Question

1. unbalanced forces cause an object to:

a. float
b. stretch
c. accelerate
d. remain in equilibrium

1. when an object is sliding, kinetic friction acts:

a. with motion
b. against motion
c. only upward
d. only downward

1. a rope pulls a box with 30 n. friction is 10 n. the net force is:

a. 0 n
b. 10 n
c. 20 n
d. 40 n

Explanation:

Step1: Analyze question 18

According to Newton's second law \(F = ma\), when there are un - balanced forces (\(F
eq0\)), the acceleration \(a
eq0\). Un - balanced forces cause an object to accelerate. Floating is related to buoyancy, stretching is related to elastic force and deformation, and remaining in equilibrium is when forces are balanced.

Step2: Analyze question 19

Kinetic friction is a force that opposes the relative motion of two surfaces in contact. When an object is sliding, kinetic friction acts against the direction of motion. It is not limited to only upward or only downward, and it does not act with the motion.

Step3: Analyze question 20

The net force \(F_{net}\) is calculated by \(F_{net}=F - f\) (assuming the pulling force \(F\) is in the direction of motion and friction \(f\) is opposite to the direction of motion). Given \(F = 30N\) and \(f=10N\), then \(F_{net}=30N - 10N=20N\)

Answer:

1. C. Accelerate
2. B. Against motion
3. C. 20 N