The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

ANSWERS TO THE REVIEW QUESTIONS OF Home Work 1

R10. 70 pounds to the right.

R11. Force has a magnitude, units and direction associated with it.

R14. 0 newtons.

R15. There is no net force acting on the object.

R17. The table provides 15 N support force in the upward direction. The net force on the book is 0 N.

R18. They are equal and opposite.

R19. Yes, because no net force is acting on the ball.

R20. If an object has zero speed or constant speed and moves in a straight line then it is in equilibrium i.e. no net force acts on the object.

R21. The friction force is 100 N in the direction opposite to the direction of motion.

ANSWERS TO EXCERISES OF Home Work 1

E7. Nothing keeps the probe moving. In the absence of a propelling force it would continue moving in a straight line.

E14. You are initially moving forward, inside the bus, at some constant speed. The bus suddenly slows, while you continue on at the same constant speed. It appears as though you lurch forward. The same idea applies when the bus picks up speed. You are initially moving slower than the bus as it speeds up, therefore, you appear to lurch backwards. This is Newtons's First Law of Motion in action.

E16. No. The force of friction between the cart and the ground is acting to slow down the cart.

E18. The maximum net force occurs when both forces point in the same direction. The net force is then 20N + 12N = 32N. The minimum net force is when the two forces point in opposite directions. The net force is then 20N - 12N = 8N.

E19. If only a single nonzero force acts on an object, it will not be in mechanical equilibrium. There would have to be another or other forces to result in a zero net force for equilibrium.

E20. No, the ball is not in equilibrium at the top of its path. In the absence of any air resistance the only force acting on the ball is gravity. Gravity slows the ball down when going up, and, speeds the ball up when coming down. At the top of its path, gravity is still acting -- there is a net downward force and the ball is not in equilibrium even though the speed is zero.

E21. If the puck moves in a straight line with unchanging speed, the forces of friction are negligible. Then the net force is practically zero, and the puck can be considered to be in dynamic equilibrium.

E22. The painting staging is in mechanical equilibrium therefore its net force is zero, i.e., F(net) = 200 + 200 - 250 - W = 0 therefore W = 150N.

E25. In the left figure, Harry is supported by two strands of rope that share his weight. So each strand supports only 250N, below the breaking point. Total force up supplied by ropes equals weight acting downward, giving a net force of zero and no acceleration. In the right figure, Harry is now supported by one strand, which for Harry`s well-being requires that the tension be 500N. Since this is above the breaking point of the rope, it breaks. The net force on Harry is then only his weight, giving him a downward acceleration of g. The sudden return to zero velocity changes his vacation plans.