Physics chapter 4: dynamics 2 forces
effects of a force on the motion of a body
a moving body increases with speed, a moving hockey ball moves faster when it is pushed, there is an increase in speed
a moving body decreases in speed, a descending parachutist slows down due to air resistance, there is a decrease in speed
a body at rest moves, a stationary football moves when it is kicked, there is a change in speed
a moving body changes direction, a moving tennis ball is returned when it is hit there is a change in direction
balanced forces and newton's first law
zero acceleration, an object is moving with a constant velocity or the object is stationary, even though the acceleration is zero, there are still forces acting on it, only the resultant force is zero
when a book is at rest on a table, there are 2 equal forces acting on the book but in opposite directions, the table exerts an upward force, the normal force, that pushes on the book the upward force is equal to the weight or the gravity acting on the book, the 2 forces acting on the book are equal but in opposite directions, thus the resultant force is zero and the book is stationary, as the book is at rest, acceleration is zero
when a force is applied on the same book and it moves across the table, the force applied on the book is the same as the friction force acting on the book which causes it to slow down and eventually stop, the 2 forces acting on the book are equal but act in opposite directions, thus the resultant force is zero, and the book continues moving at a constant velocity
newton's first law of motion: every object will continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it
inertia of an object, refers to the reluctance of the object to change its state of rest or motion, due to its mass
inertia is affected by an object's mass, an object with a larger mass will make it harder for the object to start moving, slow down, move faster or change direction
inertia is also the reason why people should wear seatbelts in cars, if the driver suddenly applies the brakes he will continue to move forward due to his inertia, without a seat belt, he would crash into the windscreen, if he wears a seatbelt, the seatbelt would hold him back as it provides the necessary opposing force that stops him
unbalanced forces and newton's second law
when a book is stationary on a table surface, the friction acting on it is equal to the force acting on it, so it does not move, when another force is applied and the force of the push is larger than the friction, the book will accelerate, when the push force is removed, the only force acting on the book is friction, this will cause the book to decelerate and stop
newton's second law of motion: when a resultant force act on an object of a constant mass, the object will accelerate in the direction of the resultant force, F=ma, this law of motion tells us, a resultant force F on an object produces an acceleration a, doubling the resultant force F on an object doubles its acceleration a, and with the same resultant force F, doubling the mass m halves the acceleration a
newton's third law
newton's third law of motion states that if body A exerts force Fab on body B then body B exerts an equal and opposite force Fba on body A, Fab and Fba occur as a pair at the same time, they are equal in magnitude but act in opposite directions, so for every action, there is an equal and opposite reaction
action and reaction forces
when a book is placed on a table, there are 2 pairs of action and reaction forces acting on the book, the forces between the book and the earth, and the forces between the table and the book
the gravitational force Feb exerted by the earth on the book and the upward force Fbe exerted by the book on the earth.
the normal force Ftb exerted by the table on the book, and the normal force Fbt exerted by the book on the table
forces always occur in pairs, and each pair is made up of an action and a reaction forces
action and reaction forces are equal in magnitude
action and reaction act in opposite directions
action and reaction forces act on different bodies
free body and vector diagrams
free-body diagrams of objects help us to visualise and identify the forces and their effects on the object
vector diagrams
Effects on resistive forces on motion
friction
the contact force that opposes or tends to oppose motion between surfaces in contact, it is the result of irregularities of the surfaces; the surfaces are very rough and have many small edges on a very deep level that our naked eye cannot see, these edges catch onto one another and causing surfaces to resist motion
friction can have good and bad effects
positive effects: we can walk without slipping and moving objects are able to slow down when needed
negative effects: cars are less efficient by up to 20%, moving parts in engines, motors, and machines suffer wear and tear
reducing negative effects of friction
using ball bearings: spherical in shape, ball bearings are used to reduce friction between moving parts of machines, cars and in-line skates, ball bearings are placed moving parts so the ball bearings can roll around, this prevents the moving parts from rubbing against each other
using lubricants and polished surface: applying a layer of lubricant, such as oil and grease, between surfaces in contact can greatly reduce friction, lubricants are frequently used between the moving parts of an engine to reduce wear and tear, this helps to prolong the engine life, polishing the surface removes surfave irregularities, this can also reduce friction between surfaces in contact
using wheels: being circular in shape, wheels reduce the friction between the object and the floor greatly, thus a smaller force can be applied to move the object around
using air cushion: the friction between two surfaces is reduced when there is a thin layer of cushion between the two surfaces
how to enhance the positive effects of friction
using parachute: air resistance is a type of friction in air, a skydiver in midair varies air resistance to change his speed, to speed up, he reduces air resistance by reducing surface area in contact with the air, to slow down, he increases air resistance by increasing the surface area in contact with the air, top achieve a safe landing, he increases air resistance greatly by opening his parachute
using chalk: rock climbers need to have a firm grip on the rock surface with their hands and feet, they usually use chalk powder on their hands to absorb perspiration and improve their grip
using threads: friction is important to the motion of vehicles, without friction, the vehicle cannot move as its wheels would just spin at the same spot, friction enables the tyres to grip the road surface tightly and roll without slipping, on a rainy day, a moving vehicle may skid on wet roads, so its tyres need more grip on the road to prevent skidding, so the tyres are designed with threads, grooves that quickly channel water out quickly this improves the grip of the tyres on the wet roads
falling with and without air resistance
without air resistance: an object can only be in free fall if the only force acting on it is its own weight, and the acceleration is the gravitational field strength
with air resistance: always opposes motion of moving objects, increases with speed of the objects, increases with surface area of object, increases with density of air
2 skydivers, one opens his arms and legs skydiver A, the other is in a head-first dive position skydiver B, skydiver A experiences a higher air resistance and falls slower than skydiver B
terminal velocity
speed of a falling object will continue to increase due to a downward acceleration, when the air resistance is not negligible, the falling object also experiences an increasing downward force, eventually, the amount of air resistance becomes the same as the weight of the object, and the velocity of the object stops increasing, the FINAL VELOCITY attained is terminal velocity