State Newton's first law of motion - Leaving Cert Physics - Question 6 - 2011

The diagram should include the following forces:

• Gravity (downwards) represented as weight (mg)
• Normal reaction force (perpendicular to the ground)
• Friction force acting opposite to the direction of motion
• Engine force (friction) propelling the car forward

Each force should be clearly labeled with arrows indicating their direction.

To calculate the acceleration, we use the formula:

$a = \frac{v - u}{t}$

Where:

• Final velocity ($v$) = 0 m/s
• Initial velocity ($u$) = 15 m/s
• Time ($t$) = 0.4 s

Substituting the values, we get: $a = \frac{0 - 15}{0.4} = -37.5 \, \text{m s}^{-2}$

Using Newton's second law, we find the net force:

$F = ma$

Where:

• $m = 1400 \, kg$
• $a = -37.5 \, m s^{-2}$

Thus,
$F = 1400 \, kg \times (-37.5 \, m s^{-2}) = -52500 \, N$

The magnitude of the net force is 52500 N acting in the direction opposite to the motion.

Kinetic energy ($KE$) is calculated using the formula:

$KE = \frac{1}{2} mv^2$

Substituting values: $KE = \frac{1}{2} \times 1400 \, kg \times (15 \, m/s)^2 = 157500 \, J$

The kinetic energy of the moving car was converted to other forms of energy during the collision, including heat energy and potential energy (as the car deforms).

During a collision, Newton's first law states that the passenger continues to move forward at the same speed as the car until acted upon by an external force (like the car stopping). This inertia can lead to injury.

To minimize this risk, safety features like seat belts are used. Seat belts exert a force on the passenger, thereby reducing their sudden acceleration and movement, aligning their motion with that of the vehicle to prevent injury.