# Balanced and Unbalanced Forces

In this worksheet, students will explore the question 'how do forces affect speed?'

Key stage:  KS 3

Curriculum topic:   Physics: Motion and Forces

Curriculum subtopic:   Motion and Forces

Difficulty level:

### QUESTION 1 of 10

For an object to move, the forces on it must be unbalanced.

In the diagram below, the duck is floating on the water. It is not moving up or down because the force acting downwards (caused by gravity) is equal to the force of the water pushing up on to the duck (upthrust).

The force of gravity (red arrow) is equal to the upthrust (green arrow) from the water.

The car shown below is accelerating because the driving force (green arrow) is greater than the drag caused by air resistance (red arrow).

The car is accelerating left to right because the driving force is greater than the air resistance

When the drag is greater than the driving force, the car will slow down; but if they are balanced and the car is already moving, it will continue to move at a constant speed. This is described in more detail in Isaac Newton's 'Laws of Motion'.

There are many forces acting on a skydiver when they leave the plane:

As a skydiver leaves the plane, they accelerate downwards due to gravity, but as their speed increases, so does the air resistance. This continues until both forces are balanced and the skydiver will fall at a constant speed, called their terminal velocity.

Complete the sentence below by typing the missing word into the text box.

When an object is stationary then the forces on it are _____________.

Complete the sentence below by typing the missing word into the text box:

When an object starts moving, then the forces on it are _____________.

Complete the following sentence by selecting one of the options below:

If a ball falls a short distance, the downwards force is........

smaller than the air resistance

greater than the air resistance

equal to the air resistance

Complete the following sentence by selecting one of the options below:

At terminal velocity the downwards force is.....

smaller than the air resistance

greater than the air resistance

equal to the air resistance

Complete the following sentence by selecting one of the options below:

A cyclist pedals to 10 m/s and then stops pedalling, which results in his speed decreasing because the driving force is....

smaller than the air resistance

greater than the air resistance

equal to the air resistance

A Harrier jump-jet is capable of hovering as well as performing many manoeuvres which require forces in all directions.

Complete the table below to give the direction of the force when the plane is moving

 forces balanced downwards force high upthrust high moving downwards moving upwards hovering

Complete the table below showing the forces on a skydiver at various points during the jump.

 forces balanced downwards force high upthrust high accelerating downwards terminal velocity parachute deployed

Complete the table below showing the forces on a car when moving.

 forces balanced drag high drive high accelerating steady speed decelerating
• Question 1

Complete the sentence below by typing the missing word into the text box.

When an object is stationary then the forces on it are _____________.

balanced
EDDIE SAYS
When an object is stationary, the forces acting upon it are balanced. That means that there isn't a greater force in one direction or the other - consequently, the object doesn't move/change direction.
• Question 2

Complete the sentence below by typing the missing word into the text box:

When an object starts moving, then the forces on it are _____________.

unbalanced
EDDIE SAYS
To start a stationary object moving, the forces on it must change from balanced (no movement) to unbalanced (initial movement). For example, kick a football that's sitting on the floor and the force of your foot against the ball provides a motive force forwards, overcoming the natural resistive force of floor and air - thus unbalancing the forces to make the ball move.
• Question 3

Complete the following sentence by selecting one of the options below:

If a ball falls a short distance, the downwards force is........

greater than the air resistance
EDDIE SAYS
The forces will be unbalanced, with the downward force being greater than the air resistance. This means that the ball will fall downwards. The forces will only be equal at terminal velocity and that won't occur over a short distance.
• Question 4

Complete the following sentence by selecting one of the options below:

At terminal velocity the downwards force is.....

equal to the air resistance
EDDIE SAYS
At terminal velocity the forces are balanced so the object will continue to move at a constant speed.
• Question 5

Complete the following sentence by selecting one of the options below:

A cyclist pedals to 10 m/s and then stops pedalling, which results in his speed decreasing because the driving force is....

smaller than the air resistance
EDDIE SAYS
Air resistance (or drag) along with friction from the road surface slows the cyclist down.
• Question 6

A Harrier jump-jet is capable of hovering as well as performing many manoeuvres which require forces in all directions.

Complete the table below to give the direction of the force when the plane is moving

 forces balanced downwards force high upthrust high moving downwards moving upwards hovering
EDDIE SAYS
Harrier jump-jets hover, to allow them to land on aircraft carriers more easily. Although recently withdrawn from service, their ability to hover like a helicopter made them invaluable in various different situations. The forces involved in taking off vertically, flying forward and hovering are an amazing part of its design. The new Lightning II is the Harrier's replacement.
• Question 7

Complete the table below showing the forces on a skydiver at various points during the jump.

 forces balanced downwards force high upthrust high accelerating downwards terminal velocity parachute deployed
EDDIE SAYS
When the skydiver pulls on the rip-cord, the parachute above them opens - this massively increases their air resistance (drag) and so they slow down rapidly due to the increased upthrust. Once they are falling at a constant speed towards the ground (terminal velocity), the forces are balanced (not speeding up/slowing down).
• Question 8

Complete the table below showing the forces on a car when moving.

 forces balanced drag high drive high accelerating steady speed decelerating
EDDIE SAYS
The drag (air/road resistance) slows the car down, but brakes increase the rate of deceleration when they are applied, increasing the force that is opposing forward motion. It's worth noting these forces' effects on you, next time you're in a car: note your body seeming to move backwards when the car accelerates (that's the forward motion of the car scooping you up and forcing you forwards). See if you can work out what the forces on you are when the car brakes, and why your body does what it does.
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