Here are the solutions to the physics problems. 9.A A body initially moving at 72 km/h accelerates uniformly to a velocity of 180 km/h in 3 seconds. Calculate its acceleration. 10.A A horizontal force

Here are the solutions to the physics problems.

9.A A body initially moving at $72 \text{ km/h}$ accelerates uniformly to a velocity of $180 \text{ km/h}$ in $3 \text{ seconds}$. Calculate its acceleration.

Step 1: Convert initial and final velocities from $\text{km/h}$ to $\text{m/s}$.

$$u = 72 \text{ km/h} = 72 \times \frac{1000 \text{ m}}{3600 \text{ s}} = 20 \text{ m/s}$$ $$v = 180 \text{ km/h} = 180 \times \frac{1000 \text{ m}}{3600 \text{ s}} = 50 \text{ m/s}$$

Step 2: Use the formula for acceleration.

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

Substitute the values:

$$a = \frac{50 \text{ m/s} - 20 \text{ m/s}}{3 \text{ s}}$$ $$a = \frac{30 \text{ m/s}}{3 \text{ s}}$$ $$a = 10 \text{ m/s}^2$$

The acceleration is $\boxed{10 \text{ m/s}^2}$.

10.A A horizontal force of $12 \text{ N}$ is applied on a wooden block of mass $2 \text{ kg}$ placed on a horizontal surface. It causes the block to accelerate at $5 \text{ m/s}^2$. Determine the frictional force between the block and the surface.

Step 1: Apply Newton's Second Law of Motion.
The net force ($F_{net}$) acting on the block is the applied force ($F_{applied}$) minus the frictional force ($F_f$).

$$F_{net} = F_{applied} - F_f$$

Also, $F_{net} = ma$.
So,

$$F_{applied} - F_f = ma$$

Step 2: Substitute the given values and solve for $F_f$.
Given: $F_{applied} = 12 \text{ N}$, $m = 2 \text{ kg}$, $a = 5 \text{ m/s}^2$.

$$12 \text{ N} - F_f = (2 \text{ kg})(5 \text{ m/s}^2)$$ $$12 \text{ N} - F_f = 10 \text{ N}$$ $$F_f = 12 \text{ N} - 10 \text{ N}$$ $$F_f = 2 \text{ N}$$

The frictional force is $\boxed{2 \text{ N}}$.

11.A A car can be brought to rest from a speed of $200 \text{ m/s}$ in a time of $2 \text{ s}$.