Chapter 4 ± Making Sense of the Universe (Motion, Energy, and Gravity)
There are 3 basic terms to describe how an object is moving:
1. Speed tells us how far an object will go in a certain amount of time.
x ie, 100km/hr is a speed, which tells us that the car will cover 100 km in an
2. Velocity tells us both the speed and direction of an object.
x ie, 100km/hr going due north.
1. Acceleration is when there is a changing velocity in any way, whether in speed or
direction or both.
x You can often feel the effects of acceleration Æ speeding up in a car would push you
back into your seat, and slowing down in a car would pull you forward from the seat,
and around a curve would push you away from the direction of your turn.
x However, when in constant velocity\RXGRQ¶WIHHOVXFKHIIHFWVLHDVPRRWKIOLJKWRQ
The Acceleration of Gravity
x Gravity accelerates all objects by the same amount, regardless of their mass.
x However, air resistance can cause a difference in acceleration.
o ie, a feather floats gently to the ground, while a rock plummets.
x The acceleration of a falling object is called the acceleration of gravity and is equal to
9.8m/s^2 Æ downward velocity increases by 9.8m/s with each passing second.
Momentum and Force
x $QREMHFW¶Vmomentum is its combination of mass and velocity (Momentum = Mass X
x The net force (overall force) acting on an object represents the combined effect of all
the individual forces put together.
o ie, when driving at a constant velocity, the force generated by the engine to
turn the wheels precisely offsets the forces of air resistance and road friction;
thus, there is no net force on your car.
x A change in momentum occurs only when the net force is not zero.
constant. Thus, the presence of a net force causes an object to accelerate.
x In other words, anytime an object accelerates, a net force must be causing the
acceleration (causing the feelings of pushes forward, backward, or to the side in a car).
Moving in Circles
x An ice skater spins in place with zero velocity and hence with zero momentum.
Simply, her spin gives her angular momentum (circling momentum or turning
x Any object that is either spinning or moving along a curved path has angular
o ie, Earth has angular momentum due both to its rotation (rotational angular
momentum) and to its orbit around the Sun (orbital angular momentum).
x The tySHRIIRUFHWKDWFDQFKDQJHDQREMHFW¶VDQJXODUPRPHQWXPLVFDOOHGDtorque