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Effects forces have on object's motions
Effects forces have on object's motions
A skier has many forces acting on him at the same time. The ground exrts forces on the him, gravity is pulling on him, and the air pushes him back. Combining these forces determine how he moves. At times, he move at constant speed or speed up or slow down.
Knowing physics of skiing helps to be skilled and gear designers to make better poles and other gears
Types of forces
Types of forces
Forces is all around us (transports, buildings, activities). Engineers are careful when designing as it's using forces to move around and lift things.
In simple words, force is a push or pull. It can cause object to change motion. Pushing a book down is very different than pushing it sideways or pull it up - so the force has directions, thus being a vector quantity.
Previous unit presented kinematics (study of object's movements without reference of motion). This unite is for dynamics: why objects move the way they do.
Measuring forces
Measuring forces
Newton (N) is a derived SI unit equal to 1 km⋅m/s². To measure force, use either a spring scale or a force sensor.
A spring scale must be at zero if not used
A force sensor is a device attached to a computer or used independetly. It gives accurate digital readings of a force and even graph how a force changes overtime, measuring both push and pulls.
a spring scale
Force diagrams
Force diagrams
To understand why an object remains at rest or moves, draw diagrams to show how forces act on objects.
A system diagram is a simple sketch of all objects present in a situation.
E.g. lifting a book in the air, its diagram shows your hand pull up the book.
Another type of force diagram is the free-body diagram (FBD), a simple drawing shown by the object (often a rectangle or dot pointing away from the center) analyzed and all forces acting on it.
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All force are labelled with the symbol F and an appropriate subscript indicating the force's type.
Everyday forces
Everyday forces
Be familiar with everyday forces. E.g. 2 kids playing outside in a wagon. 1 kid pulls forward on a rope tied to the front and the second pushes on the wagon from behind.
What forces is on the wagon?
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Applied forces (Fa) are force that occur if an object touches with another and pushes/pulls it.
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Tension forces (FT) are pulling forces from a ropes/strings on an object that always points to the rope/string.
Ropes and strings aren't rigid, so they cannot push objects.
The kid at the front causes tension on the rope by pulling forward.
An object can exert 2 different forces on object if touching it.
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Normal force (FN) s a perpendicular force on an object exerted by the surface that's touched. Its name is from the force of always being perpendicular (normal) to the surface, so they're always pointed away from a surface.
Figure 4 (b) shows the normal force from the ground on the wagon starts from a rectangle's outline and points up, perpendicular to the ground. Here the normal force supports the wagon against gravity.
Contact forces need one object to touch another, as shown on all forces from the previous page.
Some forces don't need contact and are known as action-at-a-distance forces or non-contact-forces.
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Force of gravity/Gravitational force (Fg) is the force of attraction between 2 objects due to mass, an action at-a-distance force/non-contact force.
Earth’s surface's force of gravity always points down to it scentre. Even if its surface is sloped, like the side of a mountain, the gravity force still points down toward Earth’s centre
To calculate an object's magnitude of the force of gravity, multiply object's mass by acceleration due to gravity.
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To find the force of gravity use equation: Fg = mg
where g = 9.8 m/s [down].
The force of gravity is measured in newtons and the mass in kilograms.
An internal force occurs if an object exerts a force on itself.
E.g., skater A pushes on skater B, the force on B is external and if he pulls forward on his own arm, then it's an internal force.
Calculating net Forces
Calculating net Forces
In many force problems, you often combne all forces acting on one object. The total force is also called the net force/resultant force, which is determined by using FBDs.
Note that force is a vector quantity and direction per each force must be considered before determining the ne →
force (Fnet).
Problem 1
Problem 1
floor exerts a normal force of 36 N [up] on a stationary chair.
force of gravity on the chair is 36 N [down].
Draw the chair's FBD and use the FBD to find its net force.
Solution
Step 1. Draw the object's FBD:
Step 2. Identify which directions are positive. Define "up" as positive y-axis and "down" as negative y-axis, meaning normal force exerted by the floor on the chair is positive and its force of gravity is negative.
The forces' values on the FBD are now (left):
Step 3. Add the forces on each axis to determine the net force:
The chair has no net force as it equals to 0.
Problem 2
Problem 2
This pic shows all forces acting on an object. Use the FBD to calculate the net force.
Solution
Step 1. Identify which directions are positive.
Up and east are positive and down and west are negative. The forces' value on the FBD are now:
Step 2. Define east and west forces as being along the x-axis. So up and down forces are along the y-axis. Add te forces along each axis to find the net force.
1st, find the sum of all forces parallel to y-axis.
(Fnet)y = +52 N + (-52 N) = 0 N
The net force on the y-axis is 0 N.
Now find the sum of all the forces parallel to the x-axis:
(Fnet)x = +62 N + (-45 N) = +17 N
The net force on the x-axis is 17 N [E].
Therefore, the net force on the object is 17 N [E].
Physicists have grouped all known natural forces into 4 categories called fundamental forces:
Gravitational force pulls objects down to Earth’s centre and exists between any 2 masses in the universe.
keeps the Moon in orbit around Earth and Earth in orbit around the Sun. You don't notice such force between smaller objects (like pair of basketballs) as the masses are tiny and the force is too weak compared to Earth’s gravity
Electromagnetic force is caused by electric charges
exists between charges, and magnetic force exists between magnets.
an action-at-a-distance force. Unlike gravity, this force can both attract and repel, thus, these forces often cancel each other out.
holds atoms and molecules together: it makes concrete hard and a feather soft and governs properties of chemical reactions.
In nucleus of atoms, positively charged protons are close and repel each other.
They don't fly apart as strong nuclear force of attraction between neutrons and protons keeps them in place.
A times, protons and neutrons transform into other particles.
Strong nuclear force
Weak nuclear force is responsible for interactions occured in such particle transformations.
Newton’s First Law of Motion
Newton’s First Law of Motion
Each year in Canada, about 160k car accidents occur, with 3000 being fatal, many caused for not wearing seat belts or for driving too fast.
An understanding of Newton's 1st law help to appreciate the importance of cars' safety features.
What happens to a crash test dummy in a collision for not wearing a seat belt and no airbag? Newton's law can answer this.
Inertia
Inertia
In air hockey, the plastic pucks move are close to uniform velocity after being hit. In other words, there's little friction acting on the puck to slow it down.
The puck's net force is zero as its upward force on it exerted by air and downward force of gravity cancel each other with almost no friction.
If you don't hit the puck, it sits there at rest if the air table is level.
Early scientists and philosophers saw that moving objects can spontaneously slow down for no clear reason.
They didn't know about friction and incorrectly attributed decrease in velocity to “lazy” objects and concluded constant net force is needed to move objects.
They thought bigger net forces made things move at higher constant velocity and smaller forces made them at lower constant velocity. If no net force acts on an object, they thought the object stops.
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