Surface tension is often calculated using a machine that lifts a wire ring from the surface of a liquid. In this case the ring and liquid have some cohesive forces and attract rather than repel. In order to lift a ring of radius 2.75 cm off of the surface of a pool of blood plasma, a vertical force of 2.00*10-2 N greater than the weight of the ring is required. Consider the situation just before the ring breaks contact with the blood plasma where the blood plasma makes a contact angle of approximately zero degrees along the circumference of the ring and is stretched down vertically on both sides of the ring.

Required:
Calculate the surface tension of blood plasma from this information.

Answer:

0.116 N/m

Explanation:

Since the net force acting on the ring must be greater than 2.00 × 10⁻² N, and the surface tension T = F/L where F = net force = 2.00 × 10⁻² N and L = circumference of ring = 2πr where r = radius of ring = 2.75 cm = 2.75 × 10⁻² m.

So, T = F/L

= F/2πr

= 2.00 × 10⁻² N ÷ 2π(2.75 × 10⁻² m)

= 1/2.75π  N/m

= 1/8.64 N/m

= 0.116 N/m

4. While cleaning your bedroom, you move your mattress to vacuum underneath your bed. You use a force of 48 N to move the mattress 1.5 meters out of the way. How much work was done?