1. Acceleration & Momentum
Chapter 4 Physical Science Review
Mrs. Nell

2. Acceleration
An unbalanced force acting on an object causes it to accelerate in the direction of the greater force.
Acceleration is a rate of change in velocity caused by a change in speed, in direction, or both.

3. Newton’s Second Law of Motion
Newton’s second law of motion states that a net force acting on an object causes the object to accelerate in the direction of the applied force.

4. Acceleration & Mass
Acceleration is determined by the size of the force and the mass of the object. A larger mass requires a greater force than a smaller mass would require to achieve the same acceleration. In other words, if you apply the same amount of force, it is easier to accelerate a tennis ball than a bowling ball!
Large mass…. Small acceleration.
Small mass… Large acceleration.

5. Acceleration & Force
If there is a large force there will be a large acceleration. If there is a small force, there will be a small acceleration. In other words, if you have two soccer balls, the one that you apply the greatest force to (kick the hardest) is going to have the greatest acceleration.

6. The Force Equation
The equation for Newton’s second law is F = m*a, where F is the force applied, m is the mass of the object, and a is the acceleration experienced by the object.
Or acceleration = F/m

7. Measuring Force
Since mass is measured in kilograms and acceleration is measured in m/s2, force is measured in units of kg*m/s2 or Newtons (N).
A newton is the amount of force needed to accelerate an object with a mass of 1 kg at an acceleration of 1 m/s2
1 N = 1 kg* m/s2

8. Practice Problem #1
ANSWER:
What is the acceleration of the car?
Force = m*a
10 = 5A
Acceleration = 2 m/s2
Let’s assume that the wheels of a 5-kg car apply 10 N of force. What is the acceleration if friction and drag are negligible?

9. Practice Problem #2
ANSWER:
The net force would equal 3 Newtons. The total mass = 6 kg.
What is the acceleration of the car?
Acceleration = F/M
Acceleration = 3/6
Acceleration =0.5 m/s2
What is the net force if the wheels of the 5-kg car apply 10 Newtons but a 1-kg parachute applies 7 Newtons in the other direction?
What is the acceleration of the car?

10. Practice Problem #3
ANSWER:
The net force would equal 13 Newtons. The total mass = 10 kg.
What is the acceleration of the car?
Acceleration = F/M
Acceleration =13/10
Acceleration =1.3 m/s2
A rocket is added to the car and applies an additional force of 10 Newtons. The wheels still apply 10 N. What is the net force if the parachute continues to apply 7 Newtons in the other direction? The total mass of the car, rocket and parachute is 10 kg. What is the acceleration of the car?

11. Falling Objects
Two falling objects will hit the ground at almost the same instant. Gravity is greater on a heavier object, but a larger mass also has more inertia, so more force is needed to change its velocity.
Galileo dropped a cannonball and a musketball simultaneously from a tower, and observed that they hit the ground at nearly the same time. This contradicted Aristotle's long-accepted idea that heavier objects fell faster. Please note that there is no historic evidence for the legend that Galileo has carried out this experiment at the Leaning Tower of Pisa

12. Acceleration Caused by Gravity
Near Earth’s surface, gravity causes all falling objects to accelerate at 9.8 m/s2, regardless of mass.

13. Air Resistance
Anything that moves in Earth’s atmosphere is affected by air resistance. Air resistance is the force air exerts on a moving object. This force acts in the opposite direction to that of the object’s motion.

14. Amount of Air Resistance
The amount of air resistance depends on:
Speed
Size
Shape
Density of object

15. Terminal Velocity
As an object falls through air, air resistance gradually increases until it balances the pull of gravity.
Terminal velocity is the highest velocity that will be reached by a falling object.
Air Resistance
Gravity

16. Projectiles
Anything that’s thrown or shot through the air is called a projectile. Gravity pulls the object down, but its inertia cause the projectile to follow a curved path. They have both horizontal and vertical velocities.

17. Motion Along Curves
Acceleration toward the center of a curved or circular path is called centripetal acceleration. Centripetal force is a force acting toward the center of a curved or circular path.

