Using a horizontal force F, a table is moved across the floor at a constant velocity. Class 9

Using a horizontal force F, a table is moved across the floor at a constant velocity. Class 9

Revise, Reflect, Refine (Page 112-114)

Question 1.

Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table? Class 9

Answer:

Since the table is moving at constant velocity, its acceleration is zero. By Newton's first law, the net force on it is zero. The two horizontal forces acting on the table are the applied force F (forward) and friction (backward). For net force to be zero, friction must equal F in magnitude. Therefore, the frictional force exerted by the floor on the table is equal to F (acting in the direction opposite to motion).

Question 2.

For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statements physically correct. Class 9

(i) If no net force is applied on the ball, the velocity of the ball will remain the same/ increase/decrease.

(ii) If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.

(iii) If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/ decrease.

Answer:

(i) Remain the same. By Newton's first law, with no net force, velocity does not change.

(ii) Increase. A net force in the direction of motion produces acceleration in the same direction, increasing speed.

(iii) Decrease. A net force opposite to the direction of motion produces deceleration, decreasing speed.