P5 Newton's First and Second Laws (page 211)

Question 1

In the 1660’s what was the name of the person who worked out useful ‘laws of motion’?

Answer 1

Isaac Newton.

Question 2

What is Newton’s First Law?

Answer 2

newton’s first law says that a resultant force (p.203) is needed to make something start moving, speed up or slow down:

If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity (same speed and direction).

Question 3

When a train or car or bus or anything else is moving at a constant velocity what must the resistive and friving forces on it be?

Answer 3

they must all be balanced. The velocity will only change if there’s a non-zero resultant force acting on the object.

Question 4

A non-zero resultant force will alway produce what? and what direction?

Answer 4

it will always produce acceleration (or deceleration) in the direction of the force.

Question 5

The acceleration can take five different forms, what are they?

Answer 5

starting, stopping, speeding up, slowing down and changing direction.

Question 6

On a free body diagram, will the arrows be equal or unequal? (see diagram on page 211)

Answer 6

will be unequal

Question 7

A force is needed to change what?

Answer 7

Motion

Question 8

Acceleration is Proportional to what?

Answer 8

the Resultant Force.

Question 9

The larger the resultant force acting on an object, the more the oject accelerates why?

Answer 9

the force and the acceleration are directly proportional. (You can write this as F ∝ a)

Question 10

What is the directly proportional symbol?

Answer 10

∝

Question 11

Accelleration is also inversely proportional to the mass of the object, why?

Answer 11

so an object with a larger mass will accelerate less than one with a smaller mass (for a fixed resultant force).

Question 12

There’s an incredibly useful formla that describes ‘Newson’s Second Law’ what is it?

Answer 12

F = ma

F - Resultant force (N)
m - Mass (kg)
a - Acceleration (m/s²) - see diagram on page 211

Question 13

A van of mass of 2080kg has an engine that provides a driving force of 5200 N.

At 70 mph the drag force acting on the van is 5148 N. Find its acceleration at 70 mph

Answer 13

1) work out the resultant force on the van (drawing a free body diagram may help).

2) Rearrange F = ma and stick in the values you know

Resultant force = 5200 - 5148 = 52 N

a = F ÷ m
= 52 ÷ 2080 = 0.025 m/s²

Question 14

What can you use Newton’s Second Law for?

Answer 14

to get an idea of the forces involved in everyday transport. Large forces are needed to produce large accelerations.

Question 15

Estimate the resultant force on a car as it accelerates from rest to a typical speed?

Answer 15

1) Estimate the acceleration of the car, using typical speeds from page 207. (the ~ means approximately)

2) Estimate the mass of the car

3) Put these numbers into Newton’s 2nd Law

A typical speed of a car is ~25 m/s. It takes ~10 s to reach this.

So a = Δv ÷ t = 25 ÷ 10 = 2.5 m/s²

Mass of a car is ~1000 kg.

So using F = ma 1000 x 2.5 = 2500 N

So the resultant force is ~2500 N

Question 16

Find the force needed for an 80 kg man on a 10 kg bike to accelerate at 0.25 m/s² (2 marks)

Answer 16

F = ma = (80 + 10) x 0.25 (1 mark)
= 22.5 N (1 mark).