# 2Qs - Average Frictional Force & Velocity

• glasgowm
In summary, there is a formula that can be used to calculate the velocity of a pellet before it strikes the putty, and another formula that can be used to calculate the average frictional force required for a car to come to rest after skidding on a patch of ice. Both methods involve applying previous knowledge and considering factors such as distance, velocity, and mass.
glasgowm
I'm doing revision work from the beginning of the year and I can't find the formulas for these two questions. -

1) Calculate the velocity of the pellet before it strikes the putty

2) Calculate the average frictional force it takes for the car to come to rest if its traveling at 15ms and comes to a rest after 20m.

It would be useful if you could quote the complete question(s) along with your attempts. Often one can find a formula for a particular situation by applying previous knowledge.

2) Two cars, traveling in the same direction, skid on a patch of smooth, level ice Car A, of mass 1400kg skids straight into the back of Car B of mass 1000kg.
The two cars join.
immediatly before the impact car B was moving with a speed of 8ms
Immediatly after the impact, both cars are moving with a speed of 15ms

I got the speed of car A before the impact to be 20ms using m1u1 + m2u2 = (m1m2)V

after a collision the cars leave the patch of ice and continue skidding along the road, the come to a rest in a distance of 20m after leaving the ice
Calculate the average frictional force

glasgowm said:
2) Two cars, traveling in the same direction, skid on a patch of smooth, level ice Car A, of mass 1400kg skids straight into the back of Car B of mass 1000kg.
The two cars join.
immediatly before the impact car B was moving with a speed of 8ms
Immediatly after the impact, both cars are moving with a speed of 15ms

I got the speed of car A before the impact to be 20ms using m1u1 + m2u2 = (m1m2)V
Looks good to me.
glasgowm said:
after a collision the cars leave the patch of ice and continue skidding along the road, the come to a rest in a distance of 20m after leaving the ice
Calculate the average frictional force

F=ma.

Since you know the mass, you're really looking for the average acceleration (decelleration in this case).

So what you need is an equation that, given distance, initial and final velocity, gives you acceleration.

uhh

A = v-u / t is the only one I can think of but that doesn't involve distance?

Here's what I done.

---
distance / speed
20 / 15
1.3333s = t
a = -15ms
F = ma
= 2400 * 15
= 36000N

Last edited:
glasgowm said:
uhh

A = v-u / t is the only one I can think of but that doesn't involve distance?

Have you ever seen the formula
D = (V2 - V02) / 2a

Alternatively, one could consider the kinetic energy of the cars after collision and equate this with the work done by the average frictional force. Both methods are valid.

## 1. What is the equation for calculating average frictional force?

The equation for calculating average frictional force is F = μN, where F is the force of friction, μ is the coefficient of friction, and N is the normal force.

## 2. How do you determine the velocity in the equation for average frictional force?

The velocity in the equation for average frictional force is determined by the speed at which the two surfaces are moving relative to each other. This can be measured by taking the change in position over a specific time interval.

## 3. How does the coefficient of friction affect the average frictional force?

The coefficient of friction is a measure of the amount of resistance between two surfaces in contact. The higher the coefficient of friction, the greater the average frictional force will be.

## 4. Can the average frictional force be negative?

No, the average frictional force cannot be negative. Friction always acts in the opposite direction of motion, so it will always have a positive value.

## 5. How does the surface area of contact affect the average frictional force?

The larger the surface area of contact between two surfaces, the greater the average frictional force will be. This is because there is more surface area for friction to act upon, resulting in a greater resistance to motion.

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