Motion and Force Flashcards
Define Inertia
The mass of an object is a measure of its inertia, which is its resistance to change of motion
Name 3 factors that effect drag force
The shape of the object
Its speed
The viscosity of the fluid
State the equation for an object at terminal velocity
a = g - (D/m) = 0
where a= acceleration
g= gravitational field strength
D= drag force
m= massState Newton’s first law of motion
Objects either stay at rest or remain in uniform motion unless acted on by a force.
Describe the relationship between the resultant force on an object and its acceleration
The acceleration of an object is always in the same direction as the resultant force:
F = ma
Define weight
The weight of an object is its mass multiplied by the acceleration due to gravity
W = mg
For 2 forces, F₁ and F₂ acting on a point object in opposite directions, where F₁ > F₂, give the equation for the resultant force
F₁ - F₂ = ma
For an object of mass M towing an object of mass m by a light, inflexible rope, give the equation to show the thrust force of the towing object
Force holding M back is tension T so: Resultant force = F - T = Ma Force in m is due to tension T, so: T = ma F = Ma + ma = a(M + m)
For a rocket of mass m propelling itself upwards with thrust T, give the equation for the magnitude of thrust T
T - mg = ma
T = mg + ma
For a lift, give the conditions when the tension in the cable is less than the weight
- The lift is moving up and decelerating (v>0, a
For a lift, give the conditions when the tension in the cable is greater than the weight
- The lift is moving up and accelerating (v>0, a>0)
Or - The lift is moving down and decelerating (v0)
Define terminal velocity
When the drag force is equal and opposite to its weight, for an object falling in a fluid
Give the acceleration of a powered vehicle when Fᴱ is the force created by the engine and Fᴿ is the resistive force opposing the motion of the vehicle
a = (Fᴱ - Fᴿ) / m
For a car with engine force of Fᴱ and resistive force of Fᴿ, state when the car will be at maximum velocity
When Fᴱ = Fᴿ
Define thinking distance
The distance traveled by a vehicle in the time it takes the driver to react
For a vehicle at constant velocity v, the thinking distance: s₁ = speed x reaction time = vt₀
Define braking distance
The distance traveled by a vehicle in the time it takes it to stop safely, from when the brakes are first applied
Assuming the constant deceleration, a, to 0 from speed v:
s₂ = v² / 2a since v² = 2as₂
Define stopping distance
The distance traveled by a vehicle in the time it takes it to stop safely, from when the obstruction was first present
Stopping distance = thinking distance + braking distance
Stopping distance = vt₀ + v² / 2a
where v is the velocity before the brakes were applied
Define impact time
The duration of time that the impact force has on the other object
t = 2s / (v + u)
Give the equation relating the impact force to the change in kinetic energy
F = Change in Kinetic energy / Impact distance
Name 5 car safety features and explain how they reduce the impact force
1) Vehicle bumpers - give way a little in low-speed impact so increase the impact time
2) Crumple zones - the engine compartment of a car is designed to give way in front-on collision. This increases the impact time
3) Seat belts - in a front end collision, the seat belt restrains the wearer from hitting the vehicle frame after the vehicle suddenly stops. The restraining force from the seat belt is much less than the impact force if they hit the frame. The seat belt also stretches to increase the impact time
4) Collapsible steering wheel - If the driver makes contact with the steering wheel, the impact is lessened as a result of the steering wheel collapsing in the impact
5) Airbags - acts as a cushion, increasing the impact time on the person as well as spreading the force of impact over the contact area