18. Weightlessness in Orbit?
The space shuttle and everything in it are in free-fall around Earth, which produces apparent weightlessness. Are these objects in orbit around Earth really weightless?
Answer: No- they only appear to be weightless. They are in free-fall. Gravity is still pulling everything downward.

19. Effects of “Weightlessness”
The resistance provided by gravity helps you exercise your body. Astronauts who experience the sensation of weightlessness for extended periods of time suffer some bone and muscle deterioration.

20. Exercising in Space
Scientists have developed special exercise programs for astronauts to reduce the health problems related to weightlessness.
Isometric exercises
Walking on special treadmills

21. Launching Artificial Satellites
Newton thought a satellite could be launched by blasting it horizontally from a mountaintop. It wasn’t until the 1950’s that we were able to launch a satellite using a multistage rocket system.

22. Satellites
Artificial satellites are human-made devices that orbit Earth for a specific purpose.
Earth’s Moon is a natural satellite.

23. Sputnik
In 1957, the former Soviet Union launched the first artificial Earth satellite, called Sputnik.
To remain in orbit a satellite has to move 8 km/s or 29,000 km/hr!

24. How We Use Satellites
Artificial satellites are used for a variety of purposes:
Communications (TV, radio)
Weather
Military use
Many satellites are geostationary- they appear to be stationary, but are put into orbit with a speed that matches the movement of Earth as it spins on its axis.

25. Falling Satellites
Small amounts of air resistance gradually cause satellites to lose energy and fall lower. Eventually, Earth’s gravity will pull it down, causing the satellite to burn up in the friction of Earth’s atmosphere.

26. Newton’s Third Law
Newton’s Third Law of Motion states that when one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. In other words, “to every action force there is an equal and opposite reaction force.”

27. Action-Reaction Pairs
Some examples of action-reaction pairs:
Swimmer
Rockets
Hitting a baseball
Pushing on a wall
Bowling
Action force acts on water, reaction force acts on swimmer; ball pushes pins left/pins push ball right

28. Momentum
Momemtum, or bashing power, is a property a moving object has because of its mass and velocity. Momentum can be calculated with the following equation:
p = m * v
(momentum equals mass times velocity)
Momentum is measured in kg*m/s

29. Mass * Velocity
A speeding bullet has a large momentum because of its high velocity, even though its mass is small. A semi-truck might have a low velocity but a large momentum because of its large mass.

30. Conserving Total Momentum
The momentum of an object doesn’t change unless its mass, or velocity, or both change. Momentum can be transferred from one object to another. The law of conservation of momentum states that the total amount of momentum of a group of objects does not change unless outside forces act on the objects.

31. Practice Problem #4
If a ball is projected upward from the ground with ten units of momentum, what is the momentum of recoil of the Earth? Do we feel this? Explain.
Answer: The earth recoils with 10 units of momentum. This is not felt by Earth's occupants. Since the mass of the Earth is extremely large, the recoil velocity of the Earth is extremely small and therefore not felt.

32. Practice Problem #5
A 120 kg lineman moving west at 2 m/s tackles an 80 kg football fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw a vector diagram in which the before- and after-collision momentum of each player is represented by a momentum vector. Label the magnitude of each momentum vector.
Answer:
80x8=640kg*m/s
120x2=240kg*m/s
=400kg*m/s

33. Practice Problem #6
*To solve this problem, remember that momentum is conserved!
A cannon whose mass is 1000 kg fires a cannon ball in a horizontal direction. The cannon ball has a mass of 20 kg and if fired to the right with a velocity of 25 m/s.
With what speed does the cannon recoil to the left?
P = m*v
1000*v = 20*25
1000*v/1000= 500/1000
V= .5m/s2 LEFT
Actually written as -.5m/s2

34. Practice Problem #7
A trolley of mass 1.43kg is moving at 1.45m/s when it collides with a stationary trolley, and the two move off together at 0.75m/s. No external horizontal forces act. What is the mass of the second trolley?
Answer:
Momentum before the collision is 1.43 x 1.45 = kg*m/s. Momentum is conserved, so momentum after the collision is kg*m/s.
p/v = m
2.0735/.75 = kg
This is the total mass. Must subtract mass of trolley #2.
2.765kg kg = 1.33kg