Grade 7 Science: Motion, Forces and Thermal Energy

Step 1: Average speed is the total distance divided by the total time.

Step 2: The total distance is 300 meters and the total time is 60 seconds.

Step 3: 300 meters divided by 60 seconds is 5 meters per second.

Step 4: Check the units: a distance unit per a time unit, here meters per second.

Step 1: Distance is the whole path: 30 meters plus 10 meters is 40 meters.

Step 2: Displacement is how far you ended up from where you started, and in which direction: 30 meters east, then 10 meters back, leaves you 20 meters east.

Step 3: Average speed uses distance: 40 meters in 20 seconds is 2 meters per second.

Step 4: Average velocity uses displacement: 20 meters east in 20 seconds is 1 meter per second east.

Step 1: From 0 to 4 minutes, the line rises steadily from 0 to 200 meters. Maya walks at a steady speed: 200 meters in 4 minutes is 50 meters per minute.

Step 2: From 4 to 6 minutes, the line is flat at 200 meters. Her distance does not change, so she is stopped, maybe waiting at a crosswalk.

Step 3: From 6 to 8 minutes, the line climbs steeply to 600 meters. She covers 400 meters in 2 minutes, 200 meters per minute, so she is jogging.

Step 4: For the whole trip, 600 meters in 8 minutes is an average speed of 75 meters per minute.

Step 1: Newton’s first law: an object at rest stays at rest, and an object in motion keeps moving at the same speed in the same direction, unless an unbalanced force acts on it.

Step 2: Ice has very little friction, so the forces on the sliding puck are nearly balanced. It glides on at nearly the same speed.

Step 3: When the puck hits a hockey stick, the stick pushes on it. That unbalanced force changes its motion: it slows, stops or changes direction.

Step 4: On rough concrete, friction is a strong unbalanced force against the motion, so the puck slows down and stops quickly.

Step 1: Conduction: the hot burner touches the pot, and the metal passes thermal energy into the water and along the handle. Conduction moves energy through direct contact.

Step 2: Convection: water at the bottom warms up, spreads out and rises, while cooler water sinks to take its place. This loop is a convection current. It happens only in liquids and gases, which can flow.

Step 3: Radiation: the glowing burner also sends out energy as waves that warm things nearby without touching them. Radiation can even cross empty space.

Step 1: The spoon is warmer than the water, so thermal energy flows from the spoon into the water.

Step 2: The spoon cools down and the water warms up. Energy keeps flowing as long as there is a temperature difference.

Step 3: When the spoon and the water reach the same temperature, the flow between them stops. They are in thermal equilibrium.

Step 4: Over a longer time, the cup also trades energy with the air, until everything is at room temperature.

Step 1: All particles are always moving. The energy of their motion is kinetic energy.

Step 2: In the ice water, the particles move slowly, so their average kinetic energy is low. The temperature is low.

Step 3: In the hot cocoa, the particles move much faster, so their average kinetic energy is high. The temperature is high.

Step 4: A thermometer works because the liquid inside it gains energy from faster particles around it, spreads out and rises up the tube.