Download FORCE CONCEPT WS – honors

FORCE CONCEPT WS – honors
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 x more massive, it will experience a
net force 10x greater. Those two factors will cancel each other in Fnet = 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. (Fnet = 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)
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.
10)
11)
If you want to give an object a lower terminal velocity, what should you do to it?
1)
Give it a less aerodynamic design.
2)
Take some mass away without changing its shape or size.
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
12)
v² )
A lead ball and an aluminum ball, each one inch in diameter, are released simultaneously and allowed to fall
to the earth. Which of these balls, if either, reaches the ground first? Explain.
The lead ball will reach the ground first since it will attain the greater terminal
velocity.
At their respective terminal velocities, the lead ball will be experiencing a greater air
resistance force (Fair = Fg at that point). Since both balls have the same aerodynamic
design, the lead ball will experience a greater Fair by moving through the air at a
greater speed.
13)
Two identical eagles are shot by two poachers at the same instant and fall to the ground. Eagle #1 falls with
its wings outstretched while eagle #2 falls with its wings tucked against its body. Which hits the ground first?
Explain.
Eagle #2 will hit the ground first since it will attain the greater terminal velocity.
Since both eagles have the same mass – being that they are identical – they will
experience the SAME air resistance force at their respective terminal velocities (F air
= Fg at that point). Since eagle #2 has a more aerodynamic design, the only way it
can experience the same air resistance force as eagle #1 is to be moving through the
air at a greater speed.