Download Document

Use this key only for help or to check your answers after completing the student worksheet.
Name:
Date:
Per:
Physical Science, FHS
Rundell
1. A 5,000 kg truck is exerting a forward force of 10,000 N. The truck is also experiencing a 4000 N
frictional force. Remember friction always opposes motion.
Draw a free-body diagram of the truck
What is the Net Force? (show work)
Is the truck in motion? _______ Explain:
Is the truck in equilibrium? _______ Explain:
2. The same truck is parked in a flat parking space.
Draw a free-body diagram of the truck
What is the Net Force? (show work)
Is the truck in motion? _______ Explain:
Is the truck in equilibrium? _______ Explain:
3. The same truck is travelling on a flat road at a constant speed of 50 mph. The engine is exerting a
forward force of 10,000 N. The frictional force on the truck is also 10,000 N.
Draw a free-body diagram of the truck
What is the Net Force? (show work)
Is the truck in motion? _______ Explain:
Is the truck in equilibrium? _______ Explain:
4. Is it possible for a moving object to be in equilibrium? _______ Explain:
5. The same truck is again travelling on a flat road. The engine is exerting a forward force of 10,000
N. The frictional force on the truck is also 12,000 N.
Draw a free-body diagram of the truck
What is the Net Force? (show work)
Is the truck in motion? _______ Explain:
Is the truck in equilibrium? _______ Explain:
6. So, if an object is in equilibrium it is either _____________ or it is moving with _______________.
If an object is not in equilibrium then it must be experiencing a ________________ velocity. A
change in _____________is also known as a________________!
Explain how each of the following scenarios shows the concepts of Inertia, Equilibrium and Net Force.
Discuss each concept for each scenario. Draw a free-body diagram to illustrate your answer.
ATTACH YOUROWN PAPER. WRITE THE QUESTION!
1. A cup of coffee sits just fine without spilling on the dashboard of a car moving at constant velocity.
The car and the cup are moving at constant velocity. Because the velocity is not changing, the forces
must be balanced, meaning no net force. There is no reason for the cup to spill. According to the law
of Inertia, the cup will stay in motion in a straight line unless acted on by an unbalanced force.
Ffriction)
Car
Fapp (engine)
Ffriction)
=
Fapp (engine)
therefore
Fnet = 0
2. A cup of coffee in a car slides across the dashboard and spills when the driver steps on the gas.
As the driver steps on the gas, the car’s velocity changes. It increases. This means the forward
force supplied by the engine must be greater than the frictional force. Because the car’s velocity
increases, while the cup’s inertia makes it tend to stay at the same velocity as it had before, the
coffee slides backwards and spills off the dashboard.
Ffriction)
Car
Fapp (engine)
Ffriction
<F
app (engine)
therefore
Fnet > 0
3. A cup of coffee in a car slides across the dashboard and spills when the driver quickly hits the
brakes.
As the driver applies the brakes, the force of friction on the car becomes much bigger. Assuming he
also takes his foot off the gas then there is now no forward force in the car either. These factors
cause the car to change velocity (slow down) Because the car’s velocity decreases, while the cup’s
inertia makes it tend to stay at the same velocity as it had before, the coffee slides backwards and
spills off the dashboard.
Ffriction)
Car
Ffriction)
Fapp (engine) = 0
>F
app (engine)
therefore
Fnet < 0
4. A bowling ball loses almost no speed as it rolls down the bowling alley and hits the pins.
There is very little friction between the smooth floor and ball. The ball is heavy however which gives
it lots of inertia. It takes the small frictional force a long time to overcome the great inertia and
bring the bowling ball to a stop. The bowling alley is not long enough. If the bowling alley was a mile
long the bowling ball would eventually stop. Friction would win.
Ffriction)
Ball
Fnet is slightly less than zero because friction is the only force working.
5. Given enough distance, even a bowling ball will eventually come to a stop.
There is no forward force to counteract the force of friction (Fnet is less than zero). Over time the
force of friction will overcome the balls inertia and bring it to rest.
Ffriction)
Ball
6. A spaceship in outer space, far from any atmosphere or gravitational influence, travels at a constant
50,000 mph with its engines turned off.
There is no friction force in deep space so no forward force is needed to keep the spaceship moving.
It will stay in motion at constant speed unless acted on by an unbalanced force. Since no forces are
acting on the ship, the net force = 0.
Ship
Ffriction) = 0
Fapp (engine) = 0