Unit 3 Objectives: Forces and Laws of Motion
Category One: Inertia- Newton’s First Law of Motion
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What is a force and what is the unit for representing force?
State Newton’s First Law of Motion. Give several examples.
Describe inertia. Explain what mass and inertia have in common.
What is the difference between the weight of an object and the mass of an object?
Explain what causes friction. How could one reduce the friction an object experiences?
What is the weight in Newtons of an object whose mass is 40 kg?
The gravitational attractive force on the moon is 1/6 that of earth giving objects an acceleration of 1.67
m/s2 (in other words, on the moon g = 1.67 m/s2). What would be the mass of a 150-kg TV set on the
moon?
8. Using the concept of combining vectors, determine the net force when a 6 N and 8 N interact with each
other directly against each other, at right angles, and when the forces are applied in the same direction.
9. Apply the concept of inertia to the importance of using seatbelts.
10. An astronaut happens to have a bowling ball in outer space. She throws the ball. Since there is
negligible influence of earthly gravity where she is, use the concept of inertia to describe the motion of
the bowling ball.
11. A box is on a desk. What happens to the normal force applied to the box from the deks if I
a. Push down on the box?
b. Pull up on the box?
Category Two: Newton’s Second Law of Motion
State Newton’s Second Law of Motion, write the formula and give examples.
What amount of force was required to give a 40-kg wagon an acceleration of 3 m/s2?
Why the relationship between mass and acceleration is inversely proportional?
When mass remains constant, what type of relationship will be apparent between force and acceleration?
How can an object experience more gravitational force but having the same gravitational acceleration of
an object with less mass (assuming no air resistance)?
6. Use the concept of “net force” to explain how an object reaches its terminal velocity.
7. Identify what happens to the acceleration of a freely falling object as it approaches its terminal velocity.
Describe the acceleration of an object that has reached its terminal velocity.
8. A 50- N branch falls from a tree and reaches its terminal velocity. What is the mass of the branch?
What is its weight? What is the force of air resistance against the branch when it reaches its terminal
velocity?
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Category Three:
1. State Newton’s Third law of Motion and give examples.
2. Describe and draw an example of an action and reaction force pair.
3. Identify the action and reaction force for the following: a man walking, a woman jumping, a dog sitting
on the floor, and a child throwing a ball.
4. A 600-N man sits on a chair. What is the support force provided by the chair?
5. A 100-kg man sits on a chair. What is the support force?
6. Three 100-kg triplets step onto a 1000-kg elevator. What is the tension force on the cable that is
supporting the triplets and the elevator?
7. Explain how motion is achieved if force pair interactions are always opposite and equal. (i.e.- How is
the horse able to move the cart when forces on each are equal?)
8. Applying Newton’s Third Law of Motion, diagram a bullet fired from a gun.
9. An astronaut happens to have a bowling ball in outer space. She throws the ball. Since there is
negligible influence of earthly gravity where she is, use the concept of Newton’s 3rd Law to describe the
motion of the bowling ball and the astronaut.
10. How is the gravitational attraction between two objects an example of Newton’s 3rd law of motion?
Category Four: Friction
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Identify what the following symbols represent: FW, FNET, FN, FA, Ff, µK, µS.
What is a Normal Force?
What does the coefficient of friction represent?
Why is the coefficient of kinetic friction always less than the coefficient for static friction for the same
object under the same conditions?
Construct a free-body diagram of a book sitting on a desk.
Construct a free-body diagram of a 1-kg book being pushed across a table with a force of 2 N at a
constant velocity.
Determine the coefficient of friction for the book in questions #6.
A 40-kg TV is sitting on an entertainment stand. If the coefficient of friction is 0.4, what is the force
needed to slide the TV to the other side of the stand for dusting purposes?
A 400 kg crate is pushed horizontally with a force of 1900 N. If the coefficient of friction is 0.24,
calculate the acceleration of the crate.
Problem samples
1. A 50kg box is pulled by a 4N horizontal force at a constant velocity on a horizontal surface.
a. Calculate the weight of the box
b. Calculate the normal force acting on the box
c. Calculate the frictional force acting on the box
d. Is the frictional force static or kinetic friction?
e. Calculate the frictional coefficient between the desk and the box.
2. A 50kg box is pulled by a 40N force at a 20o upward angle. It moves at a constant velocity.
a. Calculate the weight of the box
b. Calculate the normal force acting on the box
c. Calculate the frictional force acting on the box
d. Is the frictional force static or kinetic friction?
e. Calculate the frictional coefficient between the desk and the box.
3. A 12kg box at rest is pulled by a 50N force at a 30o upward angle on a horizontal frictional surface. The
coefficient of static friction is 0.4 and the coefficient of kinetic friction is 0.3.
a. Calculate the normal force acting on the box
b. Calculate the frictional force acting on the box
c. Will the box accelerate from rest? Why or why not?
d. If the box were already in motion, would the box accelerate? If so, at what rate?
4. A 15kg box is at rest on a surface that is inclined 25o above the horizontal.
a. Calculate the perpendicular force on the box.
b. Calculate the parallel force on the box.
c. Calculate the normal force on the box.
d. Calculate the frictional force on the box.
e. Calculate the coefficient of static friction between the box and the surface.
5. A 12kg box is placed on a 12o inclined frictionless surface.
a. Calculate the normal force acting on the box.
b. Calculate the rate at which the box will accelerate down the ramp.