What is friction? A car of mass 750 kg is travelling east on a level road - Leaving Cert Physics - Question a - 2007

To find the net force acting on the car, we use Newton's second law of motion:

$F_{net} = m imes a$

Where:

• $m = 750 \, kg$ (mass of the car)
• $a = 1.2 \, m/s²$ (acceleration)

So, substituting the values:

$F_{net} = 750 imes 1.2 = 900 \, N \text{ (east)}$

To calculate the force of friction, we first note the force exerted by the engine:

$F_{engine} = 2000 \, N$

Using the equation for net force:

$F_{net} = F_{engine} - F_{friction}$

We can rearrange this to find the force of friction:

$F_{friction} = F_{engine} - F_{net}$

Substituting the known values:

$F_{friction} = 2000 - 900 = 1100 \, N \text{ (west)}$

When the engine is turned off, the deceleration due to friction needs to be calculated:

Using the formula for acceleration:

$a = F_{friction} / m$

Substituting the known values:

$a = 1100 / 750 = 1.47 \, m/s²$

Next, we use the kinematic equation to find the distance:

$0 = v^2 + 2as$

Where:

• $v = 25 \, m/s$ (initial speed)
• $a = -1.47 \, m/s²$ (deceleration, negative because it's slowing down)

Rearranging for $s$ gives:

$s = \frac{v^2}{2 |a|} = \frac{25^2}{2 imes 1.47} \approx 213 \, m$

Thus, the car will travel approximately 213 meters before coming to rest.