Forces and Motion
What is a force?
A push or a pull.
What happens to an object when unbalanced forces act on it?
The object will speed up, slow down, change direction, or start moving (it will accelerate).
Which is the amount of matter in an object: mass or weight?
Mass
What is friction?
A force that resists motion between surfaces that touch, pushing opposite the direction of motion or attempted motion.
Name two things that forces can cause an object to do.
Speed up, slow down, change direction, change shape (any two).
If you push a toy car and it speeds up, which type of force is acting: balanced or unbalanced?
Unbalanced; because the applied push is greater than opposing forces, causing motion.
Does your mass change if you travel to the Moon? Does your weight change? Explain both briefly.
Mass stays the same; weight changes because gravity is different on the Moon (smaller).
When you walk on icy ground it’s harder to walk without slipping. Explain why using friction.
Ice has much less friction with boots than rough ground does, so there is less grip and feet slip more easily.
Explain why forces cannot be seen but can be detected.
Forces are invisible effects; we detect them by changes in motion or shape
A curtain hangs still because gravity pulls down and the rod pulls up. What does this tell you about the forces on the curtain?
The forces are balanced (equal and opposite), so the curtain stays still (equilibrium).
If someone’s weight on Earth is 100 pounds, approximately what would their weight be on the Moon? Show the reasoning in one sentence.
About 16 pounds because Moon gravity is about one-sixth of Earth’s: 100 X 1/6 =16.
Give two examples where friction is helpful and two examples where friction causes problems.
Helpful: walking (grip), car tires on road (traction). Problem: machine parts wearing down, heat from moving parts.
Give an example of a force provided by a living thing and describe its effect.
Muscles pushing bones to move an arm; effect: arm moves in the direction of the muscle force.
You push a swing to make it move; later the swing slows and stops. Name at least two forces involved in making the swing slow down.
Forces: gravity (pulling down), air resistance and/or friction at the pivot, and the pushing force earlier; friction and air resistance slow it down.
Explain why astronauts’ bones may weaken after spending a long time in space, using the ideas of mass and gravity.
In low-gravity (microgravity) bones don’t bear as much force, so they lose strength and mass over time.
Skateboard wheels rub the ground and slow down only a little. How can a skateboarder increase friction to stop faster? Describe the action and why it increases friction.
They can drag the back end of the board on the ground or press their foot down to create more surface contact; this increases the contact force and friction so the board slows faster.
Describe how the Sun’s gravity affects motion in the solar system
The Sun’s gravity pulls planets, keeping them in orbit around the Sun.
Describe a real-life scenario (not from the text) where two forces act in opposite directions and the stronger one causes motion. Explain which is stronger and why.
Tug-of-war where Team A pulls harder than Team B so the rope moves toward Team A; Team A’s force is stronger than Team B’s.
A student has a mass of 60 kilograms on Earth. Write one clear sentence explaining whether that mass would be different on the Moon and whether their weight would be different. (Do not calculate weight — just explain the difference conceptually.)
Mass would be the same on the Moon; weight would be less on the Moon because the Moon’s gravity is weaker.
A machine’s gears heat up because of friction. Explain one practical way engineers reduce friction inside machines and why it works.
Apply lubrication (oil) between moving parts to reduce contact friction; lubrication forms a thin film that reduces direct rubbing and heat.