Physics Exam 2 Final Conceptual Review

What does zero net force mean

Zero acceleration. The object stays at constant velocity

What does constant net force produce?

Constant acceleration for constant mass. Does not produce constant velocity

Gravity’s components on an incline

mgsin(theta) down the slope, mgcos(theta) perpendicular to the slope

(s for “sin” and “slide” because it’s gonna slide down the slope)

What if two stacked blocks begin slipping?

They can have different accelerations. Use kinetic friction and f=ma for each

What determines friction when a block is pressed against a wall?

The wall’s normal force, it’s coefficient of friction time the horizontal force into the wall

Can work be negative

yes

When is work negative/positive/zero

positive if it aids displacement, negative if it goes against it, zero if perpendicular

Work kinetic energy theorem

Net work equals change in kinetic energy

Net work = (Kinetic energy final) - (Kinetic energy initial)

What does positive net work imply?

Kinetic energy increased, so speed increased

What does constant speed imply about work?

No change in kinetic energy, so net work is zero, individual forces may still do work

What does an area under a force position graph represent?

signed work done by the force. so you add/subtract what is above and what is below

Power formulas

• power is rate of change of work

Power can be work/change in time

Power can be Force x velocity x cos(theta)

Conservative vs nonconservative forces

conservative work depends only on the endpoints (gravity and ideal spring force)

nonconservative depends on path taken (friction)

Potential energy and work done by gravity

calculated same way, just negative of one another.

magnitude of work done by gravity is mgh

Mechanical energy

kinetic + potential energy

Equation including nonconservative work

potential energy initial + kinetic energy initial + work done by nonconservative forces = potential energy final + kinetic energy final

What changes for a falling block compressing a vertical spring?

Gravity keeps doing work during compression. at maximum mg(k+x) = ½ kx²

How is work found when a spring stretches from one point to another?

½ k (x final² - x initial²)

Center of mass formula

add up all mass*position on top, then divide by total mass

How is center of mass velocity calculated?

total momentum/total mass

What causes center of mass acceleration?

Net external force

Momentum

mass * velocity

can be positive or negative

Impulse (change in momentum)

Impulse = force * time

Impulse = (momentum final) - (momentum initial)

What does the area under a force-time graph give?

Signed impulse. This is equal to the object’s change in momentum.

Does a ball’s momentum change when it rebounds at equal speed?

Yes. It’s directions reverses even though the magnitude of the momentum stays the same

Why do a small car and large truck have the same change in momentum?

They receive equal and opposite impulses during their collision

Which has the greater velocity change in the car truck collision

The smaller mass vehicle, for the same impulse magnitude

Why does deflecting a ball transfer more impulse (cause greater change in momentum) than catching it?

Reversing the ball produces a larger momentum than stopping it

When is a system’s momentum not conserved

when the system experiences a nonzero net external impulse

When is momentum conserved during a collision?

When net external impulse is negligible, whether elastic or inelastic

What makes a collision perfectly inelastic?

The objects stick together and move together afterwards, having one shared final velocity

What distinguishes elastic and inelastic collisions?

Elastic collisions conserve kinetic energy, inelastic does not conserve kinetic energy

Both conserve momentum if isolated

In 1D elastic collisions

remember approach speed equals separation speed

Explosions

Momentum is conserved if external impulse is negligible

Kinetic energy can increase using stored internal energy