Forces and Motion
Grade 4 · Science · Study Guide
Discover how forces cause objects to move, speed up, slow down, and change direction.
A force is a push or pull on an object. Forces can cause objects to start moving, stop moving, speed up, slow down, or change direction. Forces have both size (how strong) and direction (which way they push or pull). Scientists measure force in units called Newtons.
Gravity is a pulling force that attracts all objects with mass toward each other. Earth's gravity pulls everything toward its center, which is why things fall when you drop them. Friction is a force that resists movement when two surfaces rub together. Friction slows things down and produces heat. Rough surfaces create more friction; smooth surfaces create less.
When forces on an object are balanced (equal forces pushing in opposite directions), the object doesn't move — or keeps moving at the same speed. When forces are unbalanced, the object changes its motion. Isaac Newton described the laws of motion: objects at rest stay at rest and objects in motion stay in motion unless acted upon by an unbalanced force. This tendency is called inertia.
| Force | Type | Effect |
|---|---|---|
| Gravity | Pull | Pulls objects toward Earth's center |
| Friction | Resistance | Slows down moving objects |
| Applied force | Push or pull | Moves objects in the direction of the force |
| Air resistance | Resistance | Slows objects moving through air |
Key terms
- Force
- A push or pull that can change an object's motion
- Gravity
- A pulling force that attracts objects with mass toward each other
- Friction
- A force that resists motion when surfaces rub together
- Balanced forces
- Equal forces acting in opposite directions — no change in motion
- Unbalanced forces
- Forces that are not equal — causes a change in motion
- Inertia
- The tendency of an object to resist changes in its motion
Flashcards
- Force
- A push or pull that can change an object's motion
- Gravity
- A pulling force that pulls objects toward Earth's center
- Friction
- Force that resists motion between two surfaces in contact
- Inertia
- The tendency to resist changes in motion
- Balanced forces
- Equal, opposite forces — no change in motion
- Unbalanced forces
- Unequal forces — causes motion to change
Practice questions
What is friction?
- A pulling force from Earth
- A force that resists motion between surfaces
- A force that moves objects forward
- A pushing force in space
Show answer
Answer: A force that resists motion between surfaces
Friction is a contact force that acts against the direction of motion when two surfaces rub together, slowing objects down.
What happens when balanced forces act on an object?
- It speeds up
- It changes direction
- There is no change in motion
- It stops immediately
Show answer
Answer: There is no change in motion
When forces are balanced (equal and opposite), the net force is zero and the object's motion doesn't change — it stays still or keeps moving at the same speed.
Gravity is a pushing force.
Show answer
Answer: False
Gravity is a pulling force, not a pushing force. It pulls objects toward each other — Earth's gravity pulls things toward its center.
Rough surfaces create more friction than smooth surfaces.
Show answer
Answer: True
Rough surfaces have more microscopic bumps that catch on each other, creating more friction than smooth surfaces.
The tendency of an object to resist changes in its motion is called ___.
Show answer
Answer: inertia
Inertia is the property of matter that resists changes in motion. A ball sitting still resists being pushed; a rolling ball resists being stopped.
Fun facts
- The moon's gravity is about 1/6 of Earth's — you'd weigh 6 times less on the moon!
- Without friction, it would be impossible to walk — your feet would just slide out from under you.
- NASA uses the force of gravity from planets to 'slingshot' spacecraft to far parts of the solar system!
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