Forces in Motion

YOU ARE LEARNING:

Estimating Forces in Transport

Sometimes, we may need to give an estimate for the speed, acceleration and force involved in everyday transportations.

If the typical speed that a person walks at is $1.5\space m/s$ , what do you think might be a typical running speed?

If a typical running speed is $3\space m/s$ , what do you think might be the typical speed that a train travels at?

If a typical speed of a train is $30\space m/s$ , what do you think might be the typical speed that a plane travels at?

Whilst all objects travel at different speeds at different times, sometimes we need to estimate speeds when dealing with large forces and accelerations in transport. We use the symbol $\sim$ to indicate that a value is approximate.

In physics we tend to use metres per second as the unit of speed, where you might be more familiar with miles per hour.

$1mph=0.4m/s$ . Which option best describes how to convert from $mph$ to $m/s$ ?

A) Divide the value by 0.4 B) Divide the value by 2.2 C) Multiply the value by 0.6

Answer A, B or C.

A car is travelling at an average speed of $60mph$ on a motorway. Try to estimate its average speed in $m/s$ . Give your answer written as a whole number only.

An easy way to find an approximate value in $m/s$ if you have an estimate in $mph$ , is to half the value.

The actual value will be less, but you will have a close enough approximation.

Approximately, what is $30mph$ converted to $m/s$ ?

Which option is the best definition of acceleration?

A car accelerates from rest to a typical speed of $\sim\space 25\space m/s$ in 10 seconds. Estimate its acceleration.

Remember that speed and acceleration are two different quantities.

To estimate the acceleration we need to know the change in velocity, and the time taken for the change.

If the car has accelerated from rest to $\sim\space 25\space m/s$ , what is the change of velocity?

The change of velocity is the difference between the starting and end speed.

If an object is not starting at zero, you need to subtract the starting speed from the end speed.

Acceleration is change in velocity over time, so what would you do next with your value of $25\space m/s$ to find an approximate acceleration?

Estimate the acceleration of the car.

A van accelerates from rest to $\sim\space 20\space m/s$ in approximately 10 seconds. Estimate its acceleration.

We can estimate the forces acting on vehicles, using approximate or typical values. Which values do we need to estimate a force causing acceleration of a vehicle?

You can select multiple answers

A lorry has a mass of $\sim\space 30,000\space kg$ . It accelerates from rest to $\sim\space 22\space m/s$ in 60 seconds. Estimate the force needed to accelerate the lorry,

First we must find the acceleration of the lorry. Give your answer to 1 decimal place.

We know that the approximate mass of the lorry is $30,000\space kg$ , and its acceleration is $0.4\space m/s^{2}$ . Estimate the magnitude of the force which caused the acceleration using these values.

You have estimated the force because you used "approximate" or "typical values".

If you know typical values, you can estimate a force!

A typical car has a mass of $\sim\space 1200\space kg$ . Estimate the force needed to accelerate a car at a rate of $2\space m/s^{2}$ .