Year 7 Science · Unit 3 · Lesson 4
Foundation Worksheet
Learning Goals
Fill the gap
Choose the correct word from the word bank. Each word is used at most once. Two words are not needed: cross them out.
The net force is the total force on an object once every force has been combined, taking its into account. When two forces act in the same direction, you them. When they act in opposite directions, you the smaller force from the larger one, and the net force points the same way as the force. If the net force is , the forces are .
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Unbalanced forces change an object’s motion: it starts moving, speeds up, slows down or changes direction.
Correct it:
2. To find the net force, you combine all the forces on an object, taking their directions into account.
Correct it:
3. Balanced forces always mean that an object is not moving.
Correct it:
4. A car moving at a steady 60 km/h along a straight, flat road has a net force of zero.
Correct it:
5. On a free-body diagram, a longer arrow means the object is moving faster.
Correct it:
Sort it
Do the forces on each object cancel out? Write each situation in the correct box. The boxes do not hold the same number of items.
Balanced (net force = 0)
Unbalanced (net force is not 0)
1. A book sits still on a desk. Sketch the forces on it as labelled arrows drawn from a box. Then explain why it does not move, using the words balanced and net force.
Read the data
Scenario: the carnival tug of war
At a school sports carnival, the Blue team always pulls the rope to the left and the Red team always pulls it to the right. Table 1 shows how hard each team pulled in four matches.
| Match | Blue team, to the left (N) | Red team, to the right (N) | Net force (size and direction) |
|---|---|---|---|
| A | 450 | 450 | |
| B | 520 | 470 | |
| C | 390 | 465 | |
| D | 610 | 580 |
Source: made-up results for this worksheet.
2. Fill in the last column of Table 1: how many newtons are left over, and which way (left or right)? If nothing is left over, write 0 N.
3. (a) In which match does the rope stay still? (b) Jai says: “In Match D the rope will stay still too, because the teams are almost equal.” Is Jai right? Use the net force to explain.
4. A skydiver jumps out of a plane. Just after she jumps, she speeds up. Later she falls at a steady speed, called terminal velocity. Describe the forces on her at each stage and explain her motion. Use the words weight, air resistance, net force and balanced.
Fill the gap
direction; add; subtract; bigger; zero; balanced. Not used: speed, mass.
True or false
1. True. 2. True. 3. False: balanced forces mean the motion does not change; the object may be still, or moving at a steady speed in a straight line. 4. True. 5. False: a longer arrow means a bigger force; arrows show forces, not speed.
Sort it
Balanced: a parked car on a flat road; a cyclist riding at a steady speed in a straight line; a lamp hanging still from the ceiling. Unbalanced: a car pulling away from traffic lights; a rocket lifting off the pad; a car slowing down as it brakes; a ball speeding up as it rolls down a slope.
Question 1
Box with a down arrow labelled weight (gravity) (1) and an up arrow labelled normal force (desk pushing up), same length (1). The forces are equal and opposite, so balanced (1); net force is zero, so the still book stays still (1). [4 marks]
Read the data: Question 2
A: 0 N, balanced. B: 50 N to the left. C: 75 N to the right. D: 30 N to the left. 1 each; B, C and D need the direction. [4 marks]
Question 3
(a) Match A (1). (b) No (1): in Match D the net force is 30 N to the left, so the forces are unbalanced and the rope’s motion changes; it starts moving to the left (1). [3 marks]
Question 4
Marking guide: 1 weight acts down and air resistance acts up; 1 just after the jump, weight is bigger than air resistance, so the net force is downward and the forces are unbalanced; 1 so she speeds up; 1 at terminal velocity, air resistance equals weight, so the net force is zero and the forces are balanced; 1 so her speed stays the same: she is still falling fast, not stopped. [5 marks]
Model answer: Weight pulls her down; air resistance pushes up. At first weight is bigger, so the net force is down, the forces are unbalanced and she speeds up. Later air resistance equals her weight: the forces are balanced, the net force is zero, and she falls at a steady speed, still fast.
Common error: reading “net force = 0” as “not moving”. Zero net force means no change in motion.