# How to find the force of friction?

• Matthew M
In summary: So, just solving for F=MA gives me my force of friction since its in the opposite direction?Odd that it says 'smooth' surface, since that usually means no friction!I don't know why they bother telling you it's elastically powered. To solve it, you need to be able to determine the initial speed (whether or not you do so), to do that you have to assume constant acceleration, and for that to be true the elastic must be having no affect.
Matthew M

## Homework Statement

What is the force of friction on the elastic powered car as it rolls across a smooth surface?
GIVENS
Displacement= 16.25m, Change in time= 22.5s, G=9.8m/s, Mass= 1kg
UNKNOWNS
V1=?, A=? Ff=?

## Homework Equations

F=MA, A=V/T, Change displacement=((V1+V2)/2)T)

## The Attempt at a Solution

First, I used the givens using the change in displacement equation to find V1 which I got 1.4m/s , then i subbed it into A=(V2-V1)/t, I got -31.5m/s ( I am not sure why i got a negative? maybe deceleration?) and then i subbed it into F=MA to get -31.5 as my force applied...not sure why this is negative either and not sure where to go from here...

I guess the car is decelerating under the force of friction and is no longer being powered by elastic? And the figures given are for the motion from an initial velocity to rest?

You ought to see that ##-31.5 m/s## should be ##m/s^2## and that ##31.5## is way too high a number. You need to check your calculation.

But, yes, the acceleration should be negative and the car is decelerating.

How did you get V1 again? What change in displacement equation are you talking about with what inputs?
How did you get V2?

PeroK said:
I guess the car is decelerating under the force of friction and is no longer being powered by elastic? And the figures given are for the motion from an initial velocity to rest?

You ought to see that ##-31.5 m/s## should be ##m/s^2## and that ##31.5## is way too high a number. You need to check your calculation.

But, yes, the acceleration should be negative and the car is decelerating.
Hey, i checked the acceleration, i understand that is a high number, but all the numbers are correct... and yes i know it should be m/s^2 that's just a typo my bad... should I have some sort of conversion for meters --> km or is having the units in m fine? and yes the car is coming to a stop from the initial velocity

jbriggs444 said:
How did you get V1 again? What change in displacement equation are you talking about with what inputs?
How did you get V2?
V1 was calculated through the use of kinematics equations, the displacement and time are taken from a test trial. the car went 16.25 M in 22.5 seconds and V2 is zero since the car came to a stop

Matthew M said:
V1 was calculated through the use of kinematics equations, the displacement and time are taken from a test trial. the car went 16.25 M in 22.5 seconds and V2 is zero since the car came to a stop

So, it started at ##1.4m/s##, decelerated at ##-31.5 m/s^2## for ##22.5s## and ended up at rest?

PeroK said:
So, it started at ##1.4m/s##, decelerated at ##-31.5 m/s^2## for ##22.5s## and ended up at rest?
thats what it sounds like yes, it doesn't make much sense to me either...

If your calculations produce nonsense results then either your calculations are wrong, the assumptions that went into them are wrong or the inputs are wrong.

Matthew M said:
thats what it sounds like yes, it doesn't make much sense to me either...

##1.4 \times 22.5 = 31.5##

Are you sure that's the right calculation?

Heres all my calculations
F=MA

F=(1)(-31.5)

F=-31.5NA=ΔV/T

A=(V2-V1)/T

A=(0-1.4)(22.5)

A=-31.5m/s2 (F)
Δd=((V1+V2)/2)Δt

16 ¼ =((V1+0)/2)22.5

(16 ¼/22.5)(2)=V1

1.4 m/s (f) = V1

Matthew M said:
Heres all my calculations
F=MA

F=(1)(-31.5)

F=-31.5NA=ΔV/T

A=(V2-V1)/T

A=(0-1.4)(22.5)

A=-31.5m/s2 (F)
Δd=((V1+V2)/2)Δt

16 ¼ =((V1+0)/2)22.5

(16 ¼/22.5)(2)=V1

1.4 m/s (f) = V1
PeroK said:
##1.4 \times 22.5 = 31.5##

Are you sure that's the right calculation?
AHHH thank you i was supposed to divide! not multiply HAHA thanks for all your help, however from there how do i solve for force of kinetic friction?

Matthew M said:
AHHH thank you i was supposed to divide! not multiply HAHA thanks for all your help, however from there how do i solve for force of kinetic friction?

You already have the relevant equation in your first post. If you know the mass and the acceleration ..

And, force is a vector, so it can be negative too.

PeroK said:
You already have the relevant equation in your first post. If you know the mass and the acceleration ..

And, force is a vector, so it can be negative too.
so just solving for F=MA gives me my force of friction since its in the opposite direction?

Odd that it says 'smooth' surface, since that usually means no friction!
I don't know why they bother telling you it's elastically powered. To solve it, you need to be able to determine the initial speed (whether or not you do so), to do that you have to assume constant acceleration, and for that to be true the elastic must be having no affect.
I advise against finding the initial speed as a number and plugging that into the start of the next calculation. That will accumulate errors. Much better to keep everything algebraic until the end.

## 1. How is the force of friction calculated?

The force of friction is calculated by multiplying the coefficient of friction (µ) by the normal force (N). The formula is Ff = µN, where Ff is the force of friction, and N is the normal force.

## 2. What factors affect the force of friction?

The force of friction is affected by the type of surfaces in contact, the weight of the object, and the roughness of the surfaces. The coefficient of friction also varies depending on the materials in contact.

## 3. How can the force of friction be reduced?

The force of friction can be reduced by using lubricants, such as oil or grease, between the surfaces in contact. Reducing the weight of the object or using smoother surfaces can also decrease the force of friction.

## 4. What is the difference between static and kinetic friction?

Static friction is the force that resists the movement of an object when it is at rest. Kinetic friction, on the other hand, is the force that opposes the motion of an object that is already in motion.

## 5. How does the force of friction affect the motion of an object?

The force of friction acts in the opposite direction of the motion of an object, causing it to slow down and eventually come to a stop. It also helps to maintain the stability and control of an object's movement.

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