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Balanced and Unbalanced Forces — Answers

Answer sheet with model answers and success criteria for the Year 7 forces & motion worksheet.

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Success TutoringWorksheetsAnswer sheetYear 7 · Science · Forces & Motion

Balanced and Unbalanced Forces — Answers

Part A: What are forces?

Read the passage, then answer the questions.

A force is a push or a pull. Forces can make things start moving, stop, speed up, slow down, change direction or change shape. Forces are measured in newtons (N).

We draw forces as arrows. The arrow points in the direction of the force, and a longer arrow shows a bigger force.

When the forces on an object are equal in size and opposite in direction, they are balanced: a still object stays still and a moving object keeps moving at the same speed in the same direction. When they are unbalanced, the object's motion changes.

1.What unit are forces measured in?
  1. Akilograms
  2. Bmetres
  3. Cnewtons
  4. Dseconds

Answer: newtons

Forces are measured in newtons (N), named after the scientist Isaac Newton.

2.A book rests on a table without moving. Which statement is correct?
  1. AThere are no forces on the book.
  2. BGravity pulls it down and the table pushes it up with an equal force, so the forces are balanced.
  3. COnly gravity acts on the book.
  4. DThe table's push is bigger than gravity.

Answer: Gravity pulls it down and the table pushes it up with an equal force, so the forces are balanced.

Gravity pulls the book down and the table pushes up just as hard. The forces are balanced, so the book stays still.

3.What can unbalanced forces do to a moving object?
  1. Anothing at all
  2. Bonly make it stop
  3. Cmake it speed up, slow down or change direction
  4. Dmake it disappear

Answer: make it speed up, slow down or change direction

An unbalanced force changes an object's motion: its speed, its direction, or both.

4.A box slides across the floor and slowly stops. Which force slowed it down?
  1. Afriction
  2. Bmagnetism
  3. Cupthrust
  4. Delectricity

Answer: friction

Friction acts between the box and the floor, against the direction of motion.

5.A spring balance used to measure forces is also called a (answer space) meter.

Answer: force

Also accept: newton.

Part B: Calculating the net force

Show your working.

The net force is the overall force on an object. If forces act in the same direction, add them. If they act in opposite directions, subtract the smaller from the bigger. The object's motion changes in the direction of the bigger force.

6.Match each force to what it does.
  • gravity
  • friction
  • air resistance
  • upthrust
  • pushes up on objects in water
  • pulls objects towards the centre of the Earth
  • slows down objects moving through air
  • acts between two surfaces that rub or slide against each other

Answer: gravity → pulls objects towards the centre of the Earth; friction → acts between two surfaces that rub or slide against each other; air resistance → slows down objects moving through air; upthrust → pushes up on objects in water

7.In a tug of war, Team A pulls left with 200 N and Team B pulls right with 150 N. What is the net (overall) force, and which way will the rope move?

Answer: 50 N to the left — the rope moves towards Team A.

  1. 200 − 150 = 50 N
  2. The bigger pull is to the left.
8.Two friends push a heavy box to the right with forces of 30 N and 40 N. Friction acts on the box with a force of 50 N to the left. What is the net force on the box?

Answer: 20 N to the right

  1. 30 + 40 = 70 N to the right
  2. 70 − 50 = 20 N to the right
9.A boat has a weight of 500 N pulling down. The water pushes up on it with an upthrust of 500 N. Are the forces balanced or unbalanced? What happens to the boat?

Answer: Balanced (net force 0 N). The boat floats without sinking or rising.

  1. 500 − 500 = 0 N

Part C: True or false?

Write True or False.

10.A bike rolling along at a steady speed can have balanced forces acting on it.

Answer: True

Balanced forces mean no change in motion, so a moving object keeps moving at the same speed.

11.Gravity can only act on an object when it is touching the ground.

Answer: False

Gravity is a non-contact force. It pulls on objects even when they are in the air.

12.Opening a parachute increases air resistance.

Answer: True

The large canopy pushes against much more air, so air resistance increases and the skydiver slows down.

13.If a car is speeding up, the forces on it are balanced.

Answer: False

Speeding up is a change in motion, so the forces must be unbalanced (the forward force is bigger).

14.A force can change the shape of an object.

Answer: True

For example, squeezing a sponge or stretching a rubber band changes its shape.

Part D: Explain it

15.Describe the forces on a skydiver at each stage: (1) just after jumping from the plane, (2) falling at a steady top speed, and (3) just after opening the parachute. For each stage say whether the forces are balanced or unbalanced and what happens to the skydiver's speed.

Model answer: (1) Just after jumping, gravity pulling down is much bigger than air resistance pushing up. The forces are unbalanced, so the skydiver speeds up. (2) As the skydiver falls faster, air resistance increases until it equals gravity. The forces are balanced, so the skydiver falls at a steady speed. (3) When the parachute opens, air resistance becomes much bigger than gravity. The forces are unbalanced upwards, so the skydiver slows down until the forces are balanced again at a slower, safe speed.

Success criteria:

  • Names gravity and air resistance
  • Stage 1: unbalanced, speeds up
  • Stage 2: balanced, steady speed
  • Stage 3: unbalanced, slows down
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