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Compare the net force on a heavy trunk when it’s (a) at rest on the floor; (b) being slid across the floor at constant speed; (c) being pulled upward in an elevator whose cable tension equals the combined weight of the elevator and trunk; and (d) sliding down a frictionless ramp.
(a)
The net force on a heavy trunk when it is at rest on the floor.
Answer to Problem 1FTD
The net force on a heavy trunk when it is at rest on the floor is zero.
Explanation of Solution
According to Newton’s first law of motion a body will remain in rest until it is acted upon by external unbalanced force. Therefore the net force acting on a body at rest is zero.
The forces acting on a body are weight of the body which is down ward and normal reaction which is upward. For a body at rest on the floor therefore the normal reaction will be equal to that of weight of the body. Thus net force will be zero.
Conclusion:
Thus, the net force on a heavy trunk when it is at rest on the floor is zero.
(b)
The net force on a heavy trunk when it is being slid across the floor at constant speed.
Answer to Problem 1FTD
The net force on a heavy trunk when it is being slid across the floor at constant speed is zero.
Explanation of Solution
According to Newton’s first law of motion a body will remain in uniform motion until it is acted upon by external unbalanced force. Therefore the net force acting on a body in uniform motion is zero.
The forces acting on the trunk are the weight of the trunk which is acting downward and normal reaction which is acting upward, driving force along the direction of motion, air resistance opposite to the direction of force and friction opposite to the direction of motion. The normal reaction balances weight of the trunk and driving force balances the sum of air resistance and friction. Thus net force acting on a constant speed car is zero.
Conclusion:
Thus, the net force on a heavy trunk when it is being slid across the floor at constant speed is zero.
(c)
The net force on a heavy trunk when it is being pulled upward in an elevator whose cable tension equals the combined weight of the elevator and trunk.
Answer to Problem 1FTD
The net force on a heavy trunk when it is being pulled upward in an elevator whose cable tension equals the combined weight of the elevator and trunk is zero.
Explanation of Solution
According to Newton’s first law of motion a body will remain in uniform motion until it is acted upon by external unbalanced force.
The forces acting on the elevator and trunk is the weight of the body acting downward and tension of the cable acting upward. Since both are same net force acting on the system is zero. Thus total acceleration is zero. Therefore net force on elevator and trunk is zero.
Conclusion:
Thus, the net force on a heavy trunk when it is being pulled upward in an elevator whose cable tension equals the combined weight of the elevator and trunk is zero
(d)
The net force on a heavy trunk when it is sliding down a frictionless ramp.
Answer to Problem 1FTD
The net force on a heavy trunk when it is sliding down a frictionless ramp is parallel to the ramp.
Explanation of Solution
According to Newton’s first law of motion a body will move with acceleration only if there is any net force acting on it.
Normal force balances the perpendicular component of the weight. Therefore, body slides along the slope as a result of parallel component of the weight.
Conclusion:
Thus,the net force on a heavy trunk when it is sliding down a frictionless ramp is parallel to the ramp.
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