Week 13 · Axis 2, Unit 2 Motion, Concept 2.3 Energy and Collisions (A) · 3 lessons
1
When objects collide
Discovery Education Science P4: Term 1, Unit 2, pp. 158–171
GoalSay what a collision is and describe what happens to energy when two objects hit.
New words
collision · when two objects hit each otherbounce · to spring back after hitting somethinglevel crossing · a place where a road crosses a railway
Explanation
A collision happens when two objects hit each other. A cricket bat hits a ball, a car bumps a sign, a wrecking ball swings into an old wall. In every collision, energy moves. The bat has kinetic energy; when it hits the ball, some of that energy passes to the ball and the ball flies away. We can notice a collision with our senses: we see the ball change direction, we hear a bang or a click and we feel a jolt. Some of the energy leaves as sound and a little as heat. A moving object keeps moving until something stops it, so a collision can change how objects move, and sometimes it breaks or dents them. That is why drivers must stop before a level crossing. A train is very heavy and fast, and it cannot stop quickly.
moving object has kinetic energy→it hits another object→energy passes on; sound and heat too
Worked examples
Youssef rolls a tennis ball into a second tennis ball that is resting on the floor. The first ball slows down or stops, and the second ball starts to roll. Kinetic energy passed from the first ball to the second.
A boy kicks a football against a wall. The ball hits the wall and bounces back. We hear a thud, so some of the energy left as sound.
On a road near a railway, a barrier comes down and a bell rings. The cars wait. A train cannot stop quickly, so it is never safe to race it across the crossing.
Class activity
Roll and hit. On a smooth floor, a grown-up helps you set a small cardboard box as a target. Roll a tennis ball toward it gently, then with a bit more push. Watch the box and listen. Does it move? How far? Write what you see, hear and feel. Never roll balls toward people or pets.
Practice
1
What is a collision?
When two objects hit each other.
2
A bat hits a ball. What happens to the kinetic energy of the bat?
Some of it passes to the ball, and some leaves as sound and heat.
3
Which two things can you hear in a collision? Say the letters.
Letter
Thing
A
a bang
B
a colour
C
a click
A and C: a bang and a click.
4
Why must drivers always stop at a level crossing when the bell rings?
A train cannot stop quickly, and a collision would be very dangerous.
