# Bikes travelling faster

1. Jun 1, 2015

### TT0

1. The problem statement, all variables and given/known data
A cyclist riding a bicycle finds that he needs to pedal harder in order to travel faster. The main reason for this is that:

A the friction between the tyres and the road decreases with speed.

B there is greater air resistance at higher speeds.

C the bike has more inertia at high speed.

D the cyclist exerts a greater force to oppose the motion of the pedals.

2. Relevant equations
None

3. The attempt at a solution
I chose D because to pedal faster means greater force on the pedals but I am not sure.

2. Jun 2, 2015

### Simon Bridge

Lets examine D then - have you ever ridden a bike?
When you push (apply a force to) the pedals - does pushing in the opposite direction that the pedals are moving in make the bike go faster?

3. Jun 2, 2015

### TT0

No, but the other options don't make sense.
A - I think friction increases with speed, and it says you need to pedal harder to travel faster, shouldn't A have an opposite effect then?
B - There is greater resistance at higher speeds but the question says "a cyclist riding a bicycle finds that he needs to pedal harder in order to travel faster." It doesn't say it is harder to maintain a higher speed, so B is irrelevant.
C - I'm not really sure about this one but shouldn't inertia be the same for all speeds?

4. Jun 2, 2015

### CWatters

Most people would assume that "pedal harder" means he has to generate more power. What equation relates Power, force and velocity? Whats the relationship between Drag force and velocity?

5. Jun 2, 2015

### TT0

The question gave me that only

6. Jun 2, 2015

### haruspex

It doesn't say 'accelerate' either. I believe travel faster here means to maintain a higher speed, though I agree it's ambiguous.

7. Jun 2, 2015

### TT0

Ok thanks

8. Mar 21, 2016

### Newsome

A:
• Friction is the force between two objects as they move over one another such as a car's tire and the surface which it is travelling on.
• There are two types of friction, only one which with we will be dealing with. These types are known as static friction and kinetic friction. Static friction is the frictional force required to start an object moving on another surface. Kinetic frictional force is the force to keep the object in motion.

9. Mar 21, 2016

### haruspex

Are you suggesting the answer to the question is option A? Are your two bullet points intended as an argument supporting that?

10. Apr 2, 2016

### Newsome

Yes i am suggest answer is option A

11. Apr 2, 2016

### Staff: Mentor

How do you support your hypothesis that kinetic friction here decreases with increasing speed?

12. Apr 2, 2016

### haruspex

A few clarifications...
Friction is the tangential force between two surfaces which tends to oppose their relative motion.
Static friction is the force opposing attempted relative motion when there is no actual relative motion. To commence relative motion a force greater than the maximum static friction must be applied tangentially.
Kinetic friction is the force opposing actual relative motion. To keep the surfaces in constant relative motion a force equal to the kinetic friction must be applied tangentially in the direction of motion.

In the case of a tyre in rolling contact with a road, there is only static friction. If the cyclist is on the flat at constant velocity, the only reason for such friction is to counter drag. The direction of that friction would be in the direction of motion. Since the drag will be greater at higher speeds, A is certainly false.

Last edited: Apr 2, 2016
13. Apr 2, 2016

### CWatters

Sorry that's incorrect.
Unless the tyre is skidding it's not moving over the surface of the road. You appear to be confusing friction with rolling resistance. Friction is necessary or the bike cannot move (the driven wheel has no traction, try pedalling a road bike on loose gravel).
If you assume the tyres aren't skidding over the surface then only static friction applies to the tyre/road contact.