Fig. 1 shows a small electric cart travelling along a level warehouse floor. Its motor produces a driving force of 500 N. At one speed, friction and air res...
Question 1 Report
Fig. 1 shows a small electric cart travelling along a level warehouse floor. Its motor produces a driving force of 500 N. At one speed, friction and air resistance total 500 N. The cart carries boxes of water and moves past a cooling fan. The operator then adds more boxes but keeps the cart at the same speed.
(a) What is the resultant force at the stated speed? [1]
(b) Describe the motion of the cart at this speed. [1]
(c) When extra boxes are added, which force increases: the weight or the air resistance? [1]
(d) Calculate the new resultant force if resistance becomes 560 N and the motor force stays at 500 N. [2]
(e) What happens to the cart's velocity in this case? [1]
Answer Details
This question tests the link between resultant force and motion. Equal forces in opposite directions give a zero resultant force.
- (a) The motor force is \(500\text{ N}\) forwards and resistance is \(500\text{ N}\) backwards, so the resultant force is 0 N. [1]
- (b) With zero resultant force, the cart travels at constant velocity, meaning steady speed in a straight line. [1]
- (c) Adding boxes increases the cart's mass, so its weight increases. Air resistance depends mainly on motion through air, not directly on the load added. [1]
- (d) Resistance is now greater than the driving force: \[560\text{ N}-500\text{ N}=60\text{ N}\] The resultant force is 60 N backwards. [2]
- (e) A backward resultant force acts opposite to the cart's forward motion, so it slows down; its forward velocity decreases. [1]
Exam reminder: A force opposite to motion does not necessarily make an object move backwards. It first causes deceleration.
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