Download Worksheet #4 over Unit 6 reading: Determining the Net Force

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Determining the Net Force
FOR answers, see the solutions
1. When we draw forces on objects and the forces act along an axis, the forces can be summed co-linearly (forces up and
right are considered positive; forces down and left are considered negative).
When all the forces sum up = 0, then we say the object is in equilibrium. In one case of equilibrium the object is stationary.
All the forces are balanced.
Let’s say that the above box has a mass of 10 kg. On the picture above, label the two forces acting on the box with the
correct amount of force for each one. Make sure you calculate weight correctly using the weight equation.
2. However, there is another case of equilibrium: the object could be moving but at a constant velocity (not accelerating: i.e.
not getting faster or slower). This is also equilibrium.
In this case, let’s say you push the 10 kg box to the right with a force of 200 N. The box moves at constant velocity.
Friction acts to oppose your push equally. On the above picture label all of your forces and put correct amounts of forces
for each one.
3. What happens if the friction is smaller than your push force? Let’s say that friction acts at 100 N instead of 200 N. On
the picture below label all of your forces and put correct amounts of forces for each one.What is the net force now acting
horizontally? How about vertically? How will the object move now??
This situation is now called an
unbalanced (non-equilibrium)
situation. Forces balance in the
y direction but not the x
direction.
Unit 6: Determining the Net Force
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4. Let's say that you push the object but then release your hand so you are not touching it now. The friction on the 10 kg
box is 50 Newtons. On the picture below, draw your three forces, label them, and put in the correct values for each of the
forces.
Is the box in equilibrium now?
How do you know?
What is the ensuing type of motion?
5. Let’s say we have a rocket:
If the rocket has a mass of 1000 kg, what is the upward force (thrust) if
the rocket moves at a CONSTANT VELOCITY?
Draw the force vectors on the rocket (ignore air resistance).
Would this be an equilibrium or non-equilibrium situation?
However, the rocket loses mass over time as propellant is shot out of the rocket. The
thrust stays somewhat constant over time.
What happens now to the weight vector?
What happens to the thrust vector?
Would this be an equilibrium or non-equilibrium situation?
What type of motion would the rocket now have?
Unit 6: Determining the Net Force
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10/31/2013