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FORCE CONCEPT WS (P1)
1)
Two equal forces act on two different objects, one of which has a mass 10 times as large
as the other. How will their respective accelerations compare?
The less massive object will have an acceleration 10 TIMES LARGER than the more
massive object. ( a  1/m )
2)
An object moving horizontally across a table is observed to slow down.
a. Is there unbalanced force acting on the object? Explain.
Yes; since it is accelerating, Newton’s 1st Law states that an unbalanced force
must be acting on it to cause its motion to change.
b. Is the gravitational force involved in accelerating the object in this situation? Explain.
Not directly; the unbalanced force in this case is friction. (Since the amount of
friction depends upon the weight of the object in this case, Fg is indirectly
responsible since it determines the amount of Ff acting.)
3)
A ball hangs from a string attached to the ceiling, as shown in the diagram.
a. What forces act on the ball?
The earth is pulling the ball down (gravitational force) and the string is pulling the
ball up.
b. What is the total force acting on the ball?
Since the ball is not accelerating, the total force acting on it is ZERO.
c. For each force listed in your answer to (a), what is the reaction force given by Newton's
3rd Law?
The reaction force to the earth pulling the ball down is the ball pulling the earth
up.
The reaction force to the string pulling the ball up is the ball pulling the string
down.
4) The acceleration due to gravity on Mars is approximately 1/3 of the value that it has near
the earth's surface
a. If a ball is transported to Mars, will its mass change? Explain.
No, its mass will remain the same. ‘Mass’ is a measure of the amount of stuff in
an object – which does not change when you move from place to place.
b. Will the weight of the ball change in this process? Explain.
Yes, it will decrease. ‘Weight’ is a measure of the gravitational force acting on an
object. Since ‘g’ is smaller on Mars, then there will be a smaller gravitational
force on the object; therefore, it will weigh LESS.
5)
Suppose that a bullet is fired from a rifle out in space where there are no appreciable
forces of gravity. Will the bullet slow down as it gets farther away from the rifle? Explain.
No, it will continue to move forward at a constant speed since no unbalanced forces
act on it.
6)
The engine of a car is internal to the car and therefore cannot push directly on the car in
order to accelerate it.
a. What external force on the car is actually responsible for its acceleration? Explain.
The frictional force between the tires and the road surface is the force directly
responsible. The tires attempt to push the road backward by spinning; friction
causes the road to push forward on the tires, which are connected to the car
body.
b. Why is it difficult to accelerate a car on an icy road?
The coefficient of friction (µ) between rubber and ice is very small; thus, there is
very little frictional force between the tires and the road. The road is unable to
apply a large propulsive force to the car.
7)
Two identical cans are filled with different substances: one with lead shot and the other
with feathers. An astronaut standing on the moon (where there is gravity but no air
resistance) drops the cans.
a. Which can, if either, experiences the greater gravitational force? Explain.
The one filled with lead shot will be more massive and will experience the greater
gravitational force.
b. Which can, if either, experiences the greater acceleration? Explain.
The only force acting on each can is a gravitational force, which is linearly
proportional to the mass of the can. Newton’s second law states that the
acceleration of an object is linearly proportional to the net force acting and
inversely proportional to its mass. Therefore, both objects will experience the
same acceleration. (If the lead shot can is 10 times more massive, it will
experience a net force 10 times greater. Those two factors will cancel each other
in ΣF = ma, causing equal accelerations.)
c. Which can, if either, would require the greater force to stop it after it has fallen a certain
distance? Explain.
The lead shot can will require a greater force to stop it in the same time since it is
more massive. (ΣF = ma)
8)
A sprinter accelerates at the beginning of a race and then attempts to maintain maximum
speed throughout the rest of the race.
a. What external force is responsible for accelerating the runner at the beginning of the
race? Explain how this force is produced.
The ground pushes the runner’s feet forward. The runner’s foot utilizes friction
to push backward on the ground. Newton’s 3rd Law states that the ground
applies an equal and opposite force to the runner’s foot.
b. Once the runner reaches his maximum speed, is it necessary to continue pushing
against the track in order to maintain that velocity? Explain.
Yes, it is. The runner’s body experiences an air resistance force acting against
his motion. The runner must get the track to push forward on him to cancel out
this air resistance force.
9)
An adult eagle and his baby are flying across the sky at the same speed. Which bird is
experiencing a greater amount of air resistance?
The force of air resistance depends upon the speed of an object and its aerodynamic
design. Since the speeds are equal and the dad’s surface area is larger, he
experiences the greater air resistance force.
10) When will terminal velocity be reached for a freefalling object? What will be the object’s
motion after that point?
It will happen when Fair = Fg. At that point the object will continue to move
downward at a constant speed.
11) An object is moving and the air resistance on it is increasing. What does this tell you about
its speed?
Its speed must be increasing. ( Fair  v² )
12) If you want to give an object a lower terminal velocity, what should you do to it?
Give it a less aerodynamic design, or take some mass away without changing its
shape or size.
13) A solid lead ball and a solid aluminum ball, each one inch in diameter, are released
simultaneously and allowed to fall 500 feet to the earth below. Of course, the lead ball will
reach the ground first.
a. Which of these balls achieves the greater terminal velocity?
Since it reaches the ground first, the lead ball will achieve the greater terminal
velocity.
b. Which of these balls experiences less air resistance at terminal velocity? Explain your
reasoning.
The aluminum ball must weigh less than the lead ball, and terminal velocity
occurs when Fair = Fg. Thus, the aluminum ball must experience less air
resistance.