Finding the Resultant Force

Science · Forces & Motion · ages 11-12

Name ______________________ Date ____________

If two equal-sized forces pull in opposite directions on an object, the resultant force is zero.
Circle one: True False
Two people push a broken-down car forward, one with 32 N and one with 26 N. Type the size, in newtons, of the resultant force.
Answer: ______________
Two forces act on an object: 10 N to the right and 10 N to the right. What is the resultant force?
- 20 N right
- 0 N
- 10 N right
- 10 N left
Why is force called a vector quantity?
- It only has a size
- It has both a size and a direction
- It only has a direction
A book sits still on a table. What can you say about the forces on it?
- No forces act on it at all
- The forces are balanced: gravity and the normal force are equal and opposite
- The forces are unbalanced, which is why it doesn't move
Which of these statements about resultant force must be true?
- If the resultant force on an object is zero, its motion does not change
- If the resultant force on an object is zero, no forces are acting on it
- The resultant force is always found by adding every force together
A go-kart rolls along a flat track after its engine switches off. Friction keeps acting on the kart. What happens to its speed?
- It slows down, because friction pushes against its motion
- It speeds up, because friction gives it an extra push
- It stays the same, because no forces act without the engine
A crate is pulled left with 12 N and right with 5 N. Type the size, in newtons, of the resultant force.
Answer: ______________
Two people push a crate forward with 25 N and 15 N. Friction pushes back with 30 N. Type the size, in newtons, of the resultant force.
Answer: ______________
Mina puts a full water bottle on a table and says the table pushes up on the bottle. Her friend says a table cannot push anything. Who is right, and why?
- Mina: the table pushes up with a normal force equal to gravity, so the bottle stays still
- Her friend: tables are not alive, so they cannot push
- Neither: the bottle stays still because the table blocks gravity

Answer key

For grown-ups. Fold this page away before handing over the rest.

Finding the Resultant Force W1-mt_AiWlJfvC3O-s1

True · Equal and opposite forces cancel each other out, so nothing is left over to change the object's motion.
58 · Same-direction forces add: 32 N + 26 N = 58 N forward.
20 N right · Forces in the same direction add. 10 N + 10 N = 20 N in that direction.
It has both a size and a direction · A vector needs both parts. Force always pushes or pulls a certain amount in a certain direction.
The forces are balanced: gravity and the normal force are equal and opposite · Gravity pulls down and the table pushes up with an equal normal force. Balanced forces, zero resultant, no motion.
If the resultant force on an object is zero, its motion does not change · A zero resultant means balanced forces, so nothing about the object's motion changes. Forces can still act; they just cancel.
It slows down, because friction pushes against its motion · Friction acts against the kart's motion, so the forces are unbalanced and the kart slows down.
7 · Opposite forces subtract: 12 N - 5 N = 7 N, pointing left, the direction of the bigger force.
10 · Combine the same-direction pushes first: 25 N + 15 N = 40 N forward. Then subtract friction: 40 N - 30 N = 10 N forward.
Mina: the table pushes up with a normal force equal to gravity, so the bottle stays still · Surfaces push back with a normal force. The table pushes up as hard as gravity pulls down, so the bottle stays still.