Mass Matters
What is the amount of matter in an object called?
Mass.
What describes an object’s speed in a particular direction?
Velocity
What is a force?
A push or pull on an object caused by an interaction.
When two carts collide, does only one exert a force, or do both exert forces?
Both exert a force on the other cart.
The only male 8th-grade teacher
Mr. Himes
A 2 kg cart and a 6 kg cart receive equal net forces for equal times. Which has the greater velocity change?
The 2 kg cart, because it has less mass.
A cart speeds up from 2 m/s to 7 m/s while moving right. How much does its velocity change?
5 m/s to the right.
A cart experiences 12 N to the right and 7 N to the left. What is its net horizontal force?
5 N to the right.
During a collision, Cart A pushes Cart B with 20 N to the right. What force does B exert on A?
20 N to the left.
This 8th-grade teacher was a Cirque du soleil performer
Ms. Clark
A 2 kg cart and a 6 kg cart each change velocity by 3 m/s during separate, equally long pushes. Ignore other horizontal forces. Which needs the stronger force?
The 6 kg cart. Greater mass requires a stronger force to produce the same velocity change in the same time.
A bicycle travels around a circular track at a constant speed. Is its velocity changing? Why?
Yes. Its direction changes, and velocity includes direction.
A moving spacecraft’s thrusters turn off, and its net force becomes zero. What happens to its motion?
It keeps moving at the same speed in the same direction.
A 2 kg cart and a 6 kg cart collide. Ignoring other horizontal forces, which has the greater velocity change? Explain.
The 2 kg cart. The collision forces are equally strong, but the smaller mass has the greater velocity change.
This 8th-grade teacher has an engineering degree
Ms. Bhaskarapanthula
You are testing how mass affects velocity change. What should you change, measure, and keep the same?
Change: mass. Measure: velocity change. Keep the same: net-force strength and push duration.
00Two equal-mass carts receive pushes lasting the same time. Cart A goes from 1 to 5 m/s; Cart B goes from 8 to 10 m/s. All motion is rightward, and other horizontal forces are negligible. Which received the stronger force?
Cart A. Its velocity changes by 4 m/s, compared with 2 m/s for B. Final speed alone does not show force strength.
A pod moves right, but its net force points left. What happens to its speed at first?
It slows down because the force acts opposite its motion.
Collision forces are equal and opposite. Why don’t they cancel the force on each individual cart?
They act on different objects. Only forces acting on the same object combine to determine that object’s net force.
These 8th-grade teachers have kids
Mrs. Meza, and Ms. Volz
An empty pod and a loaded pod receive equal net forces for equal times. Which has the greater velocity change, and why?
The empty pod, because it has less mass. Equal forces can produce different velocity changes when masses differ.
Two equal-mass carts receive equally long pushes. Cart C slows from 8 to 4 m/s rightward; Cart D speeds up from 1 to 5 m/s rightward. Ignore other horizontal forces. Compare the force strengths and directions.
Equal strengths: both change velocity by 4 m/s. The force on C points left, and the force on D points right.
A student says, “A moving object stores force and loses that force when it slows down.” Correct this explanation.
Objects do not store force. A net force from an interaction changes an object’s velocity.
A 1,100 kg pod collides with a 415,400 kg space station. Ignore other forces. Compare their collision forces and velocity changes.
The forces are equal in strength and opposite in direction. The pod changes velocity more because it has much less mass.
This teacher's favorite band is Dirty Heads
Ms. Ostrander