Motion and Force

Question 1

Define Inertia

Answer 1

The mass of an object is a measure of its inertia, which is its resistance to change of motion

Question 2

Name 3 factors that effect drag force

Answer 2

The shape of the object
Its speed
The viscosity of the fluid

Question 3

State the equation for an object at terminal velocity

Answer 3

a = g - (D/m) = 0
where a= acceleration
           g= gravitational field strength
           D= drag force
           m= mass

Question 4

State Newton’s first law of motion

Answer 4

Objects either stay at rest or remain in uniform motion unless acted on by a force.

Question 5

Describe the relationship between the resultant force on an object and its acceleration

Answer 5

The acceleration of an object is always in the same direction as the resultant force:
F = ma

Question 6

Define weight

Answer 6

The weight of an object is its mass multiplied by the acceleration due to gravity
W = mg

Question 7

For 2 forces, F₁ and F₂ acting on a point object in opposite directions, where F₁ > F₂, give the equation for the resultant force

Answer 7

F₁ - F₂ = ma

Question 8

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

Answer 8

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)

Question 9

For a rocket of mass m propelling itself upwards with thrust T, give the equation for the magnitude of thrust T

Answer 9

T - mg = ma

T = mg + ma

Question 10

For a lift, give the conditions when the tension in the cable is less than the weight

Answer 10

• The lift is moving up and decelerating (v>0, a

Question 11

For a lift, give the conditions when the tension in the cable is greater than the weight

Answer 11

• The lift is moving up and accelerating (v>0, a>0)
  Or
• The lift is moving down and decelerating (v0)

Question 12

Define terminal velocity

Answer 12

When the drag force is equal and opposite to its weight, for an object falling in a fluid

Question 13

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

Answer 13

a = (Fᴱ - Fᴿ) / m

Question 14

For a car with engine force of Fᴱ and resistive force of Fᴿ, state when the car will be at maximum velocity

Answer 14

When Fᴱ = Fᴿ

Question 15

Define thinking distance

Answer 15

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

Question 16

Define braking distance

Answer 16

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

Question 17

Define stopping distance

Answer 17

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

Question 18

Define impact time

Answer 18

The duration of time that the impact force has on the other object
t = 2s / (v + u)

Question 19

Give the equation relating the impact force to the change in kinetic energy

Answer 19

F = Change in Kinetic energy / Impact distance

Question 20

Name 5 car safety features and explain how they reduce the impact force

Answer 20

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