Reference and Motion
A fixed object or background used to describe the position of another object.
What is a reference point?
What is the formula for speed?
Speed =Distance/Time
State Newton's Second Law
Force=Mass x Acceleration
When the road ices over, what force is weakened?
Friction
List the SI unit for force and the unit for mass used in F = ma.
Force — newton (N).
Mass — kilogram (kg).
A change in an object's position over time
What is motion.
Juanita cycles at an average speed of 7 mph for 5 hours. How far does she travel?
d = speed × time = 7 mph × 5 hr = 35 miles.
State Newton's Third Law of Motion
For every action, there is an equal and opposite reaction
Action and Reaction
What is friction
Friction is a contact force that opposes motion; it acts opposite the direction of motion (or the intended motion).
If a problem asks for speed in km/h, what units must distance and time be given or converted into? (name both units)
Distance in kilometers (km) and time in hours (h).
Give an example of a good reference point (name one)
Potential examples: tree, street sign, desk, painted line on a road.
A runner completes 40 m in 5 s. What is the runner’s average speed in meters per second?
speed = distance / time = 40m / 5 s = 8 m/s
State Newton’s First Law of Motion
An object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted on by an unbalanced external force.
or
Inertia
Gravity affects which measurement?
weight
A question will ask you to report velocity in m/s. Write the unit you should use for distance and for time.
Distance in meters (m) and time in seconds (s).
What can you not use as a reference point?
A cyclist travels 18 km in 2 hours toward the northeast. State the cyclist’s velocity including units and direction. velocity = distance / time = 18 km / 0.5 hr = 36 km/hr northeast.
velocity = distance / time = 18 km / 2 hr = 36 km/hr northeast.
What is net force?
Net force is the sum/combination of all the forces acting on an object
Name two factors that increase the amount of friction between surfaces.
Greater force pressing the surfaces together (often related to weight/mass) and increased surface roughness (rougher textures). (Also: larger normal force, heavier object.)
An elevator with a mass of 15,000 kg is accelerating at a rate of 2 m/s2. How much force is used to move the elevator? What unit will you use for force? (no calculations)
Newtons (N)
Explain the difference between distance and displacement.
Distance complete path an object travels, all the motion that occurs
Displacement is most direct path, the shortest route from point A to point B
What is the difference between vector and scalar quanities?
Scalar is only magnitude
Vector is both magnitude and direction
Explain what happens to an object with balanced forces acting on it
Balanced forces add to a net force of zero, so there is no change in the object’s velocity; no acceleration occurs.
A skydiver experiences 60 N down (gravity) and 12 N up (air resistance). State the unit you would use to report net force and indicate whether net force is up or down. (unit + direction)
Unit: newtons (N).
Net force = 48 N down (60 − 12 = 48 N down).
A physics lab asks students to measure a falling object's acceleration near Earth for comparison with gravity. What unit should the acceleration be reported in?
Meters per second squared (m/s^2).
Two cars are at a traffic light. Car A stays at the light. Car B drives around the block and returns to the light. Using the traffic light as the reference point, explain in one or two sentences how Car A’s distance and displacement compare to Car B’s.
Car A: distance = 0 and displacement = 0 because it did not move.
Car B: distance > 0 because it drove around the block, but displacement = 0 because it returned to the same reference point (the traffic light).
Two students play tug-of-war. If neither student moves, describe in one sentence whether the forces are balanced or unbalanced and what that means for the rope’s motion. Then, if one student suddenly pulls harder and the other moves toward them, describe in one sentence how the forces changed and what that says about motion.
If neither student moves, the forces are balanced — the opposing forces cancel and the rope does not accelerate. If one student pulls harder and the other moves toward them, the forces are now unbalanced — the net force is toward the stronger pull, causing the rope (and the weaker student) to accelerate in that direction.
A 2 kg toy is pushed with a net force of 6 N. Without calculating acceleration, identify which law explains how the toy's acceleration depends on force and mass.
Newton’s Second Law (a=F/m).
If an object is being pushed north with a force of 40 N at a rate of 10 m/s2 and suddenly another object traveling south enters its path, what will occur?
Acceleration
Collision
Two people push a cart to the right with 12 N and 8 N while friction resists with 5 N to the left. What is the net force and direction? (unit required)
Net force = (12 + 8) − 5 = 15 N to the right.