Kleiner's Tips for Physics Exam _1

7/14/2019 Kleiner's Tips for Physics Exam _1

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Kleiner’s Tips for Physics Exam #2 Covering Chapters 6,7

(Newton’s Second Law and Third Law)

Equations

F = 0 vf = vi + at d = vit + 1/2at2 A = F/M Ff = μFn

v = d/t vavg = (total distance)/(total time) a = ∆v/∆t

Tips and Hints for success

Chapter 6:

1. A = F/M

Acceleration is inversely proportional to mass. If you increase the mass while keeping force constant, acceleration

will decrease. Double the mass, acceleration is cut in half

Acceleration is directly proportional to force. If you increase the force, while keeping the mass constant, the

acceleration will increase. Double the force, acceleration is also doubled

2. Understand terminal velocity

A. Terminal velocity occurs when force of air resistance during free fall equals the weight of the falling object.

B. At terminal velocity acceleration is zero and speed is constant

C. The heavier an object, the greater its terminal velocity

3. Understand air resistance

A. Because of air resistance a heavy object will hit the earth before a lighter object when dropped from the same

height. Of course, excluding air resistance they will hit the ground at the same time

B. Air resistance depends on how fast an object is moving and its surface area.

4. Weight is a force! It is the measure of the force of gravity pulling on an object

5. Objects can still be moving when the net force on them is zero

When net force is zero, it does not mean an object is not moving. It simply means it is not accelerating. Hence, the

velocity is constant.

Example:

Think of a car moving at a constant speed of 60mph in a straight line. The car’s velocity is constant! The acceleration of

the car is zero, because the net force on the car is zero (force of friction trying to stop the car = force of the car’s forward

motion created by the engine)

6. Understand Pressure

Pressure = Force/Area (it is often measured in Pascals (Pa), which is N/m 2

Standing on one foot creates twice the pressure of standing on two feet because you are distributing your weight across

half the surface area

7. Understand friction

The normal force is the force pressing perpendicular to the two surfaces that are rubbing against each other

Friction depends on two things: the normal force and the coefficient of friction (μ) for the two surfaces involved



8. Objects at rest have a net force of zero acting on them, but that does not mean there are no forces. It just means the

NET force is zero.

Example: A baseball glove is sitting on the table. There are two forces acting on the glove:

the force of gravity pulling down

the force of the table pushing the glove up.

These two forces cancel each other out, creating a net force of zero and hence the baseball glove remains stationary

9. Understand why acceleration is constant for free falling objects with different masses

Question: Excluding air resistance, a 10Kg mass and a 1Kg mass will fall with the same acceleration of 9.8m/s^2. How

can this be?

Answer: A = F/M. Even though the mass (M) is 10 times larger, the force of gravity (F) pulling on the 10Kg mass is also

ten times larger, so A remains constant

Chapter 7:

1. Understand action reaction pairs!

Examples:

A pushes B (action)

B pushes A (reaction)

Foot pushes ball (action)

Ball pushes foot (reaction) Ball pushes person would be the wrong answer!!

2. Gravity is an action/reaction pair!

If the earth is pulling on you with gravity then you are pulling on the earth with gravity. You don’t just fall towards

the earth! The earth comes up to meet you a little bit!

3. Objects with different masses hit each other with the same force! What differs is how the two objects accelerate due

to their different mass.

Example: Elephant collides into mouse. They hit each other with the same force. But the mouse will decelerate to a

dead stop and the elephant will hardly decelerate at all!

4. Forces always come in action reaction pairs. Always!

Sample Problems

1. Pansy has a mass of 60kg and sits on a 15kg sled. Gus pushes her with a force of 300N acrosssnow with no friction. What will Pansy’s acceleration be?



2. A person riding a bike is not paying attention and suddenly runs into a tree! What is the reaction

force?

a. The friction of the bike on the ground b. The force the tree exerts on the bike

c. The force the biker exerts on the handle barsd. The force of the biker pushing on the bike pedals

3. In outer space a 50Kg astronaut pushes a 55Kg rock with a force of 100N. According to Newton ______.

a. The rock will push back with the same force, and the astronaut and the rock will have thesame acceleration

b. The rock pushes back with a different force, and the acceleration of the astronaut and the

rock will be the samec. The rock will push back with a different force, and the astronaut and the rock will have

different accelerations

d. The rock will push back with same force and the astronaut and the rock will have different

accelerations

4.

A 300 pound football player runs into a 150 pound quarterback so hard that the quarterback isknocked out and must be taken off the field with a stretcher. Explain.

a. The quarterback has less mass

b. The football player hit the quarterback with more forcec. The two players hit each other with the same amount of force

d. Both A and C

e. Both A and B

5. A box is sitting on the floor with bottom dimensions of 6cm by 20cm. The mass of the box is 24

Kg. What pressure does the box exert on the floor?

6. A box with a mass of 90Kg is being pushed at a constant speed with a force of 140 N. What is the

coefficient of friction between the box and the floor?

7. A jet airplane has a total mass of 12,000 kg. The thrust force of the gases escaping from the jet engines accelerates 11 kg of jet fuel gases out at the rate of 1042 m/sec2. What is the acceleration of the jet airplane

itself (ignore the airplane's 11 kg mass loss)?

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Kleiner's Tips for Physics Exam _1