Forces

Question 1

What unit are forces measured in?

Answer 1

Newtons, N

Question 2

What do two objects always exert on each other?

If a car crashes into a barrier it exerts a force on the barrier what does the barrier exert on the car?

Answer 2

Equal and opposite forces.

The barrier exerts a force on the car that is equal in size and in the opposite direction.

Question 3

Detail the forces exerted when a car travels along a road.

Answer 3

When a car is being driven forwards there is a force from the tyre on the ground pushing backwards, there is an equal and opposite force from the ground on the tyre which pushes the car forwards.

Question 4

What is the resultant force?

Answer 4

The resultant force is the single force that has the same effect on the object as all the original forces acting together.

Question 5

What two things can be said about an object if the resultant force is zero?

Answer 5

1) If the object is at rest, it will stay at rest.

2) If the object is moving it will carry on moving at the same speed in the same direction.

Question 6

What three things can be said about an object if the resultant force is not zero?

Answer 6

1) If the object is at rest, it will accelerate in the direction of the resultant force.
2) If the object is moving in the same direction as the resultant force, it will accelerate in that direction.
3) If the object is moving in the opposite direction to the resultant force, it will decelerate.

Question 7

What does it mean if the resultant force is not zero?

Answer 7

The object will accelerate in the direction of the resultant force.

Question 8

What is needed to make an object change direction?

Answer 8

A resultant force.

Question 9

How can the resultant force be calculated?

Answer 9

F = m x a
F is the force in newtons, N
m is the mass in kilograms, kg
a is the acceleration in m/s squared

Question 10

What affect does the resultant force and the mass of an object have the acceleration of the object?

Answer 10

The greater the resultant force of an object, the greater its acceleration.
The bigger the mass of an object, the bigger the force needed to give it a particular acceleration.

Question 11

If the resultant force on a vehicle is zero, it is travelling at a steady speed, what can be said about the driving and frictional forces?

Answer 11

The driving forces are equal and opposite to the frictional forces.

Question 12

How does speed of an object affect the deceleration required to stop it in a particular distance and the braking force?

Answer 12

The greater the speed of an object, the bigger the deceleration required to stop it in a given distance. So the bigger the braking force needed.

Question 13

What is the stopping distance of a vehicle?

Answer 13

The stopping distance of a vehicle is the distance it travels during the driver’s reaction time (the thinking distance) plus the distance it travels under the braking force (The braking distance).

Question 14

What is the thinking distance and how is the thinking distance increased?

Answer 14

The thinking distance is the distance a vehicle travels during the driver’s reaction time, which is increased if the driver is tired or under the influence of drugs or alcohol.

Question 15

What is the braking distance and how is the braking distance increased?

Answer 15

The braking distance is the distance the car travels under the braking force, which is increased by bad weather conditions or poorly maintained roads. The condition of the car, e.g. worn tyres or brakes will increase the braking distance.

Question 16

If an object is falling freely what is the resultant force acting on it?
How will this force make an object close to the earth’s surface accelerate?

Answer 16

If an object is falling freely the resultant force acting on it is the force of gravity, it will make an object close to the earth’s surface accelerate at 10 m/s squared.

Question 17

What do we call the force of gravity and the acceleration?

Answer 17

We call the force of gravity weight and the acceleration, the acceleration due to gravity.

Question 18

How is the force of gravity/weight calculated?

Answer 18

W = mg
W is the weight in newtons, N
m is the mass in kilograms, kg
g is the acceleration due to gravity, in m/s squared.

Question 19

If the object is not falling and is on the earth what do we call g and what are its units?

Answer 19

If the object is on earth and is not falling, g is the gravitational field strength and its units are newtons per kilogram, kg.

Question 20

When an object falls through a fluid, what does the fluid exert on the object and what affect does this have on the object?

Answer 20

When an object falls through a fluid, it exerts a drag force on the object, which resists the object’s motion.

Question 21

What affect does the speed of a falling object have on the drag force of the object, what will eventually happen to the drag force?

Answer 21

The faster the object falls, the bigger the drag force exerted on the object becomes until eventually the drag force is equal to the weight of the object.

Question 22

What happens to the resultant force and velocity of a falling object when when the drag force is equal to the weight of the object?

Answer 22

The resultant force is now zero so the object stops accelerating, it moves at a constant velocity called the terminal velocity.

Question 23

What is the extension of a spring?
When we remove weights from a spring what will happen?
What name is given to objects that behave in this way?

Answer 23

The extension of a spring is the increase in its length from the original if we hang small weights from it.
The spring will return to its original position.
Elastic.

Question 24

What can be said about the extension of a spring and the force applied to it?
What happens if we apply to big a force to a spring?

Answer 24

It is directly proportional.

The line on a graph will curve because we have exceeded the limit of proportionality.

Question 25

What does direct proportion look like on a graph?

Answer 25

A graph shows direct proportion if the line of best fit is a straight line through the origin.

Question 26

What is Hooke’s Law?

Answer 26

Hooke’s Law states that extension is directly proportional to the force applied , provided the limit of proportionality is not exceeded.

Question 27

Hooke’s law can be written as what equation?

Answer 27

F = k x e
F is the force applied in Newtons, N
k is the spring constant of the spring in newtons per metre, N/m
e is the extension of the spring in metres, m

Question 28

How does stiffness affect the spring constant of a spring?

Answer 28

The stiffer the spring, the greater its spring constant.

Question 29

What happens when an elastic object is stretched and how is the energy stored?
What happens when the stretching force is removed?

Answer 29

When an elastic object is stretched, work is done and the energy is stored as elastic potential energy.
This stored energy is released.

Question 30

What affect does reducing the speed of a vehicle have on the amount of fuel it uses to travel a particular distance?
What is this called?

Answer 30

Reducing the speed of a vehicle reduces the amount of fuel it uses to travel a particular distance.
This is called fuel economy.

Question 31

Apart from reducing the speed of a vehicle what else improves fuel economy?
What can be fitted to a vehicle to do this?

Answer 31

Reducing the air resistance by making the vehicle more streamlined also improves fuel economy.
A wind deflector.

Question 32

What are speed cameras used for?
In what two ways can they be used?
What may happen to motorists caught speeding?

Answer 32

Speed cameras are used to discourage motorists from speeding.

1) They can determine the speed of a motorist at a particular point.
2) They can also be used in pairs to determine the speed at two points and so calculate an average speed.

Motorists caught speeding are fined and may loose their drivers licence.

Question 33

When does skidding happen?

What happens when a vehicle skids?

Answer 33

Skidding happens when the brakes of a car are applied to harshly.
When a vehicle skids the wheels lock and slide along the road surface, increasing the stopping distance.

Question 34

What are anti-skid surfaces used for?
How are they different to a normal road surface?
Where are they normally used?

Answer 34

Anti-skid surfaces are used to reduce or prevent skidding.
Anti-skid surfaces are rougher than normal road surfaces, increasing the friction between the vehicle tyres and the road.
They are used in places where drivers are likely to brake, such as near traffic lights and road junctions.