forces and newtons law Flashcards
Frictional force
The force that opposes the motion of an object
(acts parallel to the surface with which the object is in contact)
Normal force
The perpendicular force exerted by a surface on an object in contact with it
Gravitational force
the gravitational force the earth exerts on any object on or near its surface
Resultant vector
the single vector which has the same effect as the original vectors acting together
resultant force
sum of all forces on an object
why will different sized objects fall and hit the ground at the same time
due to gravitational acceleration
the heavier object has more mess and hence a larger inertia
therefore the earth exerts a larger force on it to accelerate it at the same rate as the lighter object
why will a heavier object experience a greater force
object will have a greater resistance to change (mass is an indicator of inertia)
inertia
the property of an object that causes it to resist a change in its state of rest or uniform motion
how does normal force acts
acts upwards perpendicular to the surface
balances out Fg
how do you reduce friction
lubricants
what is unhelpful friction
moving parts in a car generating heat
boat on water - water slows the boat down
helpful friction
air resistance creates drag on a parachute
car tyres on a road- stops sliding and helps the car to be able to slow down
Newtons third law of motion
When object A exerts a force on object B, object B simultaneously exerts an oppositely directed force of equal magnitude on object A
what is N3 about
interaction between pairs of objects
action reaction/N3 pairs
same type of force
same magnitude
occur simultaneously
act along the same line
opposite direction (act on different objects- therefore cannot balance each other out)
Newtons First Law
An object continues in a state of rest or uniform ( moving with constant) velocity unless it is acted upon by a net or resultant force
F net = 0
a=0
what is N1 about
no forces in direction of motion
balances forces
Fnet=0. a=0
why is it important to wear seatbelts ?
With a car breaks, a net force acts on the car and is not transferred to the passenger. The inertia of the passenger is not overcome. The passenger continues in their state of motion (N1). The seatbelt exerts a force on the passenger to prevent this hence the passenger will not be injured
Why do passengers get “thrown” out to the side when a car goes round a corner?
When a car goes round the corner, the net force acts on the car and not on the passenger.The inertia of the passenger is not overcome. The passenger continues in their state of motion (N1). It appears as though they are moving to the side.
Newtons second law
With a resultant force, Fnet, is applied to an object of mass, M, it accelerates in the direction of the resultant force. This acceleration, a, is directly proportional to the resultant force and inversely proportional to the mass
what is N2 about
unbalanced forces
change in motion
how does an object behave when its velocity and acceleration act in the same direction
speeds up
how does an object behave when its velocity and acceleration act in the opposite direction
slows down
what is friction affected by
the type of surface
Fn (directly proportional)
momentum
the product of the objects mass and velocity
newtons second law in terms of monentum
the resultant force acting on an object is equal to the rate of change of momentum
impulse
the product of the net force and the contact time
airbags and impulse
an airbag’s job is to increase the time taken for the change in momentum to occur
the change in velocity is not altered, only the time is increased .
an airbag slows occupants down over a period of time rather than stopping them abruptly
In terms of newton’s laws, give an explanation of why a car which is going around a bend on a wet road might slide off the road
The car will continue in a straight line unless acted on by an unbalanced force. Under normal conditions this unbalanced force is provided by the friction between the turned wheels and the road, but when the road is slippery, this friction is not present and thus the car continues in a straight line and off the road at the curve
explain : A box, containing a ball, is pushed slowly at constant speed across a table. The box stops
abruptly as it bumps into an obstacle, but the ball rolls forward within the box.
Inside the box, initially, the ball experiences no horizontal force as there is no friction between it and the box.
• According to Newton’s first law, the moving ball (relative to the table), continues moving forward because there is no net force acting on it. Initially, the ball’s inertia is not overcome.
• When the ball hits the inside edge of the box, it will stop according to Newton’s second law, because the box exerts a net force on the ball.
explain The value of a seatbelt in an emergency stop or a head-on-collision
When the car brakes, the passenger will appear to be thrown forwards.
Explanation:
• In an emergency stop or a collision, a net force is applied backwards to the car (and hence the seat) causing it to slow down and stop rapidly according to Newton’s second law.
• However, if a person is not attached to the car via a seatbelt, the net force acting on the car will not be applied to the torso of the person.
• As a result, the person will continue to move forward in their state of constant motion according to Newton’s first law of motion. Their inertia will not be overcome. They will eventually hit the dashboard or the windscreen which is how the person receives an injury.
• If they are wearing a seatbelt, after a moment, it will apply a net force to the person’s torso.
• According to Newton’s second law, the person will then experience an acceleration which slows them down and they do not collide with the dashboard.
explain the value of an airbag in a head-on-collision
In a collision at higher speeds or with great force, an airbag provides even greater protection
against injury caused by inertia not being overcome. Explanation:
• Based on the seatbelt explanation above, the airbag applies a net force to the body and head because it moves toward the body and head.
• According to Newton’s second law, the airbag therefore changes the motion of the head, preventing a later collision with the dashboard or windscreen.
explain the value of a headrest in a collision from behind (bumper-bashing)
In a collision from behind, a net force is applied forwards to the car causing it to accelerate forward.
• The body of a person in the car will experience this net force because the car seat will apply this same net force to the body. Therefore, according to Newton’s second law of motion the body will accelerate and change its motion along with the car.
• If there are no headrests in the car, or if they are too far away from the head, there will not be a net force applied to the head of the person in the car and according to Newton’s first law of motion, the head will continue in its state of constant motion and will not increase its velocity as the rest of the body does. The inertia of the head will not be overcome initially.
• Thus, the head will move backwards relative to the body because, while the head is still moving forward, it moves forward less quickly than the body.
• At the extreme limit of the head’s extension, the muscles in the neck act to slow and stop the head’s backward motion very rapidly, resulting in the rebound of the head.
• The result will be whiplash to the neck.
explain: An unsecured smooth box on the back of a smooth flatbed truck appears to be “pushed off” the
back of the truck when the truck moves forward from rest.
When the truck moves forward there is a net force acting on the truck,
• Because there is no (or very little) friction between the box and the truck, there is no net
force on the box and so it stays where it is or continues in its state of rest according to
Newton’s first law of motion. The inertia of the box is not overcome.
• As a result, the truck moves out from underneath the box and the box stays where it is
relative to the ground.
• Obviously, it falls to the ground once the truck is not there to support it vertically.
in terms of newtons first law of motion, explain why it may be extremely important for safety belts to be worn at all times when travelling in a car
Newton I states that an object will maintain its velocity unless acted upon by an unbalanced force (object has inertia). A person travelling in a car travels at the velocity of the car. When an unbalanced force acts on the car, stopping it, this force does not stop the person in the car. Therefore the person continues at the original velocity that’s going through the windscreen. A seatbelt transfers the unbalanced force onto the person. Therefore the person stops with the car.
