Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3

SOLUTION

Step 1 : Draw a force diagram

Always draw a force diagram although the question might not ask forit. The

acceleration of the whole system is given, therefore a force diagram of the whole

system will be drawn. Because the two crates areseen as a unit, the forcediagram

will look like this:

15 kg

a = 2 m·s−^2

Applied force = 500 N

10 kg

Friction =?

Step 2 : Calculate the frictionalforce

To find the frictional force we will apply Newton’s Second Law. We are given

the mass (10 + 15 kg)and the acceleration (2m·s−^2 ). Choose the directionof

motion to be the positive direction (to the rightis positive).

FR= ma

Fapplied+ Ff= ma

500 + Ff= (10 + 15)(2)

Ff= 50− 500

Ff=− 450 N

The frictional force is 450 N opposite to the direction of motion (to the left).

Step 3 : Find the tension in therope

To find the tension in the rope we need to lookat one of the two crateson their

own. Let’s choose the 10 kg crate. Firstly, we need to draw a force diagram:

10 kg

(^13) of total frictional force Tension T
a = 2 m·s−^2
Ffon 10 kg crate
Figure 12.3: Force diagram of 10 kg crate
The frictional force on the 10 kg block is one third of the total, therefore:
Ff=^13 × 450
Ff= 150 N
If we apply Newton’s Second Law: