Chapter 7 - Newton’s laws of motion

Question 1

State Newton’s first law of motion

Answer 1

Object either stays at rest or moves with constant velocity, unless a resultant force acts on it

Question 2

State Newton’s 2nd law of motion

Answer 2

F= ma

Acceleration is proportional to the resultant force acting on it

Question 3

A vehicle of mass 600kg accelerates uniformly from rest to a velocity of 8ms^-1 in 20 seconds. Calculate the force needed to produce this acceleration

Answer 3

a = v-u/t
a = 8/20
a= 0.4ms^-1
F=ma
F= 600 x 0.4
F = 240N

Question 4

What is inertia?

Answer 4

Resistance to change motion
More force is needed to give an object a certain acceleration than to give an object with less mass the same acceleration

Question 5

Write algebraically the equation for resultant force of a rocket.

Answer 5

Thrust -mg = ma

T = mg +ma

Question 6

What does the drag force of an object depend on?

Answer 6

• Shape of the object
• Its speed
• The viscosity of the fluid (how easily a fluid flows past a surface)

Question 7

What is terminal velocity?

Answer 7

Maximum velocity attainable by an object as it falls through a fluid.
When the drag force is equal and opposite to it’s weight

Question 8

Describe the forces acting on an object falling and reacting terminal velocity.

Answer 8

Accelerates downwards due to weight, acceleration gradually decreases as drag force (air resistance) increases, until it equals the object’s weight, this is terminal velocity - there is no resultant force on the object.

Question 9

What is the equation for stopping distance?

Answer 9

Stopping distance = thinking distance + braking distance

Question 10

What is thinking distance?

Answer 10

Distance travelled by a vehicle in the time it takes for the driver to react.

Question 11

What factors affect the thinking distance/reaction time?

Answer 11

• Distractions e.g. mobile phones
• Drugs
• Alcohol

Question 12

What is braking distance?

Answer 12

Distance travelled by a car in the time it takes to stop safely, from when the brakes are applied.

Question 13

What factors affect braking distance?

Answer 13

• Speed of vehicle when brakes are applied
• Road conditions e.g. icy or greasy
• Conditions of tyres

Question 14

How do we express an impact force?

Answer 14

In terms of g.

e.g. an acceleration of -30ms^-2 is -3g in terms of g.

Question 15

How do you calculate impact time of a collision?

Answer 15

t = 2s/u+v

Question 16

How do you work out the impact force of a collision?

Answer 16

find acceleration - a=v-u/t

then find force using f=ma

Question 17

How do you calculate work done by impact force

Answer 17

F = Change in kinetic energy/ impact distance

Question 18

Give examples of some vehicle safety features that are designed to increase impact time and reduce impact force?

Answer 18

• Vehicle bumpers - give way at low speeds - increase impact time
• Crumple zones - give way in front end impact - increase impact time
• Seat belts - stops wearer crashing into vehicle frame when stopping. Restraining force much smaller than impact force would be.
• Collapsable steering wheel - impact force of driver reduced if they hit steering wheel as it collapses.
• Airbags - increases impact time and spreads impact force over the contact area.