chp 4 - essay questions
What is the acceleration of a rock at the top of its trajectory when it has been thrown straight upward? (Is your answer consistent with Newton's second law?)
10 m/s^2 (gravity) downwards. This is consistent with Newton's second law: Fnet = m*a m*a = m*g a = g
Which contains more apples, a 1-pound bag of apples on Earth or a 1-pound bag of apples on the Moon? Which contains more apples, a 1-kilogram bag of apples on Earth or a 1-kilogram bag of apples on the Moon?
A 1 lb bag of apples on the Moon contains more apples. Remember, the unit of pounds (lb) is a measurement of weight. The acceleration due to gravity on the Moon is significantly less than on Earth, so for two bags of apples to have the same weight, the bag on the moon must have more mass (apples). Both bags have the same number of apples. Remember that the kilogram (kg) is a measure of mass, so if the two bags have the same mass, they have the same number of apples.
Free fall is motion in which gravity is the only force acting. Is a skydiver who has reached terminal speed in free fall?
No, the skydiver is not in free fall. The definition of terminal velocity is the state in which the force of air friction equals the falling object's weight. So instead the skydiver is in dynamic equilibrium between the forces of gravity and friction.
Three identical blocks are pulled, as shown, on a horizontal frictionless surface. If tension in the rope held by the hand is 30 N, what is the tension in the other ropes?
Note that 30 N pulls 3 blocks. To pull 2 blocks then requires a 20-N pull, which is the tension in the rope between the second and third block. Tension in the rope that pulls only the third block is therefore 10 N. (Note that the net force on the first block, 30 N - 20 N = 10 N, is the force needed to accelerate that block, having one-third of the total mass.)
In the absence of air resistance, if a ball is thrown vertically upward with a certain initial speed, on returning to its original level it will have the same speed. When air resistance is a factor, will the ball be moving faster, the same, or more slowly than its throwing speed when it gets back to the same level? Why?
The ball will fall slower because as it falls, the ball will gain speed until air resistance equals its weight. Then, acceleration will cease and the ball will fall at a constant speed (at terminal velocity).
When a car is moving in reverse, backing from a driveway, the driver applies the brakes. In what direction is the car's acceleration?
The brakes slow the car down, which means the acceleration is in the opposite direction from the velocity. So in this case, the acceleration would be in the forward direction.
Is a satellite above the atmosphere that circles Earth in free fall?
the satellite is in free fall. It is responding to gravitational force with no opposing frictional force such as air resistance
