Name:______Date:______
Physics 11 - Chapter 4 FORCES
Question : A 4600 kg airplane must accelerate to 120 m/s on a 785 m runway. Its engine can generate 35,000 N of thrust. What additional thrust is required for it to take off on this runway?
4.3 – Types of Forces Continued
Considering objects near the earth, all experience the Force ofgravity from the earth.
The force of gravity is the weight of an object. On the earth:
Weight = Fg = mg
There is no place in the universe where you can be weightless. There is a force of gravity acting at every place in the universe.
As you are sitting in your chair, you are experiencing the force of gravity, but the force you feel is the normal force of the chair pushing up against you.
Our minds have become so used to this normal force, that we expect it. This normal force is called our apparent weight.
Gravitational Force: ______
Symbol: ______Equation: ______Direction: ______
Normal Force: ______
This is a ______force, the result of Newton’s Third Law (which was ______). Because it is a reactive force, it is only as large as it needs to be. It’s the force that prevents us from falling into the ground.
Question: What does a scale measure?
Example 1: A 72.0 kg man stands on a Newton scale in an elevator. What does the scale read if the elevator is?
a) Is not accelerating?
b) Accelerating upwards at 3.2 m/s2?
c) Accelerating downwards at 2.5 m/s2?
d) Accelerating downwards at 5.2 m/s2? What would the scale read in kg?
• Accelerating downwards at 3.72 m/s2?
Air resistance is the force of air molecules bumping into an object that is moving through the air. The force of air resistance has direction opposite to motion
Example 2: draw force diagrams for each of the following
a) a ball being thrown horizontally.
b) a rising ball (include air resistance)
c) a falling ball (include air resistance)
Example 3: In example 2b), how can the ball be rising if all of the forces acting on it are downwards? Explain using relevant physics principles.
Example 4: if the falling ball in example 4c) is speeding up, write a net force equation.
Example 5: a 90.0 kg parachutist free falls for a time and then opens a parachute. If the parachutist slows down at a rate of 1.5 m/s2
a) Compare the forces that act on the system
b) Write a force equation
c) Find the force of air resistance on the parachute system
The force of air resistance is actually not a constant value. In fact the force gets larger as speed increases. A falling object will eventually reach the point where the force if air resistance equals the weight of the object. At this point, it will hold a steady speed called the terminal velocity.
Name: ______Block: ______
4.3 – Types of Forces assignment
1. A 25.0 kg child is standing on a bathroom scale which reads in kg. The scale reads 32.0 kg. Describe the motion of the child. (2.74 m/s2 acceleration up)
2. A person stands in an elevator. Write a net force equation in each case
a) elevator at rest
b) elevator accelerates up
c) elevator moves up at constant speed
d) elevator moves down at constant speed
e) elevator accelerates down
(FN=mg; FN-mg=ma; FN=mg; FN=mg; mg-FN=ma)
3. A 55.0 kg student is in an elevator accelerating up at 2.50 m/s2 .
a) write a net force equation (FN-mg=ma)
b) what would a force scale placed under their feet read? (676 N)
4. A 25 kg mass is falling and accelerating downwards
a) Write a force equation (mg-Fair = ma; 65 N)
b) What is the force of air resistance if the acceleration is 7.2 m/s2 DOWN?
5. a) A 5.00 kg rock is moving upwards on the earth and it experiences 5.00 N of air drag. Find its acceleration.
b) A 5.00 kg rock is moving downwards on the earth and it experiences 5.00 N of air drag. Find its acceleration.
c) The same rock is thrown downwards on the moon. Find its acceleration. (Note: gmoon = 1.6 m/s2) (10.8 m/s2 down; 8.8 m/s2 down; 1.6 m/s2 down.... no air on moon!)
6. A 200. kg spaceship speeds up from rest to 250. m/s in 0.200 seconds. Assume there is no air friction and find
a) the acceleration using kinematics (1250 m/s2 )
b) the force from the engines (2.50 x 105 N )
7. A 250. gram ball is moving to the left at 25.0 m/s. A person catches it and stops it in 0.320 seconds. Find the horizontal force and direction exerted on the ball by the person's hand. (19.5 N right)
8. A 55.0 kg parachutist is falling at constant speed. What force of air resistance acts on the person? (539 N up)
Name: ______Date: ______
Drawing Free-Body Diagrams
Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
Apply the method described in the paragraph above to construct free-body diagrams for the various situations described below
1. A book is at rest on a tabletop. A free-body diagram for this situation looks like this:
2. A girl is suspended motionless from the ceiling by two ropes. A free-body diagram for this situation looks like this:
3. An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:
4. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free-body diagram for this situation looks like this:
5. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:
6. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:
7. A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. A free-body diagram for this situation looks like this:
8. A skydiver is descending with a constant velocity. Consider air resistance. A free-body diagram for this situation looks like this:
9. A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. A free-body diagram for this situation looks like this:
10. A football is moving upwards towards its peak after having been booted by the punter. A free-body diagram for this situation looks like this:
11. A car is coasting to the right and slowing down. A free-body diagram for this situation looks like this: