Physics 101/102
Forces
Fall 2009
1. State Newton’s Third law of motion in your own words and then as an expression.
2. Think about “defining the system” very carefully and answer the following questions with explanation or justifications.
(a) Can you pull up a pencil?
(b) Can you pull up yourself by your boot straps?
(c) Earth pulls Moon toward Earth. Moon pulls Earth toward Moon. This is called “Gravitational Force” which we will
study in detail in later chapters. Which object pulls more than the other, given that the mass of Earth is much
greater than that of Moon?
(d) You have mass. Therefore, just like the previous question, Earth pulls you toward Earth (downward) and you pull
Earth toward you (upward). Which force is greater?
It is very important to understand which quantities are in the “Scalar” group and which are in the “Vector” group.
We have to treat them differently when we add, subtract, and multiply them.
3. Terminology - give your definition of each:
Scalar –
Vector –
4. Forces are (scalars/vectors). More than one force can be applied to an object at the same time. Why is it important
to find out the net force?
5. Can you add vectors using regular algebra? Why or why not?
6. For Physics 101/102 level, the best way to add vectors is a: mathematically b: graphically.
To add vectors successfully, “accuracy” is the key word.
7. What are the key points to be able to draw vectors accurately if they are to be added?
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Physics 101/102
Forces
Fall 2009
An important reason to have an “accurate” vector diagram in this class and in all physics classes for that matter,
is the diagram helps us to “see” something you could not “see” before. For example, try this problem in your
head to see if you can answer. If you can’t answer in a minute, go to #8.
There is a 10-kg block on a 450 inclined plane. Also, there is 40 N of friction between the block and the slope.
What is the acceleration of the block?
It is very confusing if you try to do everything in your head. There is an easier way to do this problem. But before that,
you have to practice the following exercises. Be patient!
8. Draw all the forces applied to (or acting on) the object in each case. These are very important practices for you to be
able to see applied forces. You must determine the forces based on the conditions given.
Case 1: v = 0 m/sec, a = 0 m/sec2
Case 2: v  0 m/sec, a = 0 m/sec2
Case 3: With no friction
Case 4: With friction, a  0 m/sec2
Physics 101/102
Exercise 4 (Chapter 5)
Case 5: a = 0 m/sec2
Case 6: v = 0 m/sec, a = 0 m/sec2
Focus on the ball
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Physics 101/102
Forces
Fall 2009
Case 7: v = 0 m/sec, a = 0 m/sec2
Focus on the block
Case 8: Free fall
Case9: Terminal Speed
Case 10: Between Free fall and Terminal
speed
9. #8 was to practice determining and drawing “applied forces”. Now, practice adding the vectors to find the resultant
force. Use the parallelogram method discussed in class.
Case 1: v = 0 m/sec, a = 0 m/sec2
Case 2: v  0 m/sec, a = 0 m/sec2
Physics 101/102
Exercise 4 (Chapter 5)
Case 3: With no friction
Case 4: With friction, a  0 m/sec2
Case 5: a = 0 m/sec2
Case 6: v = 0 m/sec, a
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Physics 101/102
Forces
Fall 2009
Focus on the ball
Case 7: v = 0 m/sec, a = 0 m/sec2
Focus on the block
Case 8: Free fall
Case 9: Terminal Speed
Case 10: Between Free fall and Terminal
speed
After being able to draw all applied forces to an object, you can now answer the question that you could not answer
before.
10. There is a 10-kg block on a 450 inclined plane. Also, there is 40 N of friction between the block and the slope. What
is the acceleration of the block?
Before you attack the problem, write down the steps that you should take to solve this kind of problem.
Step1 –
Step 2 –
Step 3 –
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Physics 101/102
Forces
Fall 2009
Step 4 –
Step 5 –
(add more steps as needed)
11. Just to be certain you can do this type of problem here is another one. If you can do this one “from scratch” without
looking at previous notes, you understand. If you have to look back to your notes, you need more study.
A 10.0 kg block is on an inclined plane. If the friction between the block and the plane is 50 N, and the normal force
between the block and incline is 70.7N, what is the acceleration of the block?
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