Chapter 8 Flashcards
Friction
Force opposing the motion of a surface that moves or tries to move across another surface
Newton’s first law of motion
Objects either stay at rest or moves with constant velocity unless acted on by a force
Resultant force equation
F = m x a F = resultant force N M = mass kg A = acceleration ms-²
This equation is known as Newton’s second law for constant mass
Newton’s second law
The rate of change of momentum of an object is proportional to the resultant force
Weight
Weight w = m x g
M = mass
G = gravity
W = weight N
Object in equilibrium
When an object is in equilbrium, the support torce on it is equal
and opposite to its weight. Therefore, an object placed on a
weighing balance exerts a force on the balance equal to the weight ot the object. Thus the balance measures the weight of the object.
Gravitational field strength
Force of gravity per unit mass on a small object.
Inertia
Resistance of an object to change of its motion
Mass of an object
The mass of an object is a measure of its inertia, which is its
resistance to change of motion. More force is needed to give
an object a certain acceleration than to give an object with less
mass the same acceleration.
Two forces in opposite direction
When an object is acted on by two unequal forces acting in opposite directions, the object accelerates in the direction of the larger force.
Resultant force = F1 - F2 = m x a
Horizontal surface
If an object is on a horizontal surface and F1 and F2 are horizontal and in opposite directions the resultant force equation still applies. The support force on the object is equal and opposite to its weight
Elevator
In a elevator which is falling the up force is the tension in the metal ropes and the downward force is mass of the elevator x gravity
Towing a trailer
Car with mass of M
Trailer with a mass of m
When the car and the trailer accelerate, the car pulls the trailer forward and the trailer holds the car back. Assume the air resistance is negligible.
Engine thrust
And
Tension in the tow bar
The car is subjected to a driving force f pushing it forwards (engine thrust) and the tension T in the tow bar holding it back.
Therefore the resultant force on the car = F - T = m x a
The force on the trailer is due to the tension T in the tow bar pulling it forward
Therefore T = m x a
Combining the two equations together = F = M x a + m x a = (M+m) x a
Rocket equation
T = m x g + m x a T = thrust M = mass G = gravity A = acceleration
Lift Problem
Using upwards is positive gives the resultant force on the lift as T - m x g
Where T is the tension in the lift cable and m is the total mass of the lift and occurring.
Lift A
If the lift is moving at constant velocity then a = 0
So T = m x g
Tension = weight
Lift B
If the lift is moving up and accelerating then a > 0
So T = m x g + m x a > m x g
Lift C
If the lift is moving up and decelerating then a < 0
So T = m x g + m x a < m x g
Lift D
If the lift is moving down and accelerating then a α < 0
(Velocity and acceleration are both downwards therefore negative)
So T = m x g + m x a < m x g
Lift E
If the lift is moving down and decelerating then a > 0 (velocity downwards and acceleration upwards therefore positive)
So T = m x g + m x a > m x g
The tension in the cable is less than the weight if
the lift is moving up and decelerating (velocity > 0 and
acceleration < 0)
the lift is moving down and accelerating (velocity <0 and
acceleration < 0).
The tension in the cable is greater than the weight if
the lift is moving up and accelerating (velocity > 0 and
acceleration > 0)
the lift is moving down and decelerating (velocity <0 and
acceleration > 0).
Drag force
The force of fluid resistance on an object moving through a liquid
The drag force
The shape of the object
It’s speed
The viscosity of the fluid which is measured of how easily the fluid flows past a surface
The faster an object travel is in a fluid the greater the drag force on it.
Terminal speed
The maximum speed reached by an object when the drag force on it is equal and opposite to the force causing the motion of the object.
Object falling in a liquid
The speed of an object released from rest in a fluid increases as it falls,
so the drag force on it due to the fluid increases. The resultant force
on the object is the difference between the force of gravity on it (its
weight) and the drag force. As the drag force increases, the resultant
force decreases, so the acceleration becomes less as it falls. If it
continues falling, it attains terminal speed, when the drag force on it
is equal and opposite to its weight. Its acceleration is then zero and its
speed remains constant as it falls
Resultant force for an object falling
F = m x g - D D = drag force
Motive force
The force that drives a vehicle
Motion of a powered vehicle
The top speed of a road vehicle or an aircraft depends on its engine
power and its shape. A vehicle with a streamlined shape can reach a
higher top speed than a vehicle with the same engine power that is
not streamlined.
For a powered vehicle of mass m moving on a level surface, it Fe
represents the motive force (driving force) provided by the engine,
the resultant force on it = Fe - Fr, where Fr is the resistive force
opposing the motion of the vehicle.
Fe = engine force
Fr v resistive force
Because the drag force increases with speed, the maximum speed (the
is reached when the resistive force terminal speed) of the vehicle max
becomes equal and opposite to the engine force, and a = 0.
Acceleration with motive force
A = Fe - Fr / m Mass = m
Explain how a car arrives at max speed.
The top speed of a road vehicle depends on its engine power and shape, with a vehicle with a streamlined shape can reach a higher top speed than a vehicle with the same engine power that is not streamlined.
Free fall / parachute
1 Initially
V = 0ms-1
Fr = m x g
A = 9.81ms-2
2 Drag and m X g are involved
V > 0ms-1
Fr = m x g - drag (drag< m x g )
A < 9.81ms-2
3 eventually drag and mxg get bigger
V = max
Fr = 0N (mxg = drag)
A = 0ms-1
When parachute is deployed
Fr <0N (negative so upwards)
A < 0ms-2 (upwards)
For this The parachute is deployed and that drag is far bigger than m x g
Thinking distance
Is the distance travelled by a vehicle it takes the driver to react
Equation U = s1 / t0 U = initial speed S1 = thinking distance T0 = reaction time
Stopping distance
Thinking + braking
Braking distance
The distance travelled by a car in the time it takes top stop safely from when the brakes are first applied
V2 = U2 + 2 x a x s
Damage to a passenger
Ina collision is caused by the impact force F = m x a
Larger acceleration
Larger acceleration = a larger force
A = v-u / T
A similar time = a larger acceleration
Smaller distance =
= smaller time
T = 2 x s / u + v
Safety feature
Every safety feature in a car is designed to increase the impact time, therefore decreasing impact force on the passengers.
Seat belt = increases the distance to stop, increasing time decreases force.
Air bag
Reinforce chassis
Crumple zone
