?
Yep - can't wait!!!

So i take it that you are also doing Edexcel?
19. What other subjects are you doing by any chance?
20. (Original post by Silly Sally)
What other subjects are you doing by any chance?
Physics and Chemistry, I take it youve been revising m2 alot?

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