• Support PF! Buy your school textbooks, materials and every day products Here!

Braking then speeding

  • Thread starter chawki
  • Start date
  • #1
510
0

Homework Statement


Motorcyclist (total mass 320 kg) stops from a velocity of 36km/h in 25 s without breaking.
The same motorcyclist accelerates in the same conditions from full stop to velocity of 126km/h in 6 s.

Homework Equations


Calculate the constant forward carrying force of the engine, if the forces restricting the motion remain constant. Give the answer in 1 N precision.

The Attempt at a Solution


all i could find out is the accelerations, a1=-0.4m/s2 and a2= 0.58m/s2
and then i guess we will be interested only in a2, means only the second step of accelerating from a full stop. so F=320*0.58=185.6N
and since they asked to give the answer in 1N precision so it will be 186N.
Please tell me if i'm right
 

Answers and Replies

  • #2
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,146
491
No, you are not right, since you must read the problem carefully. There is a retarding force acting while the cycle is accelerating ...what is that retarding force?
 
  • #3
510
0
What retarding force? the friction force you mean?
 
  • #4
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,146
491
What retarding force? the friction force you mean?
Whatever the force is that you calculate in the first part, when the cycle comes to a stop without braking.
 
  • #5
510
0
you mean the friction that brought the cycle to stop? and how do we find that?
 
  • #6
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,146
491
It might be friction, or perhaps the cycle is moving uphill...or a combination thereof.. it doesn't matter....you can calculate that retarding force the same way you tried to calculate the motor force in part 2 ---find the acceleration and the retarding force in part 1 using kinematics and newton 2.
 
  • #7
510
0
Ok well!
FR is a resistive force.

-FR=-m*a1
a1=0-10/25-0
a1=-0.4m/s2

-FR=-m*a1
FR=-320*(-0.4)=128N

During acceleration from a full stop, we have:
F-FR=m*a2

a2=35/6=5.83m/s2

F=m*a2+FR
F=(320*5.83)+128
F=1993.6N
and as we have been asked to give the answer in 1N precision, then F=1994N?
 
  • #8
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,146
491
Yes, correct.:approve:
 

Related Threads on Braking then speeding

  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
7
Views
641
Replies
1
Views
798
Replies
7
Views
4K
  • Last Post
Replies
12
Views
877
  • Last Post
Replies
4
Views
19K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
3
Views
6K
  • Last Post
Replies
3
Views
1K
Top